Phase transitions and properties of ferroics

Resume : In the enhancement of the functionality of ferroelectrics, the role of local structure is very important. In lead free ferroelectrics, the investigation of Li-doped KTa1?xNbxO3 (KTN) has attracted much scientific attention owing to their colossal quadratic electro-optic coefficient, excellent photorefractive, and electrostrictive effects, which make them one of the potential candidates for not only optical but also electromechanical device applications. KTN is the solid solution of KTaO3 and KNbO3. The most important aspect of KTN is the off-center displacements of Nb ions at the B-site, and they induce polar naoregions (PNRs) related to relaxor ferroelectric behaviors. ???The precursor dynamics of a ferroelectric phase transition and the relaxor like behaviors were studied in K(Ta1-xNbx)O3 crystals grown by the top seeded solution growth method. The Burns temperature, TB, and intermediate temperature, T*, were clearly observed in K(Ta0.68Nb0.32)O3 (KTN/0.32) crystals by the acoustic emission measurement [1]. Relaxor like behaviors were studied also by the temperature variation of longitudinal acoustic (LA) phonons and a central peak (CP) in a cubic phase by Brillouin scattering [2]. The anomalous behaviors of LA and CP were discussed on the basis of the polarization fluctuations of PNRs. The order-disorder nature in the vicinity of the Curie temperature, TC, was observed by the temperature variation of a CP. ???The relaxor nature was enhanced by 5% Li-doping, and the broadening of elastic anomaly and the extension of the temperature range between TC and T* were observed [3]. The origin of the increase of random fields was discussed on the basis of the occupancy of Li ions at A and B sites in the perovskite structure. The local symmetry breaking caused by the PNRs in a cubic phase was studied using the appearance of first-order Raman scattering. In KLTN/0.05/0.27, the symmetry of PNRs was investigated by the angular dependence of the polarization plane of the incident and scattered light of Raman scattering [3]. In KLTN/0.05/0.27, the local symmetry breaking by the PNRs observed in a cubic phase is attributed to the E(x,y) symmetry of the PNRs with R3m. The origin of Fano resonance at about 196 cm-1 was discussed by the temperature and electric field dependences of Raman scattering spectra [4]. Its symmetry was also investigated by the angular dependence of Raman spectra. From these experimental results, it is suggested that the origin of Fano resonance of KLTN is the coupling between polarization fluctuations in PNRs and the TO2 mode with the E(x,y) symmetry. References [1] E. Dul?kin, S. Kojima, and M. Roth, Europhys. Lett. 97, 57004 (2012). [2] R. Ohta, J. Zushi, T. Ariizumi, and S. Kojima, Appl. Phys. Lett. 98, 092909 (2011). [3] M. M. Rahaman, T. Imai, J. Miyazu, J. Kobayashi, S. Tsukada, M. A. Helal, and S. Kojima, J. Appl. Phys. 116, 074110 (2014). [4] M. M. Rahaman, T. Imai, T. Sakamoto, S. Tsukada, and S. Kojima, Sci. Rep. 6, 23898 (2016).

Resume : A review of the recent development and application of atomistic effective Hamiltonian techniques to study Ba(Zr,Ti)O3 (BZT) [1-5] and PbMg1/3Nb2/3O3 (PMN) [6] relaxor ferroelectrics will be provided. In both cases, our calculations yield macroscopic relaxor properties and reveal the formation of polar nanoregions, but, further microscopic analyses reveal a drastic difference between the origins of these properties in BZT and PMN. In BZT, they are mostly due to the difference of the local polarizability between Ti and Zr sites, whereas, in PMN, they are caused by random fields and a subtle competition between antiferroelectric-like and ferroelectric-like interactions. We also explored the dependence of the relaxor properties on temperature, electric field and epitaxial strains. This work is financially supported by ONR Grant N00014-12-1-1034 and NSF grant DMR-1066158. D.W. also acknowledges support from National Natural Science Foundation of China (Grant No. 51390472) and National Basic Research Program of China (Grant No. 2015CB654903). 1. A. R. Akbarzadeh, S. Prosandeev, E. J. Walter, A. Al-Barakaty, and L. Bellaiche, Phys. Rev. Lett. 108, 257601 (2012). 2. S. Prosandeev, D. Wang, and L. Bellaiche, Phys. Rev. Lett. 111, 247602 (2013). 3. S. Prosandeev, D. Wang, A. R. Akbarzadeh, B. Dkhil, and L. Bellaiche, Phys. Rev. Lett. 110, 207601 (2013). 4. D. Wang, J. Hlinka, A. A. Bokov, Z.-G. Ye, P. Ondrejkovic, J. Petzelt, and L. Bellaiche, Nat. Commun. 5, 5100 (2014). 5. S. Prosandeev, D. Wang, A. R. Akbarzadeh, and L. Bellaiche, Topical Review. J. Phys.: Condens. Matter 27, 223202 (2015). 6. A. Al-Barakaty, S. Prosandeev, D. Wang, B. Dkhil, and L. Bellaiche, Phys. Rev. B 91, 214117 (2015).

Resume : NaNbO3 has probably the most complex phase diagram of any ?simple? ABX3 perovskite. It exhibits seven distinct phases as a function of temperature, several of which evaded detailed structural characterization until quite recently. I will first briefly summarise our work on two of these complex phases, which we showed to have complex octahedral tilt patterns with unit cells of 16 and 24 times the volume of the aristotype perovskite (1). Our more recent work (2,3) has concerned Li-doped NaNbO3. This phase diagram apparently shows yet more complex behavior, and we provide evidence for complex tilt phases with unprecedented superlattices involving axial lengths up to 30 times that of the aristotype perovskite. Structural analysis in terms of correlated ?symmetry modes? rather than conventional independent coordinates is shown to be crucial in unravelling such complex structures. [1] M. D. Peel et al., Inorg. Chem., 2012, 51, 6876. [2] M. D. Peel et al., Inorg. Chem., 2013, 52, 8872. [3] C. A. L. Dixon et al., in preparation.

Resume : Ferroelectric oxides are known to be very sensitive to any external "mechanical" stress. Such stress can be applied through various paths. Usually, the approaches to induce 3D, 2D or 1D stress use diamond anvil cell technique for hydrostatic pressure, thin film clamping for biaxial epitaxy stress or tensile/compression machine for uniaxial stresses, respectively. In this talk, we will present some examples showing how good stress can be very useful for functional oxides through the tuning or the improvement of their physical properties and how such stress can be smartly generated. Among the illustrations and depending on time, we will show how 1) stress (uniaxial to isotropic) can be favorably used to improve caloric responses in BaTiO3 through mechanocaloric effect being as big as the electrocaloric counterpart [1-3]; 2) stress can be used in artificial multiferroics like FeRh/BaTiO3, to generate unprecedented magnetoelectric coupling [4,5] or impact on the magnetocaloric losses [6]; 3) an unusual 3D stretching (negative pressure) can be realized in PbTiO3 nanowires via an original core-shell structure and induce enhanced ferroelectric properties [7]; 4) in case of the model multiferroic BiFeO3, in which the stress is beneficial to tune the structure as well as the ferroelectric and magnetic properties [8-11], one can use light excitation [12] and especially ultra-fast laser pulses to induce strong and useful coherent acoustic waves [13-15]. [1] Y. Liu et al., Phys. Rev. B 90, 104107 (2014) [2] Y. Liu et al., Appl. Phys. Lett. 104, 162904 (2014) [3] Y. Liu et al., Adv. Mater. 26, 6132 (2014) [4] R. O. Cherifi et al., Nat. Mater. 13, 345 (2014) [5] L.C. Phillips et al., Sci. Rep. 5, 10026 (2015) [6] Y. Liu et al., Nat. Comm., in press (2016) [7] J. Wang et al., Nat. Mater. 14, 985 (2015) [8] Y. Yang et al., Compt. Rend. Phys. 16, 193 (2015) [9] C. Daumont et al., J. Phys.: Condens. Matter 24, 162202 (2012) [10] D. Sando et al., Nat. Mater. 12, 641 (2013) [11] D. Sando et al., Adv. Mater. submitted (2016) [12] C. Paillard et al., Adv. Mater. doi:10.1002/adma.201505215 (2016) [13] M. Lejman et al., Nat. Comm. 5, 4301 (2014) [14] C. Paillard et al., Phys. Rev. Lett., in press (2016) [15] M. Lejman, et al., Nat. Comm., under review (2016)

Resume : Over the past two decades, a significant progress has been achieved in epitaxial growth of (multi-) functional oxide films. By applying epitaxial strain to thin films, ferroic transition temperatures can be increased by hundreds of degrees, new phases can be induced or the coupling between different ferroic orders can be modified. Unfortunately, due to the low film thickness and the often only subtle structural modifications, the structural characterization of (multi-) functional oxide thin films, especially in heterostructures and in the ultra-thin regime, remains challenging. Here, we present evidence that tensile and compressive strain can induce multiple phase transitions in LaNiO3 films [1] and that the different phases and subtle modifications can be traced by Raman scattering even in ultra thin layers down to 1.2 nm of thickness. [1] M.C. Weber, M. Guennou, N. Dix, D. Pesquera, F. Sánchez, G. Herranz, J. Fontcuberta, L. López-Conesa, S. Estradé, F. Peiró, J. Iñiguez, J. Kreisel, (2016) arXiv:1603.00609 [cond-mat.mtrl-sci]

Resume : Domain walls and twin boundaries currently attract enormous attention, since they can host functional properties, that are not present in the bulk crystal [1]. Modelling of functional twin walls ranges from ab-initio calculations [2], MC [3] or MD simulations to phenomenological descriptions based on Ginzburg-Landau-Devonshire free energies [4]. Although domain wall motion can significantly influence the macroscopic behaviour of a material [5] a comprehensive theory including domain freezing is still missing. Recently the notion of „domain glass“ [6] was introduced for highly twinned ferroelastic materials, which in contrast to „strain glasses“ [7] can occur in perfectly defect free crystals. We give an overview about our experimental results on domain wall motion and domain freezing in a number of ferroelastic syststems and discuss the data in the light of recent developments. Work supported by the Austrian FWF (P28672-N36). [1] Van Aert, S., Turner, S., Delville, R., Schryvers, D., Van Tendeloo, G, and Salje, E.K.H. (2012) Adv. Mater. 24, 523 (2012) [2] B. Meyer and D. Vanderbilt, Phys. Rev. B 65, 104111 (2002) [3] A. Tröster, C. Dellago and W. Schranz, Phys.Rev. B 72, 094103 (2005) [4] P. Marton, I. Rychetsky, and J. Hlinka, Phys. Rev. B 81, 144125 (2010) [5] W. Schranz, H. Kabelka, A. Sarras and M. Burock, Appl. Phys. Lett. 101, 141913 (2012) [6] E. K. H. Salje, X. Ding and O. Aktas, Phys. Status Solidi B 251, No. 10, 2061 (2014) [7] D. Wang, Y. Wang, Z. Zhang, and X. Ren, Phys. Rev. Lett. 105, 205702 (2010)

Resume : The 212 species of the structural phase transitions with a macroscopic symmetry breaking were inspected with respect to the occurrence of the ferroaxial order parameter, the electric toroidal moment. In total, 124 ferroaxial species were found, some of them being also fully ferroelectric (62) or fully ferroelastic ones (61). This ensures a possibility of electrical or mechanical switching of ferroaxial domains. For each species, we have also explicitly worked out a canonical form for a set of representative equilibrium property tensors of polar and axial nature in both high-symmetry and low-symmetry phases. This information was gathered into the set of 212 mutually different symbolic matrices, expressing graphically the presence of nonzero independent tensorial components and the symmetry-imposed links between them, for both phases simultaneously.

Resume : A full electromechanical description of a piezoelectric material requires three coefficients, for example the elastic compliance, the dielectric permittivity and the charge coefficient. Whilst the relationship between the latter two are well described by Landau-Devonshire theory, the correspondence with compliance is not. The recently proposed Equivalent Dipole Model [1] derives relationships between all three coefficients in terms of an effective dipolar charge and the asymmetry in the interatomic bond strengths in a simple 3 atom dipole. However, the model is limited by its lack of intrinsic temperature dependence. Based on literature data of the piezoelectric, dielectric and elastic properties of single crystals, the temperature dependence of the equivalent dipole parameters has been determined for barium titanate, showing that the temperature dependence of the charge coefficient, dielectric permittivity and elastic compliance are dominated by the critical softening of only one of the two dipole bonds. The relationship between the effective dipole model and the Landau-Devonshire description of piezoelectric materials is examined. References [1] A. J. Bell, Journal of Applied Physics, 118, 224103 (2015)

Resume : The concept of broken symmetry is central to many areas in physics. In particular, Landau theory (LT) is an essential cornerstone of the theory of structural phase transitions. On the other hand, the last decades have seen a tremendous success of ab initio methods in condensed matter physics. Yet, the concepts of DFT and LT are to some extent antipodal. Indeed, condensed matter broken symmetry phases usually appear at low temperature and are thus accessible by ab-initio methods. However, as a rule the high symmetry reference phase, which is the pivotal reference frame of LT, only exists at elevated temperatures. DFT and LT thus appear as complimentary concepts, and the question of how to blend these two approaches in an efficient way has been an active area of research for the last two decades. In particular, DFT calculations are indispensable for understanding high pressure phase transitions. Unfortunately, while imposing high pressure usually does not pose serious additional difficulties in DFT, an attempt to similarly extend LT to include high pressure phase transitions that involve nonlinear elasticity as a central ingredient turns out to be a non-trivial enterprise. Yet, recently we have succeeded in constructing such an extension [PRX 4, 031010 (2014)] and have demonstrated both its practical applicability as well as the tremendous increase in numerical precision over a standard Landau description by applying it to the archetypal perovskite SrTiO3. Essential for the su

Resume : Pattern formation in macroscopic systems subjected to injection and dissipation of energy, matter and/or momentum often lead to the formation of spatial patterns. Such structures can be extended, occupying the whole system, or localized, existing only on a fraction of the system. It has been long known that multilayered materials pattern forming instabilities. Changes in the stiffness of the material layers through chemical or physical action can induce these instabilities. Pattern forming phenomena poses a challenge in fabrication of stable materials, but also offers the possibility of externally modulating the surface properties of materials. Recently, it was shown that the quantitative description of wrinkling and pattern formation of curved multilayered materials might be achieved by considering a generalized Swift-Hohenberg equation, where extended patterns emerge form the homogeneous state generating hexagonal and labyrinthine structures. Here, we show that the generalized Swift-Hohenberg model supports the existence of multiple types of localized states. Interaction properties of these structures allow for the generation of a wide variety of bound states and tailored patterns in the material. Moreover, these localized states give an alternative mechanism by which extended patterns may propagate through the material, localized structure may suffer from a curvature instability elongating, leading to the formation of labyrinthine structures.

Resume : Lead zirconate titanate or PZT, with the perovskite structure, is a well-known ferroelectric and piezoelectric ceramic material with high spontaneous polarization, dielectric permittivity and piezoelectric coefficients. With the advent of nanotechnology in the recent decades, PZT nanostructures have become the main focus of research due to their potential applicability in nanoelectronic devices such as nanoelectromechanical systems (NEMS), ferroelectric nonvolatile random access memories and nanoactuators [1,2]. PZT nanoparticles have already been synthesized by the alkoxide based sol?gel technique using the hydrolysis and condensation of these precursors. However, alkoxide based synthesis method has some well know drawbacks; alkoxide-based PZT sols are less stable because they contain water [3]. Besides, successful crystallization of the perovskite phase from the calcinated alkoxide-based PZT gels requires heat treatment above 650°C [4]. This may increase the heat treatment temperature and possible evaporation of Pb. In the current research work, PZT nanoparticles, with the morphotropic phase boundary, are synthesized via a polyol-based sol?gel process. It is shown here that highly crystalline single phase PZT nanoparticles could be synthesized about 580°C via this method. Thus, this method can decrease the required final firing temperature about 70-120°C compared with the alkoxide based synthesis method, which reduces the heat treatment temperature and possible Pb evaporation. [1] A. Nourmohammadi, M. A. Bahrevar, and M. Hietschold, Journal of Alloys and Compounds, vol. 473, pp. 467-472, 2009. [2] A Gruverman, A Kholkin, Rep. Prog. Phys. 69, 2443?2474, 2006. [3] Qiu, S., Zheng, X., Gao, C., Gan, X., Chen, J., Yang, C. and Fan, H. (2009), Ceramics International, vol. 35, no. 2, pp. 733-740. [4] Q. Zou, S. Nourbakhsh, and J. Kim, Materials Letters, Vol.40, pp. 240?245, September, 1999.

Resume : The antiferroelectric lead zirconate, PbZrO3, structures are potential candidates for different technical applications, which among them, PbZrO3 nanoparticles are increasingly taken into consideration2, because a ferroelectric PbZrO3 state can be observed when a sufficiently large electric field is applied. The stabilization of the ferroelectric state is a size-dependent phenomenon in perovskite oxides. In PbZrO3, the transition occurs above 230˚C in bulk PbZrO3 material. However, size effect can reduce the mentioned Curie temperature3. After the applied electric field is removed, a large amount of electric charge is released, which is promising for many technical applications such as charge storage devices, current sources and transducers 4,5. For this reason, studying PbZrO3 nanoparticles is very important and interesting. Several groups have already synthesized PbZrO3 nanoparticles using different methods such as co-precipitation of Pb and Zr hydroxides2, solid state reaction6 and non-aqueous diol-based method7. However, as far as we know, crystallization of single perovskite phase PbZrO3 nanoparticles in the conventional alkoxide-based sol-gel method is less investigated. Here, in order to achieve single-phase perovskite , PbZrO3, nanoparticles by the conventional sol-gel method, the effect of heat treatment conditions and Pb content on crystallization of the prepared PbZrO3 nanoparticles as well as Pb-partitioning phenomenon were investigated in detail. First, thermal properties of PbZrO3 precursor gel phase was evaluated by the thermal analysis method. Then, phase composition of the synthesized PbZrO3 nanoparticles was determined by X-ray diffraction (XRD). XRD data confirmed the effect of precise heat treatment conditions, including annealing temperature and time and heating rate, as well as the impact of Pb content. Based on the achieved results, both heat treatment conditions and Pb content were precisely adjusted to improve the formation of single-phase PbZrO3 nanopowders and prevent formation of toxic lead compounds. The average size of the formed PbZrO3 nanocrystals, prepared in different conditions, was calculated by both Scherrer and Williamson-Hall methods. Finally, by optimizing the annealing temperature and time and heating rate as well as Pb content, crystallization temperature lower than 580 °C was achieved. Sol-gel processing is a simple and cost-effective method to grow nanoparticles with controlled size and morphology. Therefore, our study can help investigation of size effect in PbZrO3 nanoparticles. References (1) Singh, S.; Krupanidhi, S. Advanced Science Letters 2011, 4, 3599. (2) Chattopadhyay, S.; Ayyub, P.; Palkar, V.; Gurjar, A.; Wankar, R.; Multani, M. Journal of Physics: Condensed Matter 1997, 9, 8135. (3) Liu, H.; Dkhil, B. Zeitschrift für Kristallographie Crystalline Materials 2011, 226, 163. (4) Bharadwaja, S.; Krupanidhi, S. Materials Science and Engineering: B 2000, 78, 75. (5) Tang, X.-G.; Wang, J.; Wang, X.-X.; Chan, H. L.-W. Solid state communications 2004, 130, 373. (6) Khamman, O.; Sarakonsri, T.; Rujiwatra, A.; Laosiritaworn, Y.; Yimnirun, R.; Ananta, S. Journal of Materials Science 2007, 42, 8438. (7) Singh, S.; Krupanidhi, S. Current Nanoscience 2009, 5, 489.

Resume : Lead-based perovskite ferroelectric materials such as PbZrxTi1-xO3 or PZT are widely used in industry because of their superior ferroelectric properties. However, they are harmful and will be banned in near future, the RoHS European directive (Restriction on Hazardous substances) has prohibited lead in electric and electronic applications since July 2006. No lead-free ferroelectric ceramic has better or equal piezoelectric performance compared with PZT [1]. For this reason, a lot of research works are being performed to modify the ferroelectric and piezoelectric properties of lead-free provskite materials. barium strontium titanate (BST) BaxSr1-xTiO3 is a good lead-free piezoelectric and ferroelectric composition , Experience shows that the best room temperature dielectric properties of BST are observed for the compositions around x=0.7 [2-4]. In this research work, Ba0.6Sr0.4TiO3 nanoparticles are synthesized by the sol-gel method. The effects of the treatment condition and ethylene glycol addition on their phase structure are investigated. X-ray diffraction (XRD) analyses show formation of the perovskite barium strontium titanate nanoparticles, along with barium carbonate, after calcination.it is found that heat in forming barium carbonate in the samples. Also, experience shows that increasing the temperature causes an increase in the peak intensities of perovskite structures, and a decrease in the intensities of barium carbonate peeks. Moreover, omitting ethylene glycol, as the stabilizer in the sol preparation procedure, will decrease the peak intensities of barium carbonate. Also, by omitting water, as the solvent for the peovskite phase are obtained, free from barium carbonate phase. It is observed that a decrease in the heating rate causes better burnout of the contained organic materials and, as a result reduces the peak intensity of barium carbonate. Accordingly, single-phase perovskite barium strontium titanate nanoparticles are obtained using acidic precursor sols by decreasing the gel heating rate, increasing gel calcination temperature and omitting ethylene glycol and water in the sol preparation procedure. [1]. J-C. Carru , M. Mascot , A.Khalfallao , D.Fasquelle , and G.Velu, Int.J.Nanoelectronics and Materials 3, 9-21 (2010). [2]. S. U. Adikary, and H. L. W. Chan, Thin Solid Films 424, 70-74 (2003). [3]. S. Lahiry, and A. Mansingh, Thin Solid Films 516, 1656-1662 (2008). [4]. G. Brankovi´c , Z. Brankovi´c , M.S. G´oes , C.O. Paiva-Santos , M. Cilense , J.A. Varela , E. Longo, Materials Science and Engineering B 122, 140?144 (2005).

Resume : Barium strontium titanate (Ba1-xSrxTiO3, later BST) thin films have been intensively investigated for applications in high density dynamic random access memories. The transition temperature and hence the electrical and optical properties of BST ceramic can be tuned in a controllable way to get various functionalities for electronic and electromechanical applications. Internal fields could arise due to the lattice mismatch between the film and the substrate in the case of epitaxial films and differences between thermal expansion coefficients of the film and the substrate. The aim of this work is to investigate the electrical properties of different thickness and degree of strain BST films and estimate the relation of the strain to the activation energy of the most abundant charge carriers. BST epitaxial thin films of different thicknesses were deposited by pulsed laser deposition technique. HP4284 precision LCR-meter was used for complex impedance measurements at temperatures from 500 K to 300 K during cooling cycle at the rate of about 1 K/min and frequencies from 20 Hz to 1 MHz. It was found, that the films consist of two BST sublayers, one of them fully strained by the substrate, the other is relaxed. These layers have different activation energies of the specific resistivity. A model was proposed to explain these experimental results. This model allows to associate the activation energies with strain in the layers. The obtained results can be used to engineer the mobility of charge carriers in thin films by strain in various electronic devices, such as memristors, electromechanical devices and on-chip resistors in oxide-on-Si integrated circuits.

Resume : Bismuth manganite ceramics, which consisted of two phases, cubic I23 and orthorhombic Pbam, were obtained by standard high temperature sintering in ambient air. In the second stage, powdered ceramics were sintered in gaseous nitrogen flowing around the pellets at several stabilized temperature Ts = 870-1120 K for 4 h. Grain morphology and chemical composition was determined by scanning electron microscopy. We observed change in the grain shapes, i.e. the number and size of the hexagon-based parallelepipeds increased. The formation of orthorhombic BiMn2O5 phase was favored when sintering temperature Ts increased, which was determined using X-ray diffraction test. The local disorder was tested by X-ray photoemission spectroscopy. Multi-component lines assigned to O 1s, Mn 2p, Bi 4f were detected. The ratio of intensities of particular components was influenced by the sintering conditions. The shape of valence band was not influenced by the sintering conditions. Electric DC and AC conductivity was measured in 200–750 K range. The ceramics shown stabile semiconductor features. The activation energy increased slightly when the sintering temperature increased. The ceramics shown marked dispersion in electric conductivity and in dielectric losses. The relaxation processes were assigned to small polaron mechanism. The nearest neighbor hopping and variable range hoping models of small polarons were discussed. It was noticed that the variable range hoping model corresponded to occurrence of disorder in the studied ceramics.

Resume : Ferroelectric LiNbO3 crystal grown by the Czochralski technique is commonly used material for application in optoelectronic devices. Features of the lithium niobate can be modified by using a chemical process like proton exchange reaction, which may be useful for the applications in the surface acoustic wave (SAW) devices [1], in the fabrication of waveguides [2, 3] or for accumulation of the charge carriers [4]. In the present research we studied pure congruent LiNbO3and congruent samples modified with proton exchange reaction which led to the Li+/H+ substitution [5]. The structure of the lithium niobate was determined by using an X-ray powder diffractometer at RT, and at 200ºC for pure and etched samples. In the pure sample the rhombohedral crystallographic structure was maintained, and for the etched sample the monoclinic structure occurred. The Curie temperature was determined in terms of the Brillouin scattering. Measurements were carried out in the temperature range from RT to 1230ºC. Pure and etched samples (for 480h and 720h in acid solution) showed similar Tc=1120ºC. Anomaly in the Brillouin frequency shift was observed for both pure and protonized samples in vicinity of Tc for the longitudinal acoustic mode. [1] Hinkov V, Barth M, and Dransfeld K. Appl. Phys. A. 1985;38:269-273. [2] Gallo K, Gawith CBE, Wellington IT, Mailis S, Eason RW, Smith PGR, and Richardson DJ, Kostritskii SM. J APPL PHYS. 2007;101:014110-1-3. [3] Gallo K, Pasquazi A, Stivala S, and Assanto G. PHYS REV LETT. 2008; 100: 053901-1-4. [4] Neumayer SM, Manzo M, Kholkin AL, Gallo K, and Rodriguez BJ. J APPL PHYS. 2016;119: 114103-1-7. [5] Piecha J, Szot K, Pilch M, Gruszka I and Molak A. Ferroelectrics. 2014;466:1-7.

Resume : Gd2(MoO4)3 (gadolinium molybdate, GMO) is a well-studied multifferoic material that exhibits ferroelectric and ferroelastic orderings above room temperature. Its application on the field of multiferroic materials/devices could prove advantageous due to the large spontaneous strain that accompanies its electric polarization. The permanent spontaneous strain that GMO shows could be used to write magnetic information in multiferroic heterostructures in a more efficient manner. Regardless of its promising applicability, GMO thin films have not been yet reported on silicon substrates and the optimal stabilization parameters are still unknown. In this study GMO films were deposited on Si(001) substrates by Pulsed Laser Deposition (PLD). We studied the influence of substrate temperature and oxygen pressure on the crystalline properties of the films. Morphology, composition and crystalline structure were studied by XRD, AFM, XPS, HR-TEM and optical techniques (SHG- Second harmonic generation). Results showed that GMO films presented a granular morphology and two distinctive phases, stoichiometric GMO, with strong SHG response, and oxygen deficient GMO, without SHG signal. Finally, ex-situ annealing treatment of the samples, performed under oxidative conditions, showed the progressive reconstruction of the surface, towards stoichiometric GMO, and the improvement their optical and crystalline properties. Acknowledgements: The financial support of the National Science Centre (PRELUDIUM UMO-2015/17/N/ST5/01988) is acknowledged.

Resume : Bi2FeMnO6 (BFMO) lies between two of the most well studied compounds in the field of the so called multiferroic materials, BiFeO3 and BiMnO3, the characteristics of which are well described in the literature and still focusing much attention among researchers. Here, we report on the single phase stabilization of the BFMO double perovskite deposited on SrTiO3(001)-Nb(0.5%) substrates by pulsed laser deposition. BFMO thin films show weak room temperature ferromagnetism which was studied by VNA and SQUID techniques, revealing a magnetic susceptibility of 57,42 emu and a magnetic moment of 0.4 µB/f.u(Fe-Mn). The chemical origin of the ferromagnetism was investigated by XPS and is attributed to the high mixture of valences of Mn in the films. Dielectric measurements performed from 100 to 400K show a well-defined decoupling between intrinsic and extrinsic mechanisms. The dielectric constant was found to be ɛ=175, in close agreement with the literature. Finally, magneto impedance spectroscopy showed a direct influence of the external magnetic field on the dielectric constant of the film, showing a peak ≈ 400K. The results suggest a large magneto-electric coupling well above room temperature, although no ferroelectric response has been observed. Acknowledgements: The financial support of the National Science Centre (PRELUDIUM UMO-2015/17/N/ST5/01988) is acknowledged.

Resume : The presence in some materials of light elements, the fact that the most accesible structural determination technique, x-rays, does not discriminate between some elements (no contrast), the not-so-easy-access to best suited high-resolution techniques for the structural determination, has lead us, among some other reasons, to think on a more efficient workflow for the refinement of the patterns of perovskite materials. The Bilbao Crystallographic Server, with AMPLIMODES and related Solid State Theory Applications and the Structure Utilities, has facilitated the path to elaborate a new structural analysis workflow, less expensive, more autonomous and independent, less time-consuming to reach the final solution and more efficient. It is based on a special parametrization (P) in the refinements of some degrees of freedom. The P is done guided by the experimental results from high-resolution data on the material (or on related materials). The P is based on mode-crystallography: the structural degrees of freedom are not the atomic co-ordinates, but the amplitudes of symmetry-adapted-modes. The workflow gives a first approach to the final solution, reduces the need to perform high-resolution experiments. Makes more usable data from newly designed preliminary x-ray diffraction experiments. Thus, home-made experiments have extended capabilities for groups with no access to big facilities, but giving them reliable tools for their own systematic structural analysis or for suitable re-visit of cases subjected to controversy.

Resume : The THz--frequency range dispersion of the complex dielectric permittivity tensor of antiferroelectric lead zirconate, associated with its multiple weak infrared active phonon modes, is modeled by the oscillator formula with parameters fully determined from the recently developed atomistic shell model. The resulting realistic dielectric function is used to estimate the far--infrared reflectivity spectra of a twinned lead zirconate crystal in the limit of narrow and wide domains. It is shown that in case of the sufficiently narrow domain widths, the effective--medium average permittivity shows additional modes identified as geometric resonances (i.e., extraneous excitations created by material interfaces), possibly distinguishable in suitable experiments.

Resume : Bending a dielectric material of any symmetry generates polarization thanks to the flexoelectric effect. Bulk flexoelectricity is not the only way extract polarization from bending, however, as surface piezoelectricity, arising from the intrinsic asymmetry of interfaces, yields the same functional response to bending as bulk flexoelectricity. In addition, semiconductors can have insulating barrier layers whenever their interfaces with electrodes are not ohmic. The combination of surface piezoelectricity and interfacial charge depletion thus allows even semiconducting materials to generate charge under bending, and this represents an advantage over ?conventional? flexoelectricity, which requires dielectric insulation. As I will show in the talk, the mechanism of surface piezoelectricity in semiconductors can also be quantitatively better than bulk flexoelectricity: bending doped semiconducting oxides yields effective flexoelectric coefficients that are orders of magnitude larger than bending the same oxides in an undoped, insulating state.

Resume : The surface as a native end of the crystal is per se an inseparable part of the real crystal with properties, which are not implicitly identical than the properties of the bulk. The reduction of the surface energy, which can be for example manifested via a reduction of a number of so called dangling (broken) bonds can force the surface to relaxation, rumpling or reconstruction. This classical idealized picture of the transformation of the surface, typical for the school of the surface physics, cannot be used for the description of the modifications of the chemical composition of the surface of ternary oxides with perovskite structure. Origin for this untypical behavior of the surface is easy segregation of cations or anions in relatively broad layer (region) caused by exposition on different external parameters such as: temperature, partial pressure of oxygen, electrical gradient, bombarding with particles, illumination, mechanical stress or their combination [1]. Moreover, surface sensitive studies on the macro and nano-scale give evidence that the surface region of real crystals of the model ABO3 oxides with perovskite structure (here SrTiO3 and BaTiO3) show the astonishing transformation to lower BO2-x oxides under reducing conditions or to AO*(ABO3)n compounds during oxidation [2]. A possible driving force for this compositional transformation may be connected with different vacancy-formation energies in the surface layer with respect to the bulk or the influence of the space-charge region on the segregation of vacancies. In my talk I will demonstrate that extended defects (especially edge dislocations) should be taken into account for the observed rapid chemical transformation of the surface layer or region since they take the role of preferential paths for the segregation [1, 3]. In the literature it is accepted that the dislocations could be selectively reduced in the perovskites (for example STO), though the question of pipe diffusion of oxygen along of this type of extended defects is still controversial [4]. As this problem of oxygen diffusion and phase change is important for the phenomena of resistive switching, I will dedicate a part of my talk to an explanation of memristive phenomena in the surface layer of the model perovskite (here STO and BTO) which, in my opinion, is based on the structural and chemical transformation close to the core of single and bundled dislocations, similar than for the surface layer, and is related to the induced preferential migration of oxygen along of these extended defects [1]. 1) K. Szot, W. Speier, G. Bihlmayer, Book Series (Editors: RE. Camley, RL. Stamps), Solid State Physics, 65, 353(2014), 2) K. Szot and W. Speier Phys. Rev. B 60, 5909(1999), 3) R. Waser, R. Dittmann, G. Staikov, K. Szot, Advanced Materials, 21, 2632 (2009), 4) D.Marrocchelli?, L Sun, and Bilge Yildiz*, J. Am. Chem. Soc., 137, 4735 (2015) Permanent address: Peter Grünberg Institute, Forschungszentrum Jülich, D-52-428 Jülich, Germany

Resume : Multiferroic materials with coexisting magnetic and ferroelectric orders hold promise for the manipulation of magnetism by applied electric fields. These effects were demonstrated [e.g. Y. Tokunaga et al. Nature Materials 8, 558 (2009)] to be controlled by the dynamics of strongly interacting domain walls of different types. We report on modeling the electric filddriven dynamics of ferroelectric domain walls in improper ferroelectrics.

Resume : Multiferroics are a topic of current intense investigations: Type-II or ?improper? multiferroics exhibit strong magnetic-ferroelectric coupling, however, the ferroelectric order parameter is much smaller than robust nonmagnetic ferroelectrics such as the prototypical Ba2+Ti4+O3; The Type-I or ?proper? multiferroics with robust displacive-type ferroelectric order are rare and have very weak coupling between the order parameters. Recently new multiferroics have been discovered with strong coupling for the magnetic Sr1-xBaxMnO3 materials with x?0.45, which exhibit robust ferroelectricity and G-type antiferromagnetism originating exclusively from the Mn4+ cations. They exhibit classical displacive-type ferroelectric phase with a polarization of several µC/cm2 when the Mn ions move out of the center of the MnO6 octahedral units at TF~350 K. The Mn spins order below TN~210 K into a simple G-type magnetic structure while the displacive distortions decrease dramatically (80?100%) demonstrating that the two order parameters are strongly coupled. The ferroelectric phase transition has a signature of a crossover from displacive to order-disorder type. The phonons are coupled with a central mode but contribution to ?? is rather small. The lowest-frequency polar phonons are overdamped above TN and they exhibit pronounced softening on heating towards TF. We have recently extended investigation of manganites to the Ti-substituted system for which displacive distortions significantly exceed that of BaTiO3 and TF~420 K. The TN decreases to below 200 K and the suppression of ferroelectricity below TN is reduced to ~50 %, i.e., we achieved displacive-type multiferroic with large spontaneous polarization.

Resume : The possibility to control magnetic anisotropy by using only an electric field has rendered compounds with interacting ferromagnetic and ferroelectric order a prime target of materials research. However, both orders hardly coexist at room-temperature in a single material phase, thus efforts focused on artificial heterostructures and magnetoelectric coupling as a composite effect. Here we design a multiferroic heterostructure by growing a ferromagnet (FM) on top of a multiferroic BiFeO3 (BFO) film and we use a combination of magnetic force microscopy (MFM) and second harmonic generation (SHG) to detect the distribution of the ferromagnetic domains in the FM and the buried ferroelectric domain state in BFO. With SHG we establish a unique relation between the BFO domain distribution and the nonlinear optical response without disturbing the multilayer system [1]. MFM reveals the one-to-one coupling of the ferroelectric/ferromagnetic domains. With an external electric field, we manipulate the BFO local ferroelectric state and we use magnetic field dependent MFM to probe the resulting effect on the ferromagnetic domains and anisotropy. Previous work has unveiled in such systems that the net magnetization of the layer can be electrically reversed. Here we show that a local magnetic anisotropy rotation can be achieved and correlated with the underlying ferroelectric domain state after voltage application. [1] M. Trassin, G. De Luca et al., Adv. Mater. 27, 4871 (2015)

Resume : Multiferroic hexagonal manganites are antiferromagnetic improper ferroelectrics where unit-cell-tripling buckling of oxygen bipyramids induces polarization and, in some compounds, weak ferromagnetism. Understanding the dynamical effects controlling motion of clamped structural, ferroelectric and magnetic domain walls (DW) in these materials is critical to design devices based on controlled switching of DWs. However, the study of DW dynamics in realistic multiferroics has been mainly focused on proper ferroelectrics and ferromagnetic materials so far, and for multiferroics was mostly limited to estimating switching barriers [1,2]. We develop a model Hamiltonian to describe the driven dynamics of DWs in hexagonal manganites, with parameters extracted from ab-initio calculations. [1] Yu Kumagai, N. A. Spaldin Nature Communications 4, 1540 (2012) [2] N. A. Benedek and C. J. Fennie, Phys. Rev. Lett. 106, 107204 (2011)

Resume : RMn2O5 compounds recently attracted attention due to complex frustrated ground state and unconventional excitations. YMn2O5 with non-magnetic rare earth shows incommensurate spiral state with spins in the neighboring chains aligned at 90-degrees to each other[PRL]. Here we model the phase diagram and magnetic excitations in GdMn2O5, with magnetic rare earth.

Resume : Ferroelectric crystals are import and basic materials for technological applications in capacitors, pyroelectric, and electrooptical devices. Their nonlinear characteristics were found to be very useful, for example, in optical second-harmonic generators and other nonlinear optical devices. Recently, the molecule-based ferroelectrics, simple alkylammonium organic salts: diisopropylammonium chloride (DIPAC) and diisopropylammonium bromide (DIPAB) have been synthesized and characterized. They undergo a reversible phase transition from a high temperature paraelectric phase (2/m) to a low temperature ferroelectric phase (2) at ca. 440 K and 425 K, respectively with the spontaneous polarization resulting from the ordering of the organic cations (?order?disorder?-type ferroelectric). DIPAB is characterized by an extremely high value of the spontaneous polarization (23 ?C cm-2), a high Curie temperature (426 K), a high dielectric constant, small dielectric losses and a low coercivity field. Additional qualities of DIPAB are: facility of preparation, low cost, nontoxicity, and good thermal stability. DIPAB also shows a strong piezoelectric effect and has a well defined ferroelectric domain structure. Recently, we have synthesized and characterized the physicochemical properties of a novel biferroic material: diisobutylammonium bromide [i-(C4H9)2NH2][Br] (DIBAB) by thermal, electric and spectroscopic measurements. DIBAB exhibits unique and unexpected properties related to ordering of chains of diisobutylammonium cations. It undergoes a strongly discontinuous phase transition (at 285/286 K, cooling/heating respectively) driven by an order?disorder mechanism strongly coupled to macroscopic deformation.

Resume : Many macroscopic characteristics of ferroelectric materials are directly related to the physical properties of domains and domain walls. Therefore, it is crucial to investigate structure, size and orientation of domains and domain walls on the nanoscale in order to get a fundamental understanding of formation mechanisms and functionality of domain walls. Especially, periodic structures with monoclinic symmetry are of particular interest. Their formation has been predicted for (K,Na)NbO3 thin films grown under anisotropic in-plane strain. In this study, K0.7Na0.3NbO3 thin films were grown under anisotropic lattice strain on (110) TbScO3 substrates by metal-organic chemical vapor deposition. By means of piezoresponse force microscopy and x-ray diffraction monoclinic domains were identified with a lateral polarization component collinear with the ±[110]pc direction of the pseudocubic (pc) unit cell of the film and periodically changes by 180° in adjacent domains. The monoclinic symmetry of the domains is controlled by theelastic anisotropy of K0.7Na0.3NbO3 and the anisotropic lattice strain, which is highly compressive in one in-plane direction and weakly tensile in the corresponding orthogonal direction. With increasing thickness a 90° rotated domain variant occurs where the lateral polarization vector is aligned along ±[110]pc. Domain size and piezoelectric coefficient will be discussed in terms of film thickness and strain state.

Resume : In proper ferroelectrics, the large dielectric anomaly seen at the Curie temperature Tc is caused by softening of a polar optical phonon or by slowing down of a microwave dielectric relaxation. In the spin-order induced ferroelectrics (i.e. type-II multiferroelectrics), only small dielectric anomalies are observed at ferroelectric Tc, because these materials belong to improper ferroelectrics. Niermann et al. investigated tiny dielectric anomaly near Tc in MnWO4 and observed a critical slowing down of a Drude-like dielectric relaxation which he explained by softening of electromagnon. Many type-I multiferroics exhibit improper ferroelectric phase transition connected with a multiplication of unit cell. Here belongs as well two-layered hexagonal BaMnO3, whose unit cell triples below Tc = 130 K and a ferroelectric polarization appears. Permittivity exhibits only change of slope at Tc, because the phase transition is driven by a soft Brillouin zone-boundary phonon. Owing to the Brillouin-zone folding below Tc, the soft mode becomes a zone-center mode, which activates in the IR and Raman spectra and hardens on cooling. This phonon has a small dielectric strength, so it only partially explains the increase in permittivity below Tc. An additional dielectric relaxation, observed in the microwave region, is probably due to the motion of ferroelectric domain walls. An AFM order appears at 53 K, but short-range magnetic correlations we observed in the EPR spectra up to 230 K.

Resume : Lead zirconate/titanate PbZr_{1-x}Ti_xO3 (PZT) is probably the most studied ferroelectric compound. However, until recently the major efforts were concentrated on the compositions near the morphotropic phase boundary. Information about the structure and especially lattice dynamics of the ?low-x? compounds is less complete. In this report, we will present the results on the critical dynamics of the low-x PZT single crystals. The formation of the ?true? incommensurate phase will be discussed. Some more discussion of the incommensurate phase and corresponding peculiarities of the phonon dispersion will be presented for the case of pure PZO under high hydrostatic pressure. We also will analyze the structure modulation of the intermediate ferroelectric ?M-phase? and will show that the full pattern including the satellite peaks cannot be described in the frames of the rhombohedral symmetry.

Resume : Knowledge of crystal structure is very important to understand its physical properties. Traditionally, x-ray, neutron and electron diffractions have been used as the powerful technique to investigate macroscopic structures on crystalline materials. However, recent progress on nanoscience indicates the essential role of local structure for revealing the origin of physical properties. Here, we present recent results on crystal structural analysis on PbZr1-xTixO3 (PZT) from the view point of local and average structures. We conducted high resolution neutron powder diffraction and pair distribution function (PDF) analysis with neutron and x-ray on PZT ceramics with higher Zr concentrations at various temperatures. PDF analysis has been commonly used for amorphous materials which do not possess long-range order but it becomes popular to apply on crystalline materials. Combined PDF and Rietveld analysis confirmed the long-range rhombohedral and both long- and short-range monoclinic phases coexist across the phase diagram. Additionally, a new phase boundary which relates with monoclinic structure is found. For Zr-rich PZT, monoclinic phase shows the MB structure and it continuously transforms to MA towards the morphotropic phase boundary (MPB). It explains why the enhancement of piezoelectricity occurs near the MPB region.

Resume : The crystal structure and its relationship with the high piezoelectric performance of PbZr1-xTixO3 (PZT) have been under extensive studies for years. The long range average structures as rhombohedral-monoclinic-tetragonal phases varying with compositions have been widely accepted. However, these average structural models alone cannot fully explain the dramatic increase of piezoelectric response near the morphotropic phase boundary (MPB) compositions. Therefore it is necessary to look closely at shorter scales: local structures (local polarizations) on the atomic scale and individual domain contributions (domain wall motions) on the mesoscopic scale. Here we present our recent progress on the local structure and domain investigations on PZT ceramics and single crystals near the MPB compositions, utilizing various state-of-art high-resolution diffraction techniques, including pair distribution function analyses and in-situ synchrotron X-ray diffraction under quasi-static electric field. The presence of three types of monoclinic phases has been found on the local scale. The MA-MC first order phase transition may produce more internal strain at the MPB-tetragonal phase change region. On the other hand, three-dimensional reciprocal space volumes around several Bragg reflections from single crystals show the presence of complex domain structures. We have inspected these reciprocal space volumes under applied electric field and observed that domain switching is important contribution to the total electric-field-induced strain. With the current evidence of both intrinsic and extrinsic contributions to the piezoelectric property of PZT, we aim to separate these two effects quantitatively.

Resume : Some unexpected changes in thermodynamic properties can be observed in hydrogen bonded uniaxial ferroelectrics in the ferroelectric phase after their exposure to a prolonged electric field not parallel to the polar axis. The changes are visible even after the transverse field disconnection. However, the original properties can be easily restored by annealing the crystal above the critical temperature TC of the para-ferroelectric phase transition. The most important changes observed in triglycine sulphate (TGS) or triglycine selenate (TGSe) or Rochelle salt are the following: a gradual flattening of the hysteresis loop, quite a new rigid domain structure, suppression of low frequency dispersion, new adjustable temperature dependences of the zero field polarization, quite a new temperature dependence of specific heat as well as memory effects. Most interestingly, the classic uniaxial TGS ferroelectric crystal, formerly influenced by the transverse electric field, behaves as if it has been transversely polarized. For a transverse electric current that resembles a longitudinal pyroelectric one can be then observed. Such a current flows in the zero external electric field, below the temperature TA at which the transverse field was previously applied. There is a relation between TA , transverse field intensity ET and the time td required for the hysteresis loop to disappear. For a given ET , the smaller the TC -TA > 0 difference, the shorter the time td. At a constant TA, the stronger the field ET , the shorter the time td required for the hysteresis loop to be completely flattened.

Resume : Monoclinic symmetry and polarization rotation in ferroelectrics have been debated for nearly two decades. For the first time these concepts were introduced for PZT as a bridge between the rhombohedral and tetragonal sides of the morphotropic phase boundary. Monoclinic phases were evidenced in many other functional perovskites e.g. PMN-PT and Na0.5Bi0.5TiO3 (NBT). Polarization rotation and high complexity of monoclinic domain patterns may stand behind the enhancement of electromechanical coupling and other physical properties. Still, there is a lack of evidence of the dominating role of monoclinic phases in ferroics. This presentation shows the original experiments for probing the development of monoclinic strain in ferroelectric NBT and PMN-PT. We design in-situ X-ray diffraction experiments for the measurement of high-resolution reciprocal space volumes of multi-domain crystals under external electric field. We focused on the fine reflection splitting which were previously used as the fingerprints of monoclinic symmetry (e.g. {h00} splitting in NBT), evaluated the corresponding monoclinic strain and calculated intrinsic contribution to the piezoelectric response. The results expand the information about the emergence and the properties of monoclinic phases / polarization rotation and facilitate further evolvement of these concepts.

Resume : The so-called “metallic ferroelectrics” exhibit simultaneous metallicity and ferroelectricity or, at least, an inversion-symmetry breaking distortion. A lot of current work focuses on the discovery or prediction of new such materials [1-4]. In particular, doping regular (insulating or semiconducting) ferroelectrics with charge carriers may appear as a promising route to this end. However, free carriers will screen electrostatic (dipole-dipole) interactions and, since such interactions are known to favor the occurrence of polar order, they are generally expected to preclude the ferroelectric phase. A typical example of this behavior is prototype compound BaTiO3, whose polar distortion disappears upon doping with electrons [5, 6]. Here we address the question of whether all ferroelectrics behave like BaTiO3 or, rather, alternative behaviors are possible. By first-principles simulations, we show that the polar distortion can coexist with metallicity, and even be enhanced by the electron or hole doping, in many ferroelectric compounds. Indeed, our results actually allow us to classify ferroelectrics in two families according to their response to carrier doping, providing an atomistic explanation for this diverging behavior. As a by-product of this work, we obtain evidence that moderate levels of doping can effectively act as an applied hydrostatic pressure in many compounds, making it possible to induce a variety of structural transitions. References [1] Y. Shi et al, Nat. Mats. 12, 1024-1027 (2013). [2] A. Filippetti et al, Nat. Comm. 7, 11211 (2016) [3] D. Puggioni et al, Nat. Comm. 5, 3432 (2014) [4] T.H. Kim et al, Nature (2016), doi:10.1038/nature17628 [5] Y. Iwazaki et al, Phys. Rev. B 86, 214103 (2012). [6] Y. Wang et al, Phys. Rev. Lett. 109, 247601 (2012).

Resume : Since 2009, the investigation of the coupling of multiferroic materials with light has experienced a renewed interest, as above-bandgap photovoltages were found to occur in BiFeO3 thin films1. The so-called Bulk Photovoltaic Effect (BPVE)2,3, occurring in any material lacking inversion symmetry, has therefore attracted interest for the design of new efficient and cheap solar cells. Therefore, the lack of inversion symmetry of ferroelectrics makes them natural candidates to the design of tnew kinds of solar cells, and also provides them with interesting piezoelectric properties. The lack of inversion symmetry of ferroelectrics is also fundamental to their so-called piezoelectric properties, i.e. the coupling of their electric polarization to strain. Therefore, ferroelectrics offer a natural route towards the realization of light-induced actuators among the so-called class of photostrictive materials4. Experimental evidences towards this goal have appeared in the last five years, with intense studies of the so-called photostriction in bismuth ferrite5,6. Besides this steady state response of ferroelectrics to excitation by light, a giant shear strain induced by femtoseconds laser pulses has been detected in bismuth ferrite, and attributed to the conjunction of the BPVE and the piezoelectric effect in BFO7,8. In addition, time-resolved synchrotron diffraction showed that shifting of Bragg peaks on a picosecond time scale occurs in both bismuth ferrite9 and a more conventional ferroelectric, lead titanate10. However, until now, theory has not investigated the coupling of light with the electromechanical properties of ferroelectrics on a microscopic scale. To tackle this problem, we employ a ∆-SCF scheme11 in which the occupation numbers of the Kohn-Sham orbitals are constrained. According to our results, BFO should shrink along its pseudo-cubic and the polarization axes, with the pseudo-cubic angle getting closer to 90°. On the other hand, the directions perpendicular to the polarization, such as [1-10]pc stretches under excitation of electrons in the conduction band. Our main results is to find that photostriction originates from the combination of screening of the polarization at the unit cell scale by the photoexcited carriers, and converse piezoelectric effect12. This work is mostly supported by the Department of Energy, Office of Basic Energy Sciences, under contract ER-46612. L.B. also thanks the support from the Air Force Office of Scientific Research Under Grant FA9550-16-1-0065 and the DARPA grant HR0011-15- 2-0038 (MATRIX program). C. P. thanks acknowledges support from a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program (reference: ANR-10-LABX-0035, Labex NanoSaclay). References 1 S. Y. Yang, J. Seidel, S. J. Byrnes, P. Shafer, C.-H. Yang, M. D. Rossell, P. Yu, Y.-H. Chu, J. F. Scott, J. W. Ager, L. W. Martin, and R. Ramesh, Nature Nanotechnology 5, 143 (2010). 2 B. I. Sturman and V. M. Fridkin, The Photovoltaic and Photorefractive Effects in Noncentrosymmetric Materials, Gordon and Breach Science Publishers (1992). 3 V. M. Fridkin, Crystallography Reports 46, 654 (2001). 4 B. Kundys, Applied Physics Review 2, 011301 (2015). 5 B. Kundys, M. Viret, D. Colson, and D. O. Kundys, Nature Materials 9, 803 (2010). 6 B. Kundys, M. Viret, C. Meny, V. Da Costa, D. Colson, and B. Doudin, Physical Review B 85, 092301 (2012). 7 P. Ruello, T. Pezeril, S. Avanesyan, G. Vaudel, V. Gusev, I. C. Infante, and B. Dkhil, Applied Physics Letters 100, 212906 (2012). 8 M. Lejman, G. Vaudel, I. C. Infante, P. Gemeiner, V. E. Gusev, B. Dkhil, and P. Ruello, Nature Communications 5, 4301 (2014). 9 D. Schick, M. Herzog, H. Wen, P. Chen, C. Adamo, P. Gaal, D. G. Schlom, P. G. Evans, Y. Li, and M. Bargheer, Physical Review Letters 112, 097602 (2014). 10 D. Daranciang, M. J. Highland, H. Wen, S. M. Young, N. C. Brandt, H. Y. Hwang, M. Vattilana, M. Nicoul, F. Quirin, J. Goodfellow, T. Qi, I. Grinberg, D. M. Fritz, M. Cammarata, D. Zhu, H. T. Lemke, D. Walko, E. M. Dufresne, Y. Li, J. Larsson, D. A. Reis, K. Sokolowski-Tinten, K. A. Nelson, A. M. Rappe, P. H. Fuoss, G. B. Stephenson, and A. M. Lindenberg, Physical Review Letters 108, 087601 (2012). 11 R. M. Martin, Electronic Structure: Basic Theory and Practical Methods, Cambridge University Press (2008). 12 C. Paillard, B. Xu, B. Dkhil, G. Geneste and L. Bellaiche, Physical Review Letters, in press (2016).

Resume : Perovskites with lone-pair-active atoms and tolerance factor leading to rotations of oxygen octahedra often exhibit complex behaviour with different structural variants being energetically close to the ground state. For example, recent first principles calculations for BiFeO3 and PbZrO3 [1-3] have indicated competing ferroelectric and antiferroelectric phases in these compounds. We show, also by means of ab-initio calculations, that similar feature can be attributed to ordered PbSc1/2Nb1/2O3 (PSN), where a possibly antiferroelectric (AFE) phase has an energy only 1 meV/formula unit higher than the ferroelectric (FE) ground state. We analyze distortions (including their dynamics) leading to both FE and AFE states and assess the role of oxygen octahedra tilts. The impact of chemical disorder is also studied with the supercells having different possible arrangements of B-site cations. The results are discussed in the context of relaxor behaviour of PSN as well as older experimental results, especially those indicating local structural features in ordered and disordered crystals [4,5]. [1] O. Diéguez, O. E. González-Vázquez, J. C. Wojdeł and J. Íñiguez, Phys. Rev. B 83, 094105 (2011). [2] S. E. Reyes-Lillo and K. M. Rabe, Phys. Rev. B 88, 180102 (2013). [3] J. Íñiguez, M. Stengel, S. Prosandeev, and L. Bellaiche, Phys. Rev. B 90, 220103 (2014). [4] N. Takesue, Y. Fujii, M. Ichihara and H. Chen, Phys. Rev. Lett. 82, 3709 (1999). [5] K. Z. Baba-Kishi and M. Pasciak J. Appl. Cryst. 43, 140 (2010). [6] M. Paściak, T. R. Welberry and J. Hlinka, submitted to Phase Transitions.

Resume : The world of ferroelectrics is dominated by the perovskite-type (ABO3) materials with the A site occupied by Pb and substitution disorder on the B site because of their excellent dielectric and piezoelectric properties. However, the restrictions on the use of Pb have increased over the past years, due to its potential environmental toxicity during disposure. As a consequence, ferroelectric ABO3-type solid solutions without or at least with a reduced amount of Pb are gaining continuously growing attention and are becoming subjects to extensive studies. Unfortunately, the so far produced Pb-free solid solutions compare poorly to Pb-based solid solutions regarding materials properties and performance. Thus the ultimate fundamental question is to understand the relation chemistry-structure, including on a mesoscopic length scale, because the compositional disorder primarily induces local elastic and/or electric fields that in turn lead to various ferroic clustering, which may be the key factor for materials properties at the morphotropic phase boundary. In this contribution the differences in the composition- and temperature-driven structural transformations in two representative solid solutions with reduced and none content of Pb: (1-x)PbTiO3-xBiMg0.5Ti0.5O3 and (1-x)Na0.5Bi0.5TiO3-xBaTiO3, will be discussed on the basis of combined pair-distribution-function analysis and Raman spectroscopy at room temperature as well as of temperature-dependent Raman spectroscopy.

Resume : Ferroics compounds are known to present efficient electro-mechanical coupling coefficients which are at the core of many devices in telecommunications and sensors technologies (piezoelectric, electro-optic and acousto-optic devives). In particular, LiNbO3 is nowadays currently employed in many acousto-optic devices for light processing but with limited device bandwidth of tens of MHz (CMOS electronic limits). To go beyond this limitation, it is necessary to explore new ultrafast acousto-optic processes. The ability to generate efficient GHz-THz coherent acoustic phonons with femtosecond laser offers new route to explore original acousto-optics processes. Here, using GHz coherent acoustic waves generated by a femtosecond laser pulse, we evidence new ultrafast Brillouin processes with in particular the demonstration of the existence of ultrafast mode conversion process (odinary-extraordinary) in two ferroelectric materials BiFeO3 and LiNbO3. By allowing the manipulation of light polarization with GHz coherent acoustic phonons, our results highlight new capabilities in using ferroelectrics in modern photonics.

Resume : We present a first-principles-based parametrization of the Landau potential for BiFeO3 (BFO). BFO is a multiferroic material, which has a rhombohedral ground state with large ferroelectric polarization, accompanied by strong tilts of the oxygen octahedra. It is assumed that the polarization is strongly influenced by tilts. Normally, BFO exists in a polydomain state; the domain-structure related properties are interesting for a wide range of potential applications, but their better understanding is needed. Due to the atomic-level complexity of BFO the theoretical investigations remain mostly limited to properties of a bulk material, or to small supercells representing domain walls. There were attempts to derive phenomenological potentials [1,2], which would facilitate simulation of domain interaction on a larger scale. Nevertheless, a trustworthy parametrization allowing to reproduce well the local properties and yet enable to study domain walls of different types and their interaction is, to our opinion, still missing. For this sake we conduct series of first-principle-based calculations for selected configurations of the BFO and fit the obtained data with standard form of the Landau potential for ferroelectric materials. Ferroelectric polarization, tilting of oxygen octahedra, and mechanical strain are treated as order parameters. Capability of the obtained parametrization of the potential to predict BFO's properties will be presented. References: [1] F. Xue, Y. Gu, L.Liang, Y.Wang, and L. Q. Chen, Phys. Rev. B 90, 220101 (2014). [2] N. E. Kulagin, A. F. Popkov, S. V. Soloviov, K. S. Sukmanova, and A. K. Zvezdin, Phys. Sol. State 57, 933 (2015).

Resume : By means of a theoretical approach based on Density Functional Theory, different chemical compositions of the multifunctional Ba(1-x)Ca(x)Ti(1-y)Zr(y)O3 (BCTZ) solid solution have been investigated, focusing on the energetics and lattice dynamical properties. High-performance piezoelectrics are key components of various smart devices and, recently, it has been discovered that BCTZ shows enhanced piezoelectric properties. Our goal is to understand better the microscopic mechanisms involved in the large electrochemical response of BCTZ in order to rationalize the search of optimal compositions. At first, a comprehensive study of the four parent compounds has clarified the energy landscape and the competitions at play between various phases. Then, a description of BCTZ based on the Virtual Crystal Approximation (VCA), has revealed interesting competitions between different ferroelectric phases when increasing the stoichiometry of Ca. At variance, it has been found that VCA fails to reproduce the energetics and ferroelectricity of the system when increasing the Zr stoichiometry, whereas a standard DFT approach based on supercells more properly works. A better understanding of the environment required to activate ferroelectric distortions in BCTZ is provided by a simple electrostatic model. This first analysis allows us to predict that appealing ferroelectric and piezoelectric properties characterize a wide-range of compositions of BCTZ and that advanced DFT-based studies are required to completely describe the physics of this system.

Resume : The (1-x)PbZr0.70Ti0.30O3-(x)BiMn2O5 was studied for x = 0, 2, 5, 11, 15, 22 %. The lead zirconate titanate (PZT) is a compound with a perovskite structure. It is a standard ferroelectric showing good piezoelectricity. The second component, bismuth manganite BiMn2O5 (BMO) is a non-polar material. The aim of our work was to determine the electrical properties of PZT influenced by BMO addition. The electric impedance was measured for f = 0.1 Hz–1 MHz in the 100–600 K range. It was analyzed with use of permittivity, conductivity and electric modulus formalism. The temperature between ferroelectric phases shifted towards higher temperature for 0.98PZT ̶ 0.02BMO ceramics. The eps'(T) shown marked dispersion for ceramics with 11 and 15% BMO. Results were analyzed using modulus formalism due to high conductivity. One relaxation process occurred in for pure PZT sample. The BMO addition induced the second relaxation process in other samples. The processes were assigned to participation small polarons in electrical conductivity dispersion. Characteristic relaxation times shown values about 1ns and 10 ps. The slower relaxation process corresponded to PZT and the faster to BMO contribution. Estimated activation energy was 0.81 eV for pure PZT and it changed from 0.36 to 0.89 eV when BMO content increased. We discussed competition of the two relaxation processes in studied samples.

Resume : The electrical, crystallographic and magnetic properties of ultra-thin magnetite (Fe3 O4 ) have been studied in detail, employing superlattice structures of Fe3O4/MgFe2O4 and Fe3O4/MgO on a variety of substrates. By careful analysis of their properties the influence of substrate stoichiometry, Fe3 O4 thin film thickness, antiphase boundaries on the magnetic properties can be separated. In particular, the origin of the controversial enhanced magnetic moment in ultra-thin films can be assigned to the substrate stoichiometry, specifically the migration of oxygen vacancies into the Fe3 O4 thin films. The multi-layer concept can be employed with many other such systems and offers new methods of tuning the properties of thin magnetic oxides. We present a comprehensive analysis of the films stoichiometry and geometry using TEM, Raman spectroscopy, X-Ray reflection and discuss the impact of film structure on the magnetic moment but also the Fe3O4 metal insulator transition (Verwey transition).

Resume : This work is focused on the study of the optical properties of barium hexaferrite and barium hexaferrite doped with Ti or Co thin films. Samples were deposited by radio frequency magnetron sputtering in an argon atmosphere on two side polished crystalline calcium fluoride (CaF2) and crystalline silicon (Si) substrates. The annealing temperature ranged from 300 to 900 °C in steps of 100 °C and took place in a quartz tube at atmospheric pressure. The same samples were used for the next thermal treatment. UV-Vis spectroscopy allowed the determination of the refractive index, absorption coefficient and thickness of the samples. From the absorption coefficient, the optical bandgap and Urbach energy were calculated. FTIR and XRD allowed the identification of barium hexaferrite and the formation of the tetrahedral (Fe-O4) and octahedral (Fe-O6) complexes with increasing annealing temperature. The composition of the samples was assessed by energy dispersive spectroscopy (EDS). The analysis determined that the formation of the amorphous hexaferrite network, as well as the beginning of the crystallization process of the network with the annealing temperature happens between 600°C and 750°C. This value is consistent with the literature [1]. The observed decrease of the bandgap with increasing annealing temperature suggests the expansion of the network and therefore an increase of the average lattice constant. This expansion is the result of the formation of octahedral and tetrahedral complexes. In addition, typical Tauc-gaps are around 2.89 eV [2]. The refractive index and extinction coefficient are in the same range of values reported in the literature [3]. Regarding the study of hexaferrite doped with Ti or Co, FTIR revealed a decrease of the Fe-O bonds with increasing doping. Also a decrease of the Urbach energy is observed, suggesting a decrease of electronic defects states (Fe and / or O) in the gap. Finally, the variation of bandgap, refractive index and extinction coefficient as a function of dopants were studied. Keywords: Barium Hexaferrite, Optical Properties, Amorphous Phase References: [1] W. Y. Zhao, P. Wei, H. Bin Cheng, X. F. Tang, and Q. J. Zhang, J. Am. Ceram. Soc. 90, 2095 (2007). [2] M. S. Rafique, S. Anjum, and K. Siraj, Thin Solid Films 545, 608 (2013). [3] I. Harward, Y. Nie, D. Chen, J. Baptist, J. M. Shaw, E. Jakubisova? Lis?kova?, S. Vis?n?ovsky?, P. S?iroky?, M. Lesn?a?k, J. Pis?tora, and Z. Celinski, J. Appl. Phys. 113, 043903 (2013).

Resume : The best piezoelectrics used in industry, medical and military applications are solids, usually ceramics. However, hard material are useless in some applications, e.g. in touch sensitive devices or in cases, when big areas are to be covered with a piezoelectric material. In such cases, piezoelectric polymers may be used. However, all soft piezoelectric materials have one common disadvantage: they need to be polarised in a very high electric field, which is one or two orders of magnitude higher than in the case of ceramics. Thus, there is a strong demand for materials that combine properties of both soft and hard piezoelectrics. Piezoelectric composites seem to be an easy way to combine advantages of both ceramics and polymers.Despite the ongoing efforts, the properties of the composites are not yet fully understood. In this work we present experimental study on the impact of piezoelectric fillers on the dielectric and ferroelectric properties of a ferroelectric copolymer based composite. It is shown that the electrical properties of the composites depend on the volume fraction of the fillers. This dependence is well described by Licthenecker's mixing formula. Moreover, the electric field is nearly zero in the filler. Thus, most of the effective properties of the composites are determined by the polymer matrix. The obtained results allow to tailor the composition to obtain the desired properties.

Resume : Transition metal oxides with an AMO3 perovskite structure develop a wide range of functionalities that originates from the interplay between lattice, electronic, and magnetic degrees of freedom. With their electronic degeneracies, rare-earth titanates RTiO3 are often expected to be a text book example of such a subtle interplay. Titanates are Mott insulators and adopt an orthorhombic Pbnm structure at low temperature. They all undergo a magnetic phase transition to either a ferromagnetic or G-type antiferromagnetic ordering as a function of the rare-earth. However, this scarce ferro to antiferromagnetic transition remains controversial and debated. Here we revisit the ground state properties of titanates on the basis of first-principles calculations and symmetry mode analysis. We show that an antipolar motion involving the rare-earth results in a change of orbital-ordering when going from systems with low tolerance factors to systems with large tolerance factors. Moreover, this motion is also demonstrated to favor a purely antiferromagnetic solution. We further evidence that the Jahn-Teller distortion does not play any role, although these systems are nominally Jahn-Teller active. Our simulations therefore provide strong support to the model of Mochizuki et al [PRL 91 167203], underlying the importance of the crystal-field of the rare-earth in the spin-orbital properties of titanates. Work supported by the European Research Consolidator grant MINT (Contract 615759).