Phase transitions and properties of ferroics in the form of single crystals, ceramics and thin films II

Resume : The most characteristic signature of antiferroelectric materials is their electric-field induced phase transition to a polar phase and the associated response of polarization as a function of field P(E). In practice, a very large body of published work revolve around a few end compounds with the perovskite structure both lead-based (PbZrO3 and PbHfO3) and lead-free (AgNbO3 and NaNbO3). The antiferroelectric character of these end compounds are basically known for decades even though they may not be understood in details because of the complexity of the microscopic mechanisms involved. In principle, it would be desirable to identify other families of compounds with different crystal structures in order to enrich this portofolio of potential antiferroelectrics. This is desirable both from a theoretical perspective, in order to better appreciate the variety of microscopic mechanisms responsible for antiferroelectricity, but also from application perspectives for a broader choice of end compounds. The search for new antiferroelectric materials however is impeded by a number of difficulties, starting with the somehow ambiguous definition of the notion itself. This includes also difficulties in identifying characteristic symmetry criterion or experimental signatures that could be used to screen efficiently large families of crystals. In this talk I will describe our theoretical and experimental approaches to this problem and our attempts to identify such signatures. This has led us to identify a unique example of a displacive antiferroelectric transition and explore the potential for antiferroelectricy in scheelites.

Resume : Calcium titanate is the original compound to which the name of perovskite was attributed. Although showing a ferroelectric instability in its aristotype cubic reference structure, it is not ferroelectric. Instead, like many other oxide perovskites, it adopts an orthorhombic Pnma ground state, which emerges from the condensation of oxygen-octahedra rotation and tilts through a phase transition sequence that remains partly controversial. It was sometimes suggested that calcium titanate could be an incipient ferroelectric like strontium titanate. It can also be questioned why it is not antiferroelectric like lead zirconate. Here, using a second-principles approach, we will reinvestigate the phase transition sequence of calcium titanate with temperatures. We will provide a possible explanation to its apparent incipient ferroelectric character, sometimes observed experimentally. We will also question why it is not antiferroelectric and try to rationalize what makes it so different from lead zirconate showing however a very similar tolerance factor. Work done in collaboration with M.M. Schmitt, L. Bastogne, H. Zhang and C. H. Chao and supported by F.R.S.-FNRS Belgium (PDR project PROMOSPAN) and by the European Union’s Horizon 2020 research and innovation program under grant agreement N° 766726 (TSAR).

Resume : Antiferroelectrics are among the most promising materials for applications as high energy storage capacitors. Yet, the number of known antiferroelectrics is relatively small, which hinders their usage. Moreover, the optimization of their performance for energy storage is still somewhat limited, partly due to our incomplete understanding of the physical mechanisms yielding antiferroelectric order. In this talk we present our latest theoretical works on antiferroelectrics. Lead zirconate is considered the prototypical antiferroelectric perovskite. However, several experimental and theoretical works hint at a partially polar behaviour in this compound, indicating that the polarization may not be completely compensated. We propose a new structure for this material with uncompensated dipoles following a ferrielectric up-up-down pattern. First-principles calculations reveal this state to be more stable than the commonly accepted antiferroelectric state at low temperatures, possibly up to room temperature, suggesting that PbZrO3 may not be antiferroelectric at ambient conditions. We discuss the implications of this discovery, how it can be reconciled with the experimental observations and how the predicted ferrielectric phase could be obtained in practice. We also explore the possibility of designing artificial antiferroelectrics using ferroelectric/paraelectric superlattices. When surrounding a ferroelectric by dielectric layers, a competition between the polar distortion and the depolarizing field arises, often resulting in a rupture of the ferroelectric into domains of alternating polarization. Such system can be considered antipolar and can switch into a polar state under an electric field. We study the antiferroelectric-like response of PbTiO3/SrTiO3 superlattices by means of high-throughput second-principles calculations. By optimising their energy density and efficiency, we show that these systems are competitive with state-of-the-art antiferroelectrics for energy storage and offer a platform for further systematic optimization.

Resume : Antiferroelectrics exhibit unique electrical properties, which originate from the complex antiparallel arrangement of the spontaneous polarization. Typically, a ferroelectric phase with a comparable free energy to the antiferroelectric phase exists and it can be induced by different external stimuli (electric field, temperature, stress). Compositions in which this transition is reversible exhibit characteristic double polarization loops and could be used for high-energy storage capacitors or electrocaloric cooling [1,2]. NaNbO3 is a prototypical lead-free antiferroelectric material that serves as the basis for the development of new compositions. However, the field-induced transition in pure NaNbO3 is irreversible and thus the material retains the ferroelectric order after field removal [3]. To understand this behavior, we first investigated the nature of this phase transition using electromechanical and structural characterization. The macroscopic increase in polarization/strain and inducement of piezoelectricity were accompanied by microscopic disappearance of the translational domain structure and changes of the local Na environment. Moreover, in situ high-energy X-ray diffraction measurements revealed that the transition is subdivided into three stages [4]. In order to obtain a reversible field-induced phase transition, a series of (1-x)NaNbO3-xSrSnO3 (x=0-0.06 mol.%) compositions were designed under the guidance of first-principles calculations and processed using the solid-state reaction route [5]. Although the global structure was maintained, 23Na solid-state nuclear magnetic resonance study showed an increase in the overall disorder and a less distorted local environment of Na in the NaO12 cuboctahedra. Double polarization hysteresis loop indicating the reversible nature of the phase transition were observed in the SrSnO3-modified samples, with a 14-times higher energy storage density (1.7 J/cm3) as compared to pure NaNbO3 and an efficiency of 33%. [1] Hao et al., Prog. Mater. Sci. 63, 1 (2014) [2] Novak et al., Phys. Rev. B 97, 094113 (2018) [3] Zhang et al., Acta Mater. 200, 127 (2020) [4] Zhang et al., Appl. Phys. Lett. 118, 132903 (2021) [5] Zhang et al., Chem. Mater. 33, 266 (2021)

Resume : Recent studies indicate that spontaneous symmetry breaking may be common in a wide range of dielectric materials, even if the physical origins of the phenomenon are different. By symmetry breaking we here consider appearance of a macroscopic piezoelectric effect or macroscopic polarization in a nominally centrosymmetric material. In this talk we shall discuss such symmetry breaking in canonical relaxor, Pb(Mg1/3Nb2/3)O3 (PMN), paraelectric phase of prototypic ferroelectric BaTiO3 and (Ba,Sr)TiO3, cubic Gd-doped CeO2 with fluorite structure and organometallic halide perovskites. To study macroscopic asymmetry we apply different techniques, including pyroelectric measurements, thermally stimulated currents, measurements of nonlinear polarization and strain, mechanical poling and illumination. These techniques are complementary and together reveal complexity of this emerging phenomenon. Our results suggest that there are at least two competing mechanisms contributing to polarization dynamics in PMN. The mechanism dominant at high fields is consistent with the dynamic switching of polar entities while the weak-field mechanism leads to a response resembling the weak-field motion of domain walls in hard ferroelectrics. It is possible to show that the transition between these two dynamic regions happens at alternating fields which are comparable in amplitude to the strength of the static field needed to switch the direction of spontaneous macroscopic polarization. Interestingly, the PMN single crystals could be poled by mechanical force. Our results question the ergodic nature of relaxor PMN below the Burns temperature. Electric-field induced symmetry breaking in Gd-doped CeO2 leads to huge values of the apparent piezoelectric d coefficient at low frequencies of the probing alternating field. The atomistic mechanism of symmetry breaking in this material is related to a high mobility of ionic defects (oxygen vacancies) and the resulting strain is a combination of redistribution of oxygen vacancies, chemical expansion effects and electric field-induced phase transition. In methylammonium (MA) lead halide single crystals with cubic structure (MAPbX3, X=Br, Cl) we revealed the appearance of symmetry breaking which is strongly dependent on defect structure and history of the samples. Some resemblance exists between electrostrictive and piezoelectric behavior in Gd-doped CeO2 and these organometallic perovskites, underlying ionic origin of the electro-mechanical coupling. An interesting aspect of MAPbX3 is that the generated piezoelectric strain depends on UV illumination. We discuss whether the macroscopic symmetry breaking is essentially related to the presence and mobility of defects or the symmetry can be broken in “perfect” crystals, as suggested by recent first-principles studies.

Resume : Domain walls have been recognized as exciting objects, due to their ability to carry functional properties, that are absent in the surrounding bulk domains [1, 2]. In the present talk we discuss the symmetry and properties of domain walls of various perovskites (e.g. SrTiO3, PbZrO3, PbTiO3) using a recently developed method of combining layer group analysis [3] with order parameter symmetry and Landau-Ginzburg theory [4, 5] and compare the results with some experimentally determined properties of the corresponding materials [6,7] and with recent results from computer simulations [8]. [1] G. Nataf, M. Guennou, J. Gregg, D. Meier, J. Hlinka, E. Salje, and J. Kreisel, Nature Reviews Physics 2, 634 (2020). [2] P. Sharma, Q. Zhang, D. Sando, C. H. Lei, Y. Liu, J. Li, V. Nagarajan, and J. Seidel, Science advances 3, e1700512 (2017). [3] V. Janovec, Ferroelectrics 12, 43 (1976). [4] W. Schranz, I. Rychetsky and J. Hlinka, Phys. Rev. B 100, 184105/1-14 (2019). [5] W. Schranz, C. Schuster, A. Tröster and I. Rychetsky, Phys. Rev. B 102, 184101/1-12 (2020). [6] W. Schranz, A. Tröster and I. Rychetsky, Journal of Alloys and Compounds 890, 161775 (2022). [7] I. Rychetsky, W. Schranz and A. Tröster, Phys. Rev. B 104, 224107 (2021). [8] A. Tröster, C. Verdi, C. Dellago, I. Rychetsky, G. Kresse and W. Schranz, Hard Antiphase Domain Boundaries in Strontium Titanate unravelled using Machine-Learned Force Fields, submitted to Phys. Rev. Lett.

Resume : An artificial neural network is basically an ensemble of neurons connected by weighted synaptic connections allowing superior computing performances over classical von Neumann-based systems in processing cognitive and data intensive tasks, such as real-time image recognition, data classification or natural language processing, to cite a few. Typically, the information represented by a weight for each synapse is transmitted from the pre-synaptic neuron to the post-synaptic neuron. The network is then trained by updating its synaptic weights to perform a specific task. In the race for efficient materials to emulate neuronal and synaptic functions, classical ferroelectrics, both oxides and PVDF-based polymers have been considered as good candidate materials. Here, we take advantage of the polar instabilities and the flat energy landscape characteristic of relaxors to exploit this special class of ferroelectrics for mimicking neuromorphic elements. We show that field-induced transitions are useful for tuning the capacitance and polar responses along multiple states and in a non-volatile manner to reproduce mem-ristor/capacitor behaviors. We use such component to emulate some fundamental learning rules including long-term memory or spike-timing-dependent-plasticity. Using modelling, we also show that ultrafast THz electric field excitations enable both neuronic and synaptic behaviors, via the creation of out-of-equilibrium hidden states of polarization, resulting from the ultrafast response of polar nanoregions. These findings may open a new field of research dedicated to employ relaxors for the design of neuromorphic architecture and computing.

Resume : Compound phases often display properties that are symmetry forbidden by their nominal, average crystallographic symmetry, even if extrinsic reasons (defects, strain, or imperfections) are not apparent. An example of such disallowed property is piezoelectricity in nominally centrosymmetric phases and shows great potential to be exploited in device applications. In this work, Resonant Piezoelectric Spectroscopy (RPS) was used as the method of choice and the piezoelectric effect was quantified in unpoled ferroelectrics and a large number of nominally centrosymmetric materials, including paraelectrics, relaxors, ferroelastics, and isotropic materials with low defect concentrations. Values range from ~1 pm/V to 10-5 pm/V. The former is comparable to that of piezoelectric quartz whereas the latter is 3 orders of magnitude below the detection limit of conventional piezoelectric measurements. Polar nanostructures, such as precursors of ferroelectrics in the paraelectric phase, polar nano regions in relaxors above the freezing temperature, twin walls in ferroelastics, and coherent dipole complexes at cryogenic temperatures, are yet to be experimentally confirmed as the cause of symmetry breaking; however, they make a dominant contribution to piezoelectricity, with the piezoelectric effect significantly enhanced below a characteristic temperature. This temperature appears to be analogous to the coherence temperature T* which is commonly used for relaxors and is associated with the development of quasi-static regions and nanotwinning within PNR’s. Considering an obvious role of polar nanostructures in the enhanced piezoelectricity below T* and recent observations of static nano clusters in the paraelectric phase of BaTiO3, the influence of DC electric fields on the paraelectric phase of ferroelectric PbSc0.5Ta0.5O3 (PST) (with 65% B-site cation order) was also investigated. The application of the DC electric field results in remnant piezoelectricity, which disappears after the sample is heated above the coherence temperature T*~450 K. In a temperature region between the ferroelectric transition temperature Tc = 295 K and 355 K, even electric fields below the coercive field of PST lead to strong remnant piezoelectricity, whereas above ~355 K remnant piezoelectricity is drastically reduced. These results are attributed to poling of ferroelectric precursors, which manifest themselves as macroscopic switchable piezoelectricity, and suggest that increased coherency of precursors leads to a microscopic transition at 355 K, presumably to form a stable tweed structure.

Resume : The stability of piezoelectric/metal interfaces is particularly important for multilayer ceramic capacitors and decides about the choice of suitable electrode materials. Noble metal interfaces such as Pt or Ag-Pd are suitable for sintering of capacitors in air but are important cost factors. Non-noble metal electrode materials such as Cu or Ni will oxidize during sintering in air and thereby loose their conductivity, hence sintering in reducing conditions is required. This may lead to a decomposition of piezoelectric. In this contribution, it will be shown the instability of the piezoelectric is caused by an electrochemical instability, induced by a too high Fermi energy of electrodes with low work function. Photoelectron spectroscopy is used to analyze the evolution of the Schottky barrier heights and chemical composition at interfaces between various piezoelectrics and Sn-doped In2O3 model electrodes (ITO). The Fermi energy in the 3 nm thick ITO electrode is continuously raised during XPS measurement either by heating in vacuum or by cathodic polarization of the ITO electrode at elevated temperature. Thereby we observe a limitation of the Fermi energy at the interface, which is accompanied either by the observation of metallic Pb (PZT and related compounds), metallic Bi (NBT-BT), or by diffusion of Na or K through the ITO electrode (KNN). Together with the previously observed electrochemical reduction of BiFeO3 [1], it is concluded that the stability of piezoelectric interfaces is fundamentally determined by the Schottky barrier at the contact. [1] N.S. Bein, AK, et al, J. Phys. Chem. Lett. 10 (2019), 7071.

Resume : The discovery of ferroelectricity and antiferroelectricity in ultrathin films of the fluorite system Hf1-xZrxO2 (HZO) triggered a lot of attention because of its potential integration with current semiconductor technology that would facilitate the development of low-energy consuming devices [1]. Despite polycrystalline films are the main ingredient for potential applications, epitaxial single-phase highly-crystalline thin films would enable a deeper understanding and control of the ferroic properties of HZO. Most of the work in this area has so far been focused on integrating fluorite HZO on the well-known perovskite oxide architecture [2]. Unfortunately, in these kind of heterostructures the HZO give rise to texturing and coexistence of different phases that can inhibit a clear understanding of the structure-property relationship [3]. To mitigate this phase coexistence, we use a different epitaxial design and directly grow the HZO on buffered and pristine YSZ fluorite substrates. To further minimize any disorder related with Hafnium-Zirconium mutual doping, we focus on the x = 1 composition (ZrO2). Our films grow epitaxial and with a single phase, with different growth parameters (thickness, temperature, buffer layer) affecting the resulting ZrO2 fluorite polymorph. Direct integration of HZO ceramics on buffered fluorite substrates has the potential to become a new platform for understanding how epitaxial strain, confinement and different interfaces can affect the properties of HZO and related systems. [1] J. Müller et al., “Ferroelectricity in simple binary ZrO 2 and HfO 2,” Nano Lett., vol. 12, no. 8, pp. 4318–4323, 2012. [2] I. Fina and F. Sánchez, “Epitaxial Ferroelectric HfO 2 Films: Growth, Properties, and Devices,” ACS Appl. Electron. Mater., p. acsaelm.1c00110, Apr. 2021. [3] T. Song et al., “Positive Effect of Parasitic Monoclinic Phase of Hf0.5Zr0.5O2 on Ferroelectric Endurance,” Adv. Electron. Mater., vol. 2100420, 2021.

Resume : Bismuth ferrite (BiFeO3, BFO) is one of the few multiferroic materials with both, ferroelectric and antiferromagnetic ordering at room temperature. The bulk photovoltaic (BPV) properties of BFO were first reported in 2009. Due to the high remanent polarization (~100 μC/cm2) and its “narrow” bandgap compared to other ferroelectrics, BFO has been the focal point in most research on ferroelectric photovoltaics. Besides, the high Curie temperature of BFO (TC = 1013 K) being compatible with high-temperature applications, its high birefringence and its lead-free nature make BFO a candidate for electro-optic (EO) applications. The understanding of both the BPV and EO response of BFO is essential for the study of the photorefractive properties in BFO that are generally suppressed by its high conductivity. Here, we present highly (100)-textured polycrystalline BFO thin films fabricated by solution processing. Low leakage films with epitaxial-like ferroelectric properties were obtained by co-doping with manganese and titanium and by precise control of the microstructure.[1] The photovoltaic properties were measured using interdigitated electrodes. We show that the main light-induced charge transport mechanism is the BPV effect, the first such demonstration for a solution-deposited polycrystalline film.[2] The EO properties were quantified by a modified Teng-Man setup in transmission geometry, obtaining EO coefficients (reff) up to 3 pm/V, reaching values about 25% of those reported for epitaxial films.[3] The results on the BPV effect and the EO properties are used to assess the potential of using solution-processed films for photorefractive applications. [1] A. Blázquez Martínez, N. Godard, N. Aruchamy, C. Milesi-Brault, O. Condurache, A. Bencan, S. Glinsek, T. Granzow, J. Eur. Ceram. Soc. 41 (2021) 6449–6455. [2] A. Blázquez Martínez, P. Grysan, S. Girod, S. Glinsek, T. Granzow, Scr. Mater. 211 (2022) 114498. [3] D. Sando, P. Hermet, J. Allibe, J. Bourderionnet, S. Fusil, C. Carrétéro, E. Jacquet, J.-C. Mage, D. Dolfi, A. Barthélémy, P. Ghosez, M. Bibes, Phys. Rev. B 89 (2014) 195106.

Resume : Polarons are quasiparticles that form in a wide variety of materials as a result of electron?phonon coupling, and they typically have significant effects on materials properties. Although the concept is known since about 75 years, the origin of polarons is not yet fully elucidated. To date the most popular explanation relies on Fröhlich's theory: an electron placed in a crystal lattice will induce local polar screening achieved from the activation of a polar phonon mode and resulting in the formation of a polaron. WO3 is a well-known prototypical system for studying polarons since its polaronic nature is expected to be at the origin of its remarkable electrical and chromic properties. Here, using first-principles density functional calculations, we successfully predict the formation of medium size polarons in WO3. Amazingly, our calculations reveal however that, when an extra electron is added into WO3, it will dominantly activate non-polar rather than polar modes. This observation implies that, in addition to conventional Fröhlich's view, there should be another mechanism to stabilize the charge. Developing a simple model, we unveil and rationalize how this is intimately related to changes of hybridization between W 5d and O 2p orbitals. Our findings might be relevant to the whole family of ABO3 perovskites, to which WO3 is closely related.

Resume : Broadband dielectric spectra (from Hz up to THz and infrared frequencies) of the xBaZrO3–(1-x)BaTiO3 (BZT) ceramic solid solutions will be presented. We studied the BZT system in a broad (almost complete) concentration range and in a broad temperature range from 10 to 700 K. BZT is a model lead-free system which, depending on the concentration x, behaves as incipient, relaxor, diffuse or classical ferroelectric material. The behaviour is compared to that of pure BaTiO3 and BaZrO3 end members. Based on the results of the broadband fits of the spectra, we will discuss different processes and their origins that play role in the dynamics, such as (soft) phonons and relaxations. The spectra are characterized by an overdamped central mode in the microwave range which weakens on cooling. Except for BaTiO3, the soft mode and central mode do not soften appreciably and do not contribute substantially to the low-frequency permittivity maximum. The most important dielectric contribution is brought by Cole-Cole relaxation assigned to the hopping of Ti ions in the BaTiO3 clusters, which obeys the Arrhenius law. We acknowledge the support from the grant of the Czech Science Foundation (Project LA 20-20326L).

Resume : We utilized in-situ thermal, electric-field and illumination excitations to expose the domain-scale, origin of functionality in ferroelectric systems that exhibit augmented electromechanical and photoelectric properties. Specifically, using variable-temperature piezoresponse microscopy, direct imaging of the nanodomain dynamics and local hysteresis spectroscopy measurements in the seminal relaxor PMN-PT, demonstrated the role of these nanodomains around the ferroelectric-to-paraelectric transition. Likewise, introducing variable-temperature photoconductive atomic force microscopy, revealed that the origin of photoconductivity in the seminal hybrid-halide perovskite, MAPbI3 is the existence of polar domains [1]. The mechanisms in which ferroelectric domains contribute to the material functionality will also be discussed. [1] R. Saraf, C. Saguy, H. Elangovan, V. Maheshwari and Y. Ivry, Appl. Phys. Lett. 118, 151903 (2021)

Resume : Recently, it was reported that the cubic quadruple perovskite BiMn3Cr4O12 exhibits subsequent ferroelectric and antiferromagnetic phase transitions at 135 K and 125 K. Moreover, second magnetic phase transition associated with the antiferromagnetic order of the Mn3+ spins occurs at 48 K and it is accompanied by a 20% increase in ferroelectric polarization due to a strong magnetoelectric coupling. For that reason, Zhou et al. [1] concluded that BiMn3Cr4O12 exhibits features of both type-I and type-II multiferroicity. Here, we report heat capacity, magnetic and low-frequency dielectric properties together with IR, THz, and Raman spectra of BiMn3Cr4O12 ceramics. The IR and THz spectra reveal a polar soft phonon near 26 cm-1 at 300 K, whose softening down to 13 cm-1 is responsible for the ferroelectric phase transition, confirming the theoretical prediction that this transition is of the displacive type.[2] Furthermore, the optical mode softens according to the Cochran law towards 125 K and the Curie-Weiss fit of dielectric permittivity also results in ferroelectric TC = 125 K. Since only one anomaly in heat capacity is seen near 125 K [1], we propose that both magnetic and ferroelectric transitions coincide. It seems that the structural phase transition triggers the antiferromagnetic order of Cr3+ magnetic moments due to variation of exchange striction interaction. Detailed pyrocurrent and magnetization studies allow to distinguish between intrinsic and extrinsic contributions in polarization, as well as to determine the values of electrocaloric and magnetocaloric effects near the phase transitions. Raman spectra reveal a deviation of the temperature dependence of some phonon frequencies from the classical anharmonic behavior due to spin-phonon coupling below 130 K. The observed phonon activities in Raman and IR spectra are compared with the predictions from factor-group analysis and first-principles calculations. [2, 3] [1] L. Zhou et al., Advanced Materials 29, 1703435 (2017) [2] J. Q. Dai and C. C. Zhang, J. Amer. Ceram. Soc. 102, 6048 (2019) [3] J.Q. Dai, X.L. Liang, and Y.S. Lu, Chem. Phys. 538, 110924 (2020)

Resume : Electric control of magnetism and magnetic control of ferroelectricity can improve energy efficiency of magnetic memory and data processing devices. However, the necessary magnetoelectric switching is hard to achieve, and requires more than just a coupling between spin and charge degrees of freedom. We show [1] that an application and subsequent removal of a magnetic field reverses the electric polarization of the multiferroic GdMn2O5, thus requiring two cycles to bring the system back to the original configuration. During this unusual hysteresis loop, four states with different magnetic configurations are visited by the system, with one half of all spins undergoing unidirectional full-circle rotation in increments of ~90 degrees. Therefore, GdMn2O5 acts as a magnetic crankshaft converting the back-and-forth variations of the magnetic field into a circular spin motion. This peculiar four-state magnetoelectric switching emerges as a topologically protected boundary between different two-state switching regimes. Our findings establish a paradigm of topologically protected switching phenomena in ferroic materials. [1] L. Ponet, S. Artyukhin, Th. Kain et al., Nature (2022) doi: 10.1038/s41586-022-04851-6

Resume : Some novel puzzling and unexplained observations regarding the cubic to “tetragonal” phase transition at TS=282K in the almost multiferroic perovskite EuTiO3 have been reanalyzed in order to obtain deeper insight into the true structure and magnetic activity evolving below TS. For this purpose earlier and new birefringence [1] and high resolution XRD data [2] have been used where zero magnetic field data are compared to data under the influence of a direction dependent magnetic field. The XRD structural data have been taken as input to derive the magnetic exchange constants, the related Néel temperatures TN and the energies of the possible magnetic ground states. Taken both results together we conclude that below TS the symmetry cannot be tetragonal but only monoclinic followed by another symmetry lowering transition around T*≈210K. Furthermore, the exchange interactions, TN, and the energies evidence that any kind of magnetism stems from a competition of the various ground state energies and must be a consequence of frustration. References 1. Bussmann-Holder, A. et al. Transparent EuTiO3 films: A possible two-dimensional magneto-optical device. Sci. Rep. 7, 40621 (2017). 2. Bussmann-Holder, A., Liarokapis, E., & Roleder, K. Intriguing spin-lattice interactions in EuTiO3. Sci. Rep. 11, 18978 (2021).

Resume : Just as the apparent incompatibility between ferroelectricity and magnetism prompted the renaissance of multiferroics1, the research on « ferroelectric » metals – conjectured in the 1960s by Anderson and Blount2 – was recently revitalized. Yet, their experimental demonstration remains very challenging due to the contra-indication between the presence of free charge carriers and switchable electric dipoles. In this talk we will report on two-dimensional electron gases (2DEGs) formed on Ca-substituted SrTiO3 (STO). Signatures of the ferroelectric phase transition near 30 K are visible in the temperature dependence of the sheet resistance RS and in a strong, reproducible hysteresis of RS with gate voltage3. In addition, spectroscopic explorations of the 2DEG region indicate the presence of switchable ionic displacements. Beyond their fundamental interest in materials physics, ferroelectric 2DEGs offer opportunities in spin-orbitronics: we will show how their spin-charge conversion properties, caused by the inverse Rashba-Edelstein effect, can be electrically tuned in amplitude and sign in a non-volatile way4. These results open the way to a whole new class of ultralow-power spin-orbitronic devices operating without the need for magnetization switching. Finally, I will show how one can introduce magnetism into such systems to achieve multiferroic 2DEGs displaying magnetoelectric coupling. 1 N.A. Hill, J. Phys. Chem. B 104, 6694 (2000). 2 P.W. Anderson and E.I. Blount, Physical Review Letters 14, 217 (1965). 3 J. Bréhin et al, Phys. Rev. Materials 4, 041002 (2020). 4 P. Noël et al, Nature 580, 483 (2020).

Resume : Magnetic Rare-earth orthoferrites RFeO3 exhibit a rich playground ranging from multiferroicity to spin-reorientation, strong magnetostriction and ultrafast light-driven manipulation of magnetism, which can be exploited in spintronics. Such properties stem from the strong spin-orbit coupling (SOC) interaction combined with lattice vibrations (phonons) due to the presence of two magnetic ions (R3+, Fe3+) in different sublattices. This is the case, for instance, in SmFeO3, where recent experiments attempted to clarify its non-collinear ground-state, the origin of its spin-reorientation and low-temperature magnetization compensation. Particularly, anomalous evolutions of vibrational modes, the emergence of new modes and possible electro-magnons have been detected in Raman spectroscopy. Nevertheless, a conclusive picture has not yet been achieved, and theoretical studies to clarify microscopic mechanisms at play are still rare. Here, combining Raman spectroscopy and fully non-collinear first-principles simulations, we explore the interplay between magnetic states and phonons. We highlight the fundamental role of the Sm f-orbital SOC in the magnetic and structural anisotropies, and samarium’s contribution to the spin-phonon coupling.

Resume : Rare-earth orthomanganites are outstanding compounds because they exhibit macroscopic properties arising from the correlation between spins, charge, orbital and lattice degrees of freedom. Among them, only TbMnO3 and DyMnO3 have Mn–O–Mn bond angles which fall in the narrow range that is required for multiferroic properties where ferroelectricity arises from cycloidal magnetism [1]. GdMnO3 has attracted much attention with the aim to strain-engineering its properties [2], as its Mn-O-Mn bond angle sets it at the border line, next to spontaneous multiferroics TbMnO3 and DyMnO3. It is clear that sensitivity to the A-site cation size and the lattice distortions which follow, are a vital component of the structural and magnetic stability relationships even though strain is not the functional property of primary interest. This contribution addresses the experimental study of elastic and anelastic properties across the low temperature magnetic phase transitions in GdMnO3 and TbMnO3 through a temperature dependent resonant ultrasound spectroscopy in the 100 kHz - 2 MHz range [3]. The comparison of the temperature dependence of the elastic moduli and acoustic losses, and their anomalous behaviour across the magnetic phase transitions of GdMnO3 and TbMnO3 enable us to get information concerning the strain/magnetic order parameter coupling form, and the effect of the specific magnetic ordering on this interaction. Magnetoelastic loss peaks at lower temperatures demonstrate that aspects of the magnetoelectric structure remain mobile down to at least ∼5 K. The results clearly evidence the key role of the magnetism on the strain properties of these materials. This contribution will also address the implications of the coupling between strain, magnetic and electric dipoles in thin films and multiferroic domain walls. References: [1] T. Goto, et al., Phys. Rev. Lett. 92, 257201 (2004) [2] P. Machado, et al., Scientific Reports 9, 18755 (2019). [3] M. A. Carpenter et al., J. Phys.: Condens. Matter 33, 125402 (2021).

Resume : Rare-earth orthomanganites (RMnO3) and orthoferrites (RFeO3) have renewed the scientific interest due to their remarkable magnetic properties, spin-reorientation transitions and, more recently, by the discovery of low-temperature ferroelectricity and multiferroicity in some of them [1]. While in the RMnO3, the Mn3+ spins present a TN typically below 50 K, into magnetically ordered phases that strongly depend on the rare-earth, in the RFeO3, the Fe3+ spins have a TN above 600 K into a common canted AFM phase [1]. Despite being heavily studied materials, there are recent reports of a deviation to the Curie-Weiss law in the paramagnetic phase of TbMnO3 well above TN (between 150 and 200 K), accompanied by a negative thermal expansion of the c-axis, and anomalies on the optical phonon energies measured by THz, FTIR and Raman spectroscopies [2]. This magnetostructural effect has been interpreted on the basis of short-range magnetic ordering and/or crystal-field excitations of the Tb3+ cation. In this work, we study both TbMnO3 and TbFeO3, showing that similar anomalies on the magnetization, lattice parameters and phonon energies, to those observed in the paramagnetic phase of TbMnO3, also occur in TbFeO3 at the same temperatures, well within its antiferromagnetic phase [3]. Combining neutron powder diffraction obtained at ILL and Raman spectroscopy, we show this magnetostructural effect is a result of shift of oxygen positions below 200 K, which further increase the octahedra rotation angle by 0.15º in TbFeO3. We also show that in other RMnO3 and RFeO3 such anomalies are not found. To understand whether short-range magnetic ordering or crystal-field excitations underlie this phenomenon, we performed muon spin spectroscopy on TbMnO3 at PSI. For this, we take advantage of using the muons to measure the local magnetic fields through the Knight shift, at 1 Å distance from the oxygen position. Our results show that, although the local magnetic field increases below 200 K, the Knight shift is perfectly scaled by a paramagnetic law down to TN = 40 K. This result allows us to conclude that no short-range order occurs in the paramagnetic phase of TbMnO3 that would explain the observed magnetostructural effect. We then suggest that in fact an interplay between the oxygen position and the crystal-field excitation levels of the Tb3+ occurs, leading to a further deformation of the crystallographic structure, which then affects the macroscopic magnetic properties. [1] T. Kimura et al., Nature 426, 6962 (2003); E. Bousquet et al., JPCM 28, 123001 (2016) [2] K. Berggold et al., PRB 76, 094418 (2007); D. O'Flynn JPCM 26, 256002 (2014); S. Mansouri et al., JPCM 30, 175802 (2018) [3] R. Vilarinho et al., Scientific Reports, in press (2022)

Resume : Scanning Thermal Microscopy (SThM) is a promising Atomic Force Microscopy technique for mapping the thermal properties of materials at the nanoscale; surface temperature distributions can typically be mapped out with temperature resolution on the order of 10 mK and with spatial temperature resolution that is sub-100nm. In this talk, I will discuss how SThM could be a powerful tool for studying the role of microstructure on heat flow in ferroelectric materials, specifically for (i) mapping the thermal properties of domain walls and (ii) understanding the microscopic behaviour of electrocaloric effects. Interest in using ferroic domain boundaries to enable active control of heat flow has been steadily growing over the last decade. Domain walls are already known to inhibit thermal transport through phonon scattering [1] and the fact that some domain walls can exhibit enhanced electrical conductivity suggests that enhanced thermal transport within the wall is also a possibility. However, direct, local measurements of either of these domain wall thermal behaviours have yet to be reported. In this talk, I will describe our efforts to locally map variations in thermal response associated with such microstructural inhomogeneity. Similar to the approach used in [2], a thin gold bar deposited on the sample surface is periodically Joule heated and the resulting temperature oscillations are mapped by using the scanning probe as a temperature sensor. To validate that spatial variations in the thermal properties of the underlying sample of interest can be imaged, we attempt to image thermal contrast across the metal/dielectric layers of a multilayer ceramic capacitor (MLCC). Our attempts to image the thermal response of conducting domain walls in LiNbO3 using this approach will also be discussed. The electrocaloric effect is a well-known phenomenon where adiabatic application of an electric field to a ferroelectric material results in a temperature change. While it can be well described macroscopically through thermodynamics, and is understood to arise fundamentally from changes in dipolar configurational entropy, the effect is not as well characterised at the microstructural level [3]. In this regard, we build on previous SThM based studies [4] and demonstrate how local electrocaloric response can be imaged with sub-micron spatial resolution, here demonstrated in a MLCC. Using this approach, 2D spatially resolved maps of electrocaloric heating and cooling can be generated. We intend to further use this technique to elucidate the influence of microstructural inhomogeneity on electrocaloric response in other material systems. [1] Ihlefeld, J. et al. Nano Lett. 15, 1791 (2015); Langenberg, E. et al. Nano Lett. 19, 7901 (2019). [2] Menges, F. et al. Nat. Commun. 7, 1, (2016). [3] Y. Liu, Appl. Phys. Rev. 3, 031102 (2016). [4] Kar-Narayan, S. et al. Appl. Phys. Lett. 102, 032903 (2013); Shan, D. et al. Nano Energy 67, 104203 (2020).

Resume : Functional properties that can, for instance, be of electrical, magnetic, or elastic origin and that can be used in applications have driven materials science research in the past decades. Multifunctional materials, in particular, that exhibit several functional properties simultaneously which can be coupled together, are of even higher applicative interest, notably for their transducing possibilities. The control and tunability of functional properties have been increasingly investigated, including the possibility of tuning properties under strain. In particular, epitaxial strain, which is now routinely implemented, has yielded numerous ground-breaking results, especially in multiferroic materials [1,2]. We present here a novel strain engineering technique, based on Helium implantation, that allows to obtain an effective negative pressure in implanted materials, making it available to probe entirely new regions of materials’ phase diagrams. We first studied the structural effect of Helium implantation in epitaxial thin films of BiFeO3, a model multiferroic compound exhibiting multiferroicity well above room temperature. We demonstrated the effective negative pressure effect and showed that we could increase the lattice parameter of the films under implantation. We could trigger the structural transition towards the supertetragonal polymorph with increasing Helium dose, achieving an increase of the out-of-plane lattice parameter of up to 8.9% [3]. We now focus on structurally-textured sol-gel films, less costly to produce and more suited for application processes, where we demonstrate the tunability of functional properties. In BiFeO3, we show the influence of Helium implantation on the ferroelectric properties. We modify significantly the coercive field (up to 68% increase) and the low-field conductivity of the films under Helium dose (two orders of magnitude decrease at higher Helium dose), as well as the piezoresponse force microscopy (PFM) signal of the implanted regions. References: [1] Infante et al., Phys. Rev. Lett. 105, 057601 (2010) [2] Sando et al., Nature Materials 12(7), 641-646 (2013) [3] Toulouse et al., Phys. Rev. Materials 5, 024404 (2021)

Resume : A quarter of the UK’s CO2 emissions can be attributed to space heating and cooling. This is primarily due to heating with natural gas and cooling with compression of greenhouse gases, which are neither environmentally friendly nor energy efficient. Therefore there is great interest in developing energy-efficient solid-state heat pumps that can replace these environmentally damaging technologies. Barocaloric materials are at the core of novel solid-state heat-pump technologies. During this talk I will describe our work on mechanically responsive barocaloric materials for heating and cooling applications.

Resume : Hybrid organic-inorganic have been extensively studied for their tuneable properties such as photovoltaic, photoluminescent, switchable dielectric or ferroelectric. Properties of these materials are often induced or tuned by onset of structural phase transitions. Therefore, in order to obtain deep insight into phase transition mechanisms and structure-property relation, temperature-dependent studies are usually performed. However, pressure is emerging as another external parameter that can be used for tuning crystal structures and properties of hybrid perovskites. For instance, the application of pressure can enhance existing properties or lead to discovery of polymorphs with novel functional properties. Recently, we have discovered that methylhydrazinium cation (MHy+) can be used for synthesis of both 3D (MHyPbX3, X=Cl, Br) and 2D (MHy2PbX4, X=Cl, Br, I) perovskites, which exhibit multiple linear and non-linear optical phenomena [1-4]. Furthermore, MHy2PbBr4 is a ferroelectric materials with high Tc~351 K [4]. In this talk, I will present results of temperature- and pressure-dependent studies of methylhydrazinium-based lead halide perovskites. In particular, I will show that centrosymmetric MHy2PbI4 transforms at higher pressure to a ferroelectric Pmn21 phase, which is stable for the bromide at ambient conditions. Thus, pressure induces ferroelectricity in the studied compound. Interestingly, on further compression both MHy2PbBr4 and MHy2PbI4 exhibit unprecedented type of structural phase transitions into P21 phases, when MHy+ cations are pushed into the gaps in inorganic layers. This transport of counter cations leads to a significant increase of Pb-NH2 interactions, an unprecedented three-fold increase of positive linear compressibility along the layer stacking direction and large negative linear compressibility along the stacking direction, not reported for any 2D analogue. Acknowledgements: This research was supported by the National Science Center (Narodowe Centrum Nauki) in Poland under the project no. 2018/31/B/ST5/00455. [1] M. Mączka, M. Ptak, A. Gągor, D. Stefańska and A. Sieradzki, Chem. Mater. 31, 8563 (2019). [2] M. Mączka, M. Ptak, A. Gągor, D. Stefańska, J. K. Zaręba and A. Sieradzki, Chem. Mater. 32, 1667 (2020). [3] M. Mączka, A. Gągor, J. K. Zaręba, D. Stefańska, M. Drozd, S. Balciunas, M. Simenas, J. Banys and A. Sieradzki, Chem. Mater. 32, 4072 (2020). [4] M. Mączka, J. K. Zaręba, A. Gągor, D. Stefańska, M. Ptak, K. Roleder, D. Kajewski, A. Soszyński, K. Fedoruk and A. Sieradzki, Chem. Mater. 33, 2331 (2021).

Resume : Organic-inorganic hybrid ferroelectrics (OIHF) are a new member of ferroelectrics family which comprised of both organic and inorganic parts as building block. Since 2014, tens of OIHF structures have been explored with striking progress made in electromechanical properties such as high ferroelastic strain output and piezoelectricity. Recently, two orders of magnitude higher strain (21.5%) and d33 (1540 pC/N) higher than those of Lead zirconate titanate (PZT) were achieved in OIHF, suggesting their bright future as lightweight and high-performance electroactive materials. Different with widely used piezoelectric enhancement methods such as morphotropic phase boundary (MPB) effect in piezoelectric ceramics, the approach for OIHF to realize these excellent electromechanical are involved with special molecular and bond engineering. With our work, I will introduce the space-confinement method and molecular engineering strategy that are employed to realize non-180 degree ferroelastic switch and thus achieved large strain and piezoelectricity in hybrid systems. Reference: Hu, Yuzhong, et al. "Ferroelastic-switching-driven large shear strain and piezoelectricity in a hybrid ferroelectric." Nature Materials 20.5 (2021): 612-617.

Resume : There is currently an increasing enthusiasm towards long-range magnetic order and multiferroicity in two-dimensional (2D) materials both from the fundamental and applicative point of view. In this respect, by means of first-principles-based simulations, we investigated magnetic properties in a single-layer of some of the triangular 3d transition metal dihalides [1], which belong to the promising class of van der Waals materials, with a special focus on the exotic NiI2 case [2,3]. Through also supporting Monte Carlo simulations, we show that a thermodynamically-stable skyrmionic lattice with a well-defined topology and chirality of the spin texture in absence of Dzyaloshinskii?Moriya (DM) and Zeeman interactions, can be stabilized by the anisotropic part of the short-range symmetric exchange, related to the relevant spin-orbit coupling (SOC) of the heavy ligands, assisted only by the exchange frustration. In detail, the symmetric anisotropic exchange tensor, also referred to as two-ion anisotropy (TIA), shows large off-diagonal terms, which - due to the peculiar non-coplanar arrangement of the spin-ligand plaquettes- induce frustration in the relative orientation of spins by setting non-coplanar principal axes. Combined with the exchange frustration from competing ferromagnetic first- and antiferromagnetic third-neighbor interactions, this results in a not-trivial spin-configuration with well-defined topology, making the TIA acting as an emergent chiral interaction. Particularly, in the centrosymmetric NiI2 monolayer, we predicted a spontaneous high-Q antiskyrmionic lattice (|Q|=2) with fixed chirality, undergoing a topological phase transition, under magnetic field, to a more conventional skyrmion lattice (|Q|=1). Additionally, we predicted a competing noncollinear single-q helimagnetic state, exhibiting low-dimensional magnetoelectric and multiferroic properties. Such a multiferroic state in single-layer NiI2 has been also experimentally detected by combined birefringence and second-harmonic-generation measurements [4]. Such findings, on the one hand, propose a novel mechanism able to drive stabilization of topological spin structures in magnetic semiconductors with short-range anisotropic interactions. On the other hand, they open the street towards new low-dimensional magnetoelectrics and/or multiferroics. References: [1] K. Riedl, D. Amoroso et al. ?Microscopic origin of magnetism in monolayer 3d transition metal dihalides?. ArXiv:2206.00016 (2022) [2] D. Amoroso, P. Barone & S. Picozzi, ?Spontaneous skyrmionic lattice from anisotropic symmetric exchange in a Ni-halide monolayer?. Nature Communications 11, 5784 (2020) [3] D. Amoroso, P. Barone and S. Picozzi. ?Interplay between Single-Ion and Two-Ion Anisotropies in Frustrated 2D Semiconductors and Tuning of Magnetic Structures Topology?. Nanomaterials, 11(8), 1873 (2021) [4] Q. Song, [?] , D. A. et al. ?Evidence for a single-layer van der Waals multiferroic?. Nature, 601, 602 (2022)

Resume : Ferroelectric domain walls (DWs) formed at the boundaries between domains have been reported for electrically conductive phenomena. Since DW can be artificially controlled by external electric field and structure, DWC non-volatile memory application is possible by forming and removing DW with high DW conductivity (DWC). Here, we introduce the recent research results related to DWC non-volatile memories and present the recent research results of our research group. We fabricated and evaluated two types of DWC nonvolatile devices with two electrodes and three electrodes using epitaxial PbTiO thin films on single crystal (100) Nb:STO substrates. For the two-electrode device, the change of the current-voltage curves was investigated depending on the formation of the ferroelectric domain walls. As for the resistance of DWC, a systematic increase in current was observed as DW was changed to 0, 2, and 4. In particular, in a threeelectrode DWC nonvolatile device having three electrodes having a structure similar to that of a flash memory, the state of the source-drain currents could be changed into two states by the gate voltage. In addition, the source-drain currents exhibited a nonvolatile memory characteristic in which a low current state and a high current state were maintained even when a gate voltage was not applied. Research results on two types of DWC non-volatile devices having two electrodes and three electrodes may provide opportunities for new nonvolatile memory devices that can replace the existing flash memory.

Resume : We have shown that the existence of optical activity enhances significantly the acousto-optic (AO) figure of merit. The effect arises due to nonzero ellipticity of the interacting optical eigenwaves. More specifically, the above enhancement takes place because additional elasto-optic (EO) tensor components become superimposed in the effective EO coefficient. The latter just occurs because the ellipticity of the optical eigenwaves approaches unity in the vicinity of optic axis. Using the Pb5Ge3O11 crystals as an example, we have demonstrated that enhancement of the efficiency of AO diffraction is always typical for all the types of isotropic and anisotropic AO interactions, whenever the incident optical wave propagates close to the optic axis. In the particular case of diffraction in the interaction plane XZ of Pb5Ge3O11 crystals, the maximal enhancement of the AO figure of merit takes place under conditions of the isotropic diffraction of ordinary and extraordinary optical waves with a pure transverse acoustic wave polarized parallel to the Y axis (a so-called AW PT2), with the AO figure of merit increasing from zero up to 7.5×10?17 s3/kg, and the types of anisotropic AO diffraction on the transverse (PT2) and quasi-transverse QT1 acoustic waves, when the AO figure of merit increases twice (e.g., from 7.6×10?17 up to 1.5×10?16 s3/kg). The maximal AO efficiency in the interaction plane XZ is reached at the types I and II of the isotropic AO interactions of ordinary and extraordinary optical waves with a quasi-longitudinal acoustic wave. In these cases, due to a nonzero ellipticity of optical eigenwaves, the AO figure of merit in these cases increases respectively from 6.8×10?15 up to 12.4×10?15 s3/kg and from 5.6×10?15 to 12.4×10?15 s3/kg. The results of theoretical analysis have been proved experimentally by the studying EO coefficients for the Pb5Ge3O11 crystals at the optical wavelength 632.8 nm, using the Dixon?Cohen method for the case of isotropic AO interaction between circularly polarized optical waves and a quasi-longitudinal (QL) acoustic wave propagating along the X axis. It is supposed that the AO efficiency can be enhanced not only in the non-centrosymmetric chiral crystals revealing the natural optical activity. Centrosymmetric materials can also manifest induced optical activity which is due to either the Faraday effect or electrogyration. To achieve these situations, a magnetic or electric field must be applied along the optic axis. This leads to practical possibilities for operating the efficiency of AO diffraction with the external electric and magnetic fields.

Resume : PbZr1-xTixO3 (PZT) solid solutions occupy an exceptional position among the perovskite-structured materials because of their many practical applications. In this work, we report the grown PZT single crystals with 0?x?0.13 and good Ti homogeneity. Recent studies describe new phases [1-4] and significantly modify Jaffe's diagram [5]. Because most reports on the phase diagram of PZT solid solutions were constructed based on investigations of ceramics, mainly due to technological difficulties in growing the PZT single crystals, we undertook investigations of the Zr-rich PZT single crystals. Based on dielectric and optical studies, the tricritical point was confirmed for x close to 0.06 [6]. Temperature changes of the optic, dielectric, and strain properties of the PZT single crystal have confirmed the existence of additional phase transitions. Earlier, additional phase transitions have been reported above x>0.06 for ceramics [1]. In PZT crystals, such additional transition has been observed already in a PbZr0.95Ti0.05O3 single crystal. [1] F. Cordero, F. Trequattrini, F. Craciun and C. Galassi, Phys. Rev. B 87, 094108 (2013). [2] M. J. Li, L. P. Xu, K. Shi, J. Z. Zhang, X. F. Chen, Z. G. Hu, X. L. Dong and J. H. Chu, J. Phys. D:Appl. Phys. 49, 275305 (2016). [3] N. Zhang, H. Yokota, A. M. Glazer, D. A. Keen, S. Gorfman, P. A. Thomas, W. Rena and Z.-G. Ye, IUCrJ 5,1 (2018). [4] N. Zhang, H. Yokota, A. M. Glazer, Z. Ren, D. A. Keen, D. S. Keeble, P. A. Thomas and Z. - G. Ye, Nat. Commun. 5, 5231 (2014). [5] B. Jaffe, W. R. Cook and H. Jaffe, Piezoelectric Ceramics, Academic, London (1971). [6] R. W. Whatmore, R. Clarke and A. M. Glazer, J. Phys. C: Solid State Phy.s, 11, 3089 (1978). Acknowledgements: This work was supported by the National Science Centre, Poland, grant number 2020/37/B/ST3/ 02015

Resume : Rare-earth nickelates, RNiO3, are challenging compounds due to their intriguing physics, a consequence of the strong correlation between electronic, charge, spin and lattice degrees of freedom [1]. Among these compounds, NdNiO3 has raised some controversy regarding the nature of its metallic to insulator transition (MIT). NdNiO3 exhibits a first-order MIT, adopting a metallic and paramagnetic Pnma symmetry above TMI = 200 K and, in the insulating phase, transits into the P21/n symmetry, with the stabilization of a E’-type antiferromagnetic phase [2]. In RNiO3, a close relationship between structure and MIT has been proposed due to the strong dependence of the TMI from the rare-earth cation size [3]. The symmetry lowering at MIT is supposedly accompanied by NiO6 breathing distortion that once coupled to in-phase and anti-phase octahedra rotation distortions would trigger the MIT. However, contrarily to smaller rare-earth cations, in NdNiO3, the amplitude of these oxygen rotations is not enough to stabilize the charge ordering and open the bandgap of the eg-Ni orbitals, and the magnetic ordering helps to promote the occurrence of MIT [3]. Therefore, in NdNiO3, TNéel = TMI is expected. While some experimental studies evidence the concomitant nature between structure and MIT [4], others completely reject it, assigning the magnetic ordering to the triggering mechanism instead [5]. The question still remains concerning the mechanisms that actual trigger MIT in NdNiO3. Towards searching for an answer to this demand, we have carried out an experimental study in NdNiO3 ceramics and thin films of 260 nm and 110 nm deposited onto (001) oriented LaAlO3 substrate. In this work, we report temperature-dependent Raman scattering and magnetization measurements to follow the structure and magnetic order evolution across the MIT, which was identified from resistivity measurements. The experimental results point out for a decoupling between the structural and electronic orders but evidence a coupling between electronic and magnetic orders in NdNiO3, independently on the used sample type. [1] G. Catalan, Ph. Transit. 81, 729 (2008) [2] S. Catalano et al., Rep. Prog. Phys. 81, 046501 (2018) [3] A. Mercy et al., Nat. Commun. 8, 1 (2017) [4] J. Y. Zhang et al., Sci. Rep. 6, 23652 (2016) [5] D. Meyers et al., Sci. Rep. 6, 2793 (2016)

Resume : Complex oxides host a rich variety of functional phenomena - including multiferroicity, dielectricity, superconductivity, metal-insulator transition, and topology. Therefore, understanding these phenomena and their corresponding phase transition is of both fundamental and technological importance to further stimulate the search for novel material designs and manipulations. The latter pathway involves the epitaxial growth of these oxides with a lattice-mismatch-induced biaxial strain, which has led to the enhancement of these functional properties. In this work, we go beyond this conventional epitaxy and examine the use of a 3D stress/strain on these materials by first-principle methods. We focus on (negative) pressure-induced isosymmetric phase transitions – involving no change in their structural space group – in a ferroelectric (BaTiO3) and Mott (V2O3) material. For the former, we have shown the presence of a first-order transition upon negative pressure from its tetragonal phase to a novel supertetragonal polymorph with a c/a ratio of 1.3, resulting in unprecedent polarization values. On the other hand, V2O3 is well-known to have a room-temperature metal-insulator transition (MIT) upon Cr doping, however, we observe a similar MIT in pure V2O3 upon negative pressure. Both pressure-induced transitions show a discontinuity in their c/a ratio, which can be related to their corresponding distortion; c/a representing the tetragonal polar distortion for BaTiO3, while correlating to a trigonal distortion in V2O3. Firstly, a detailed density functional theory (DFT) study sheds light on how these structural discontinuities translate to their electronic and dielectric properties. Secondly, an ab initio local auxiliary atomic orbital description (crystal orbital Hamiltonian population analysis) is applied to study the changes in the chemical bonds associated with their structural distortion. We show that both structural discontinuities can be ascribed to a critical change in their orbital overlaps which triggers an abrupt electron re-ordering. Finally, it can be shown by Landau theory that all isosymmetric transition are a priori first-order in nature. Hence, we identify the general features of earlier-reported (pressure-induced) isosymmetric phase transitions, and unify these within this local atomic orbital description.

Resume : Antiferroelectrics possess ordered dipole moments, which are arranged antiparallel so they cancel out within one unit cell. Upon application of sufficiently large electric field, ferroelectric phase is induced and characteristic double-hysteresis polarization loops can be measured. An archetypal antiferroelectric is PbZrO3, which is orthorhombic in its antiferroelectric phase and it is supposed to be rhombohedral in its ferroelectric phase. It is also a very complex material. For instance, thermodynamically stable ferrielectric phase was recently proposed in this material by first-principles studies [1]. Chemical Solution Deposition (CSD) is a well-known thin-film deposition technique, characterized by low cost, efficient scale-up and ability to produce high-quality polycrystalline thin films. Our group has been studying strongly (001)pc oriented polycrystalline PbZrO3 thin films prepared via CSD, with thicknesses ranging between 85 and 255 nm. Electromechanical properties were characterized on nanoscale using piezoresponse force microscopy and on macroscale by measuring polarization-electric field loops. In both cases presence of polar phase at low electric fields has been detected. While it could be of intrinsic origin, it could also arise from the presence of PbTiO3 (ferroelectric) seed layer, which was used to enhance crystallization of PbZrO3 and induce (001)pc orientation [2,3]. CSD has been traditionally seen as inferior technique for the growth of epitaxial films. In this contribution we will challenge this view by presenting our recent work on solution-grown epitaxial-like PbZrO3 thin films, which are deposited on Nb-doped (100) SrTiO3 substrates. According to theta-2theta X-ray diffraction the films are single-phase perovskites with mixed (120)o/(001)o out-of-plane orientation. Epitaxy is indicated by phi-scans corresponding to (110)o plane, in which four sharp peaks, separated by 90°, are observed. Polarization-electric field loops reveal antiferroelectric-ferroelectric phase transition at ~700 kV cm-1 and spontaneous polarization Ps of ~20 ?C cm-1. Smaller switching peaks are observed at low electric fields (~200 kV cm-1) with remanent polarization 2Pr of 10 ?C cm-2, pointing towards presence of polar phase. These peaks are rather minor in the pristine state and they become stronger with electric-field cycling. Further microstructural and electrical characterizations will be presented in the contribution and possible origins of the observed behaviour will be discussed. [1] H. Aramberri et al., NPJ Comp. Mater., 196 (2021). [2] H. Lu et al., Adv. Funct. Mater., 2003622 (2020). [3] C. Milesi-Brault et al., Appl. Phys. Lett, 118, 042901 (2020).

Resume : Antiferroelectric materials exhibit electric field-induced phase transitions between anti-polar and polar states, which enable their use in energy storage capacitors and electrocaloric devices. Here, we investigate the effect of this phase transition on the birefringence change 1 mm thick polycrystalline PbZr0.95Ti0.05O3 film processed by chemical solution deposition on a transparent PbTiO3(13 nm)/HfO2(23 nm)/SiO2 substrate. We show that, despite the polycrystalline nature of the film and its moderate thickness, the field-induced phase transition produces a sizeable effect observable under a polarized microscope. The azimuthal angle-dependence and the time evolution of this field-induced birefringence are further discussed in order to better understand the observed effect. The study not only facilitates further understanding of the physics of the antiferroelectric phase transition but also opens the door for oxide antiferroelectric materials in electro-optic applications such as optical switching and display.

Resume : Photo-induced ferroelectric characteristics are receiving tremendous attention in research and technological aspects due to various fascinating phenomena exhibited by them, such as photovoltaic (PV), photostriction, photo-ferroelectric, and photo-electrocatalytic effects. In this regard, exploring the photo-induced multifunctional characteristics of ferroelectric materials is crucial for a detailed understanding of the dynamics associated with these phenomena and for implementing these characteristics in optoelectronic applications. Several advantages and drawbacks are associated with the ferroelectric system as shown by current research progress. For example, ferroelectric material exhibits anomalous PV response, which can be higher than the equivalent bandgap voltage. However, the wide-bandgap characteristic of ferroelectrics restricts the absorption of the solar spectra in the system, and therefore limits overall device efficiencies. To resolve some of the current issues, we plan to combine ferroelectric materials with conventional photo-absorbers. In this respect, porous BaTiO3 (pBTO) synthesis by spin coating technique is used as a substrate for electrodeposition, and the photo-absorber Fe2O3 is deposited on the pBTO thin film. The nanoscopic ferroelectric properties of the combined structure is demonstrated through piezoresponse force microscopy which exhibits typical ferroelectric behaviour. In this presentation, I will explain the outcomes of PV and photo-electrocatalytic characteristics of the combined system by tuning several external parameters such as porosity percentage, strain engineering, geometrical modification, etc. Overall, I will discuss a general overview of the current and future direction of the photo-induced ferroelectric characteristics for advanced optoelectronic applications.

Resume : The present study aims to enhance the optical properties of barium titanate through narrowing its band gap energy to be effective for photocatalytic reactions in sunlight and be useful for solar cells. This target was achieved through growth of the hollandite phase instead of the perovskite phase inside the barium titanate crystals. By using solvent thermal reactions and thermal treatment at different temperatures (250 ◦C, 600 ◦C, and 900 ◦C), the hollandite phase of barium titanate was successfully obtained and confirmed through X-ray diffraction (XRD), Raman spectra and scanning electron microscopy techniques. XRD patterns showed a clear hollandite phase of barium titanium oxides for the sample calcined at 900 ◦C (BT1-900); however, the samples at 600 ◦C showed the presence of mixed phases. The mean crystallite size of the BT1-900 sample was found to be 38 nm. Morphological images revealed that the hollandite phase of barium titanate consisted of a mixed morphology of spheres and sheet-like features. The optical properties of barium titanate showed that its absorption edge shifted to the visible region and indicated band gap energy tuning ranging from 1.75 eV to 2.3 eV. Photocatalytic studies showed the complete and fast decolorization and mineralization of green pollutants (naphthol green B; NGB) in the prepared barium titanate with hollandite phase after illumination in sunlight for ten minutes. Finally, it can be concluded that the low band gap energy of barium titanate having the hollandite phase introduces beneficial structures for optical applications in sunlight.

Resume : Lead-free perovskite solid solution systems with morphotropic phase boundary (MPB), which can be produced using conventional ceramic routes or crystal growth techniques, have already been investigated in detail and, in spite of numerous chemical modifications attempted, are hardly possible to make a real alternative to lead zirconate-titanate (PZT). In this respect, high-pressure synthesis is a fresh approach to search for new lead-free piezoelectrics and multiferroics. In the (1-x)BiMg0.5Ti0.5O3-xBiZn0.5Ti0.5O3 system [(1-x)BMT-xBZT], in which the orthorhombic BMT is a structural analogue of PbZrO3, while the tetragonal BZT is isostructural to PbTiO3, neither the end members nor their solid solutions can be obtained in bulk form using the conventional synthesis routes. The as-prepared (unannealed) compositions with a relative BZT content x < 0.75 are orthorhombic (space group Pnnm), while those with a BZT content above this value are tetragonal (P4mm). In the solution with x = 0.75, both phases coexist forming an MPB. We have recently revealed a phenomenon of annealing-stimulated irreversible transformations of the high-pressure stabilized phases (conversion polymorphism), and demonstrated that this is a new and promising approach to produce novel multiferroic materials. One of the most remarkable features of the high-pressure prepared complex perovskite compositions, which demonstrates the effect of conversion polymorphism, is that the patterns of their phase diagram depend on the maximum annealing temperature. This implies that by means of controlled annealing, materials with different combinations of the perovskite phases can obtained. This feature can be used, e.g., to design new materials with a morphotropic phase boundary. Annealing of the (1-x)BMT-xBZT samples at the temperatures below the decomposition points of their metastable perovskite phases (650-700oC depending on composition) was found to induce irreversible structural transitions of the perovskite phases over a wide compositional range. The compositions with 0.60 ≤ x ≤ 0.75 transform into the tetragonal structure upon heating and this structure remains upon cooling down to room temperature. The compositions with 0.20 ≤ x ≤ 0.60 irreversibly transform upon heating to the rhombohedral R3c structure. The compositions with x ≤ 0.10 keep the orthorhombic structure over the annealing. The new location of the MPB in the (1-x)BMT-xBZT system, where the rhombohedral and the tetragonal phases coexist, is at x = 0.60. The compositional dependence of the primitive perovskite unit-cell parameters is essentially similar to that of PZT. We report results of the in situ temperature X-ray diffraction study and the Piezoresponse Force Microscopy measurements. The results are discussed in comparison with those found in PZT. The authors acknowledge the financial support of the bilateral Portugal-Germany (FCT-DAAD) project PZT-FREE (grants 2021.09702.CBM and 57610755, respectively).