Crystallography in materials science: novel methods for novel materials

Resume : X-ray diffraction has been longtime applied to identify various single crystal structure based on reciprocal information. In this talk, X-ray imaging methods to visualize defects of single crystals in real space will be introduced. First, high resolution X-ray diffraction microscopy that is capable of coupling grain orientation with its spatial location in textured thin-films will be presented. The principle is based on the combination of X-ray topography with high resolution X-ray diffractometry. This approach is applied for analytic determination of the three-dimensional distribution of dislocations, warping mechanism of Si wafer. Secondly, a bright-field X-ray imaging method will be presented. Bright-field x-ray images carry information both from diffraction/scattering phenomena and from absorption and phase contrast. Specifically, synchrotron x-ray transmission micrographs simultaneously yield diffraction-based information on strain effects and information on structural inhomogeneities when (0001) 4H-SiC wafers are set for a strong reflection in the Laue geometry. This approach offers interesting advantages with respect to the separate study of strain and inhomogeneity effects for a variety of crystalline systems.

Resume : The development of techniques like spin and dip coating, physical or chemical vapor deposition during the past decades offers now the opportunity for tuning the composition of polycrystalline layers and coating at the nanometric scale. The knowledge of the structure and the microstructure of different layers allow to predict their long term stability under harsh environments. Glancing Incidence X Ray Diffraction technique, based on the variation of the refractive index across the interface between material and air, provides an efficient tool to collect diffraction patterns as a function of the probed depth [1]. As the experimental setup in GIXRD exhibits peculiar features, many corrections need to be applied to extract information about the structure and the microstructure from Rietveld refinement [2]. In this talk, we will firstly discuss those corrections implemented in a Rietveld refinement code and illustrate by different examples[3]. References: [1] see for instance Brunel, De Bergevin, Acta Cryst A42, 299 (1986); [2] D. Simeone, G. Baldinozzi, D. Gosset, G. Zalczer, and J. F. Berar. J. Appl. Crystallography, 44 :1205–1210, 2011 [3] G. Baldinozzi, G. Muller, C. Laberty-Robert, D. Gosset, D. Simeone, and C. Sanchez. J. Phys. Chem.C, 116(14) :7658–7663, 2012.

Resume : Growth polarity strongly affects the surface structure and chemistry of group III-V semiconductors such as nitrides or GaAs and hence it is very important for the development of devices based on these materials. Strain and polar direction of GaN film determines the direction of the piezoelectric field that is crucial to the device performance. Therefore, knowledge of the growth polarity of a particular material is of high importance. One of the best methods that can be used to determine the growth polarity is convergent beam electron diffraction (CBED). This method can be applied to very small volumes that cannot be studied by other methods. A dynamic approach with full CBED pattern of the [1100] pole is usually used for GaN and [110] pole for GaAs, however a kinematic approach using a systematic (0002) row can be also applied. Since both these crystals are non-centrosymmetric the intensity distribution within the particular discs such as (0002) and (0002) for GaN or (002) and (002) for GaAs is different and this information can be used to determine the crystal polarity. As a reference, computer-simulated CBED patterns need to be calculated for particular zone axes for different sample thicknesses and the acceleration voltage of the microscope used for the experiment. Advantages of this method and comparison to determination of particular growth polarity by the high resolution microscopy will be discussed.

Resume : We present the structural and crystallographic characterization of inclined self-formed GaN nanowires (NWs) grown on r-plane sapphire by plasma-assisted Molecular Beam Epitaxy. Direct deposition took place after excessive nitridation of the sapphire substrate. The NWs were grown along the c-axis, subtending a 61o angle to the sapphire. A thin nonpolar a-plane GaN film was also formed between the NWs. The NW orientation was identified by selected area electron diffraction, and their polarity was determined using convergent beam electron diffraction. High resolution transmission electron microscopy (HRTEM) techniques were employed for elucidating the NW structural characteristics, especially their nucleation sites. By combining cross-sectional and plan-view observations, the 3D crystallographic model of the NWs was constructed. The HRTEM and topological analysis of the NW-nonpolar GaN interfaces showed a high symmetry relative orientation relationship. Their coexistence was accommodated by low-energy grain boundaries comprising interfacial disconnections. Using Moiré fringe analysis and Bragg filtering of HRTEM images, it was determined that NW emanation is promoted by the steps on the sapphire substrate that are formed by the nitridation pre-treatment. Geometrical phase analysis confirmed the direct emanation of the NWs from semipolar GaN nucleated at such sites. Acknowledgments Research co-financed by the EU and Greek national funds - Research Funding Program: THALES.

Resume : We have recently developed a new titanium oxide material dispersed on silica particles [1]. Although titanium oxide and related materials are well known for their photocatalytic activity, the newly developed material exhibited functionalities such as the ability to decompose environmental pollutants, or kill germs and viruses, without the need for light irradiation. We have already applied this material to environments that are difficult to irradiate with light [1]. In order to understand the mechanism that confers such activity under dark conditions, it is important to determine the crystal structure of the material, including the local structure around specific ions. Therefore, in the present study, a structural evaluation was performed using a combination of X-ray absorption near edge structure (XANES) spectroscopy and first-principles density functional theory calculations, paying particular attention to the local structure in the vicinity of Ti ions. The Ti K-edge XANES spectra exhibited peaks in the pre-edge region that differed from those for anatase TiO2. However, in the post-edge region, the observed peaks were similar to those for anatase TiO2. These spectral characteristics were well reproduced by a spectrum calculated for Ti ions on the {001} surface of anatase TiO2. Such ions have a five-fold coordination with oxygen, in contrast to the six-fold coordination found within the crystal. These results indicate that in this material, a larger fraction of Ti ions are at the surface than in the bulk, from which it can be concluded that the material is composed of anatase TiO2 particles with nanometer dimensions on the surfaces of the silica particles. [1] T. Irie et. al., Jpn, Unexam. Patent Appl. No. 2013-126623, (JP2013126623A).

Resume : Intermetalic compound with the Ti2Ni and derivative types gain a lot of attention as hydrogen storage materials. Modified by oxygen alloys are characterized by improvement of hydrogenation activation parameters and increased stability to disproportionation in hydrogen. Therefore, crystal structure of Zr3Cr3O and Ti3Cr3O compounds and their hydrides as well as hydrogen absorption-desorption properties were investigated. The samples with the Zr3Cr3O and Ti3Cr3O nominal compositions were synthesized by arc melting of pure elements (≥99.8 wt. %) and compacted powders of TiO2 or ZrO2 oxide under an argon atmosphere, annealed at 870 K during 720 h. Crystal structure of these compounds were refined by the Rietveld method using the X-ray powder diffraction pattern of the prepared samples collected on the DRON–3M diffractometer (CuKα radiation). The structure was successfully solved in the Fd-3m space group and Ti2Ni structure type (a = 11.914(3) Ǻ for Zr3Cr3O and a = 11.289(1) Ǻ for Ti3Cr3O compound). Further hydrogenation was performed at room temperature in an autoclave under 0.1-0.15 MPa hydrogen pressure after preliminary activation of the samples by heating at 500°C during 15 min. The amount of absorbed hydrogen was measured volumetrically. Calculated hydrogen storage capacity was ~5 and ~3 D/f.u. for Zr3Cr3O and Ti3Cr3O accordingly. During hydrogenation the metal matrix of the compounds preserves the cubic structure with unit cell expansion. Statistical occupation of 96g, 48f and 16d sites by Ti(Zr) and Cr atoms was observed for parent compounds as well as for the hydrides.

Resume : The advantage of stochastic genetic algorithms (GA) is a parallel evolutionary search in the population of trial structural models. The disadvantage of GA is a frequent stagnation of the population in the local minima on R-factor hypersurface. A perspective direction is a using the multi-population parallel GA (MPGA) with exchange of structural models between populations to prevent of stagnation. This work is devoted to the research of MPGA convergence relatively single-population binary and real GA. The optimal size of the computational grid for binary GA is 5 bits. The real GA using an atomic coordinates as a floating-point numbers. Both GA includes the operation of selected structural models refinement by DDM method [1] in addition to conventional genetic operations of selection, crossover and mutation. Basic operation of MPGA is a controllable migration of structural models between populations. By several examples statistically shown that the convergence of MPGA in 2-3 times higher, than for single- population GA. For example, in a case of ab initio search of test triclinic structure K4SnO4 convergence for both single GA is 30-35%, and MPGA convergence on 4-cores computer is about 90%. The report will also present examples of more complex structures solution by MPGA on a multiprocessor cluster. 1. Solovyov L.A., The Derivative Difference Minimization Method, Chapter 10 in: Powder Diffraction Theory and Practice, ed. R.E. Dinnebier and S.J.L. Billinge. Royal Society of Chemistry, 2008.

Resume : A new ternary phase of ZrSTe single crystals have been grown at 873 K from pure elements in a stoichiometry ratio by chemical vapor transport method. The crystals are characterized by single-crystal X-ray diffraction technique. The compound of ZrSTe crystallizes in the trigonal space group P31 with lattice parameters, a=4.430(5) Å, b=4.430(5) Å, c=5.923(5) Å, V=100.67(18) Å3. The structure of ZrSTe features a new structural type that consists of alternate two-dimensional zigzag infinite chain of Zr-Te extending along the c-axis. The consecutive 2-dimensional chains of Zr-Te are bridged via weak van der Waals bonds perpendicular to the c-axis.

Resume : Magnetic or diluted magnetic semiconductors have been intensively studied in the past, but recently, the possibility of applications in spin electronic devices has renewed the interest for these materials, especially when they exhibit magnetic properties at room temperature. Among binary manganese compounds MnTe with a hexagonal crystal structure of NiAs type is particularly interesting because of its relatively high Neel temperature (TN = 310 K). The successful growth of such MnTe thin layers by molecular beam epitaxy [1] opened a possibility of potential applications of this material in multilayer structures. The relevant MnTe literature data published in past were limited to narrow temperature range (170- 350 K) only [2]. The authors of both mentioned papers demonstrated a sharp decrease of the c parameter value with temperature below TN unaccompanied by any significant change in the a (T) parameter dependence. The goal of present paper was to check the finding mentioned above in a wide temperature range. The high quality MnTe crystals were grown in the Institute of Physics PAS and investigated by the X-ray diffraction methods with the use of synchrotron radiation at Hasylab in the temperature range from 15 K to 1100 K. The linear expansion coefficients were determined for the a and c parameters. Decrease of a distance between Mn ions observed in plane perpendicular to the hexagonal crystal axis in spite of the ferromagnetic exchange integral between the nearest neighbours in plane [3] is discussed. [1] E. Przeździecka, E.Kamińska, E. Dynowska et al., Phys. Stat. Sol. C 2, 1218 (2005). [2] N.P. Grazhdankina and D.I. Gurfel’, Soviet Phys. JETP 35, 631 (1959). [3] W. Szuszkiewicz, B. Hennion, E. Dynowska et al., Phys. Rev. B 73, 104403 (2006).

Resume : The interest in hybrid compounds has grown, based on the assumption that through the combination of different building blocks into a single material, one can combine and sometimes enhance selected properties (advantages). Because hybrid materials are built of organic and inorganic parts, they possess features typical for both. Thus, they combine the structural diversity, processability, tuneability and flexibility characteristic of the organic part with the rigidity and thermal stability of the inorganic component [1]. Furthermore, the inorganic fragment may add unusual and interesting optical, electronic and magnetic properties. Compounds of the type MeXn(MXDA)m, where Me = Cd, Co, Ni, Mn, X = I, Cl, Br, -SO4-2 and MXDA- m-ksylylenediamine, belongs to new class of hybrid materials [2]. Its representatives do not possess the outstanding optical properties (like ‘blue shift’) of the ‘semiconducting’ family MeQ(amine)1/2. Nevertheless, the incorporation of magnetic elements (Mn, Cu, Co, Ni, Fe) in their layers may produce new and unusual magnetic properties. In this case, their range of potential applications may be very broad as for example catalysis, photo-catalysis and sorption. The potential applications are correlated with dimensionality of obtained materials. Thus efforts devoted to optimization of synthesis conditions and to reach real crystal engineering level in these systems, played crucial role in our study. All obtained compounds were characterized by X-ray diffraction, SEM/EDS and TG/DSC methods. [1] H.R. Heulings IV, X. Huang, J. Li, Nano Lett. 1 (2001) 521 [2] K. Luberda-Durnaś, D. Mucha, P. Sanz-Camacho, W. Łasocha, Z. Anorg. Allg. Chem. 639 (2013) 2195

Resume : Dipole moment, polarizability, and first-order hyperpolarizability of polyphenyl derivatives have been investigated using ab initio and density functional theory calculations. Geometries of all molecules were optimized at the Hartree–Fock and DFT levels in a series of steps, first with the 6-31G(d) and 6-31+G(d) basis set, then with the 6-311G(d) and 6-311+G(d) basis set. The dipole moment , polarizabilities and The first static hyperpolarizabilities of these molecules were calculated using HF and DFT levels using same basis sets using GAUSSIAN 03 W. In general, the first hyperpolarizability is dependent on the choice of method and basis set. In order to understand this phenomenon in the context of molecular orbital picture, we examined the molecular HOMOs and molecular LUMOs generated via HF and DFT/6-311+G(d) level. The study reveals that the polyphenyl derivatives have large β values hence in general may have potential applications in the development of non-linear optical materials.

Resume : Lithium niobate is a well known crystal material with its unique combination of special properties – especially acoustooptic and electrooptic effect. Diamond on the other hand has outstanding physical properties – extremely high chemical and thermal resistance. Erbium (III) ions belong to rare earths with the emission around the wavelength of 1.5 μm. The properties and effects of the crystals in the combination with ion doping are used to simultaneous amplification and modulation of light. For the characterization and prediction of the resulting properties of these materials it is important to create theoretical models. In our study we have focused on the structural description of the crystal systems. According to the crystals defect structures we have developed several lithium niobate and diamond structure models containing erbium ions in the most probable site locations. With the models we calculated geometry optimizations and overall energies of the structures. The theoretical results were also compared with the experimental structural results. For the accurate calculations of total energies and crystal field parameters we utilised DFT (Density Functional Theory) ab initio method. The results of theoretical simulations can be further used to predict the ground and excited state energy levels of RE dopants and to estimate the related optical properties of the materials. We acknowledge the Czech Science Foundation project GA 14-05053S and the CANAM infrastructure.

Resume : Lithium ion batteries(LIBs) are important power storage system for many applications from portable electronic devices to electric vehicles. Meanwhile, graphite has been used as the anode material for LIBs by virtue of its low working voltage and their good cycle ability. However, the theoretical capacity is limited to 370mA/g that may be not enough for high performance LIBs in the future. It means that alternative anode materials with the higher capacity have to be developed. Regarding this, silicon (Si) is most attractive anode material to replace the graphite due to its high theoretical capacity of 4,200 mA/g. However, intercalation of Li into Si accompanies a drastic volume expansion leading to mechanical stress, and irreversible amorphorization of host Si. In this study, we developed Si nanosheets (SiNSs) as anodes materials for LIBs. The SiNSs with thickness and diameter of < 2 nm and > 2 um, respectively, were grown on graphite foil by chemical vapor deposition process. A coin-type half-cells using SiNSs as anodes were then fabricated. The half-cells were charged/dischared with Li ion upto 50th cycles. Meanwhile, the structure of SiNSs electrodes was studied by using in-situ X-ray diffraction and high-resolution transition electron microscopy. It was reavealed that SiNSs cell performed good coulombic efficiency of 98% up to 50th cycle without capacity fading. Interestingly, our structural study showed that SiNSs are reversibly transformed from amorphous to crystalline state in the course of discharging of Li ions. The correlationship between reversible phase transition and high performance of SiNSs was studied intensively and will be discussed in details.

Resume : The present method permits the in situ functionalisation of hydroxyapatite nanoparticles by gluconic acid in a microwave solvothermal synthesis (MSS) with high energy density. The hydroxyapatite nanopowder functionalized by gluconic acid was a crystalline hexagonal hydroxyapatite with needle-like morphology. The functionalized material was characterized by: average particle length of 8 nm, length-to-width ratio of 4.4, specific surface area of 245 m2/g, and a calcium-to-phosphate molar ratio equal to 1.57. Compared to a pure hydroxyapatite nanopowder synthesized in an analogous process, the in situ functionalization enabled a significant reduction in the agglomeration rate of the obtained nanoparticles. The reported method is an effective approach to preparing a homogeneous nano hydroxyapatite by applying a soft template in situ forming step. The obtained hydroxyapatite is a promising material for fully resorbable bone implant fabrication, and also delivers the necessary mechanical properties.

Resume : At third generation synchrotrons focused X-ray beams have been used for about two decades adding spatial resolution to diffraction experiments. Whilst the early days the typical beam size was at the 10 micron scale, today nano-beams ranging from few tens of nanometers to a few hundred nanometers are available to investigate micro-/nano-crystal arrangments. We will discuss instrumental aspects of nano-beam scanning diffraction, exemplified on the ID13 nano-probe. This comprises the production of nano-beams, requirements of positional stability, the selection of different types of area detectors, the layout of the nano-goniometer, and sample manipulation. Many scientific topics that can in principle be addressed with nano-diffraction can be enhanced by the use of complementary methods or sample environments in order to perform the experiment under in-situ/in-operando conditions such as working at elevated temperatures [1] and mechanical deformation. Examples from a wide range of scientific applications from material science to polymer research will be presented in the second part of this contribution. Nano-diffraction can be used, for instance, to investigate texture, strain, particle size, and phase composition of thin films and other materials. Mapping the orientation of its crystalline component, the longitudinal morphology of high performance polymer fibres can be imaged in detail at the 100 nm scale. We will conclude with an outlook on future opportunities of nano-diffraction in view of the availability of high performance pixel array detectors and substantial upgrades foreseen of the 3rd generation synchrotrons at the ESRF and elsewhere. [1] Rosenthal, M., et al (2014) J. Synchrotron Radiat. 21 223-228

Resume : Investigation of photovoltaic and photo-catalytic materials by surface photovoltage techniques Abstract: Charge separation in space is crucial for the application of photovoltaic and photo-catalytic materials. Surface photovoltage (SPV) measurements in the Kelvin-probe and fixed capacitor arrangements give information about spectral, time and temperature dependent charge separation in space in semiconducting materials and in ultra-thin charge-selective layers or layer systems. SPV signals contain information about mechanisms of charge separation, electronic transitions from which charge separation is possible, charge transport and recombination. Advantages of SPV techniques are (i) the local sensitivity in one dimension related to regions of charge separation, (ii) the high sensitivity up to charge separation in molecular monolayers, (iii) the ability to bridge the pressure gap between ultra-high vacuum, gas atmospheres and electrolytes and (iv) the special access to materials specific properties. The complex dependence of SPV signals on photo-generation, carrier dynamics and photo-chemical processes makes a straight forward interpretation of SPV signals often difficult so that the investigation of model systems is required. A brief introduction into SPV techniques will be given. Examples of mechanisms of charge separation will be demonstrated for the characterization of charge separation in surface treated semiconductors and across organic and quantum dot monolayers and nano-composites.

Resume : Several groups of inorganic compounds are recently investigated in respect to their thermoelectric properties. To found new ways for designing of thermoelectrica, crystallographic features characterizing structural complexity belong to the factors influencing thermoelectric ability of substances. In general, structural complexity of materials and thermoelectrica in particular may be discussed taking into account either their basic crystallographic characteristics. To get more insight into the thermoelectric behavior, the bonding descriptors were found to be suitable analytical tool. Spatial separation of the regions with different chemical bonding in the crystal structure is a fingerprint for enhanced thermoelectric ability. For the compounds with the characteristic structural and bonding features, e.g. clathrates with their cage structures or oxides with crystallographic share planes, structural and bonding complexity opens an opportunity to influence more directly the thermal conductivity separating - at least partially – its lattice and electronic parts. E.g., the lattice thermal conductivity of Ba8TMxGe46-x-y.□y is reduced with respect to the defect-less clathrates and to Ba8Ge43□3 with ordered vacancies, additional Ba-TM interactions further reduce thermal conductivity.

Resume : Most of matter in Universe exists either under huge pressure inside stars and planets, or in very low pressure of interstellar space, and there are relatively materials at ‘normal’ conditions of 0.1 MPa and 298 K. However, at these normal conditions most of functional materials are obtained and practically applied. However, presently more diamonds for jewelry and industrial applications (mainly cutting, abrasive materials and heat sinks) are made than mined. Can also other types of materials synthesized in high-pressure conditions be more advantageous than those obtained traditionally? Pressure considerably changes the balance between intermolecular interactions in the crystals of organic compounds, which affects their preference for spontaneous crystallization: at 0.5 GPa thiourea preferentially crystallizes as a hydrate, above 0.7 GPa another hydrate is formed and above 1.0 GPa anhydrous thiourea is obtained again [1]. Likewise, 1,4-diazabicyclo[2.2.2]octane hydroiodide (dabcoHI) above 0.5 GPa forms two polymorphs of the monohydrate [2]. Intermolecular interactions changed by pressure induce phase transitions and reverse the preferential crystallization [3,4]. For example, in halomethanes high pressure favours the CH•••halogen hydrogen bonds to halogen•••halogen interactions [5]. Envisaged applications of high-pressure crystallization will be discussed. [1] H. Tomkowiak et al. Cryst. Growth Des. 13 (2013) 121-125. [2] A. Olejniczak, A. Katrusiak, CrystEngComm, 12 (2010) 2528-2532. [3] A. Olejniczak et al. Cryst. Growth Des. 10 (2010) 3537-3546. [4] E. Patyk et al. Angew. Chem. 51 (2012)2146-2150. [5] M. Podsiadło et al. CrystEngComm. (2014). DOI 10.1039/C4CE00241E.

Resume : Thanks to the combined use of synchrotron radiation and diamond-anvil cells, powder x-ray diffraction (XRD) has become a technique widely used to study the physical structure of materials under high pressure (HP). This technique is particularly relevant for earth sciences, high-pressure materials synthesis study or optimization, and detailed studies of properties of complex materials. In this presentation, I will review studies carried out during the past years in different compounds, which represent breakthroughs in the frontier area of HP research. They will include ferroelectric materials (CuWO4), photocatalityc materials for hydrogen production (InVO4), and scintillating materials (BaWO4) among others. Tacking advantage of these examples, the latest developments in powder XRD at high pressure will be described. Future challenges will be also commented as well as the importance of technical details such as the choice of pressure-transmitting media. Examples of structural solution of complex structures, structural refinements of HP polymorphs, and collection of accurate compressibility data will be described. The importance of combining powder XRD with other characterization techniques such as Raman spectroscopy and ab initio calculations will be also outlined. The particular case of CuWO4 will be used to show limitations of HP powder XRD in low-symmetry materials and the need of single-crystal XRD to accurately solve the crystal structure of their HP phases.

Resume : Although everything seems to be already well known in the field of routine structural single crystal X-ray analysis and scientists working in field have solved and refined ca. 1 mln structures so far, even commonly used approaches and models should be critically re-evaluated. It is incredible that the Independent Atom Model (IAM) effectively introduced a century ago is still used and successful in structural analysis. One would even say that it its success has dominated the whole field. On the other hand, within the past century there is an overwhelming progress in design and production of X-ray hardware which is made for needs of both small laboratories and large scale facilities. This progress in sophisticated X-ray hardware also accelerates progress in the quality and complexity of models used to interpret experimental results. The main ideas of experimental charge density studies going well beyond IAM will be presented. Accuracy and precision of structural data obtained from routine and charge density studies will be discussed. Some practical suggestions will be presented how to estimate and improve the quality of single crystal X-ray diffraction structural results. Also some new applications of experimental charge densities showing their potential will be presented.