High-resolution Synchrotron X-ray Powder Diffraction Research Articles

Accurate unrestrained DDM refinement of crystal structures from highly distorted and low-resolution powder diffraction data.

The structure of benzene:ethane co-crystal at 90 K is refined with anisotropic displacement parameters without geometric restraints from high-resolution synchrotron X-ray powder diffraction (XRPD) data using the derivative difference method (DDM) with properly chosen weighting schemes. The average C-C bond precision achieved is 0.005 Å and the H-atom positions in ethane are refined independently. A new DDM weighting scheme is introduced that compensates for big distortions of experimental data. The results are compared with density functional theory (DFT) calculations reported by Maynard-Casely et al. [(2016). IUCrJ, 3, 192-199] where a rigid-body Rietveld refinement was also applied to the same dataset due to severe distortions of the powder pattern attributable to experimental peculiarities. For the crystal structure of 2-aminopyridinium fumarate-fumaric acid formerly refined applying 77 geometric restraints by Dong et al. [(2013). Acta Cryst. C69, 896-900], an unrestrained DDM refinement using the same XRPD pattern surprisingly gave two times narrower dispersion of interatomic distances.

Reaction of Pb(II) and Zn(II) with Ethyl Linoleate To Form Structured Hybrid Inorganic-Organic Complexes: A Model for Degradation in Historic Paint Films.

To investigate soap formation in drying oils in historic paints, the reaction between metal acetates (K+, Zn2+, Pb2+) and ethyl linoleate (EL) was studied using optical microscopy, X-ray powder diffraction, and electron microscopy. Pb(II) and Zn(II) react rapidly with EL to form highly structured, spherulitic, luminescent crystallites that aggregate. Evidence from Fourier transform infrared (FTIR) and scanning electron microscopy/energy dispersive X-ray analysis and high-resolution synchrotron powder X-ray diffraction indicates that these are organic–inorganic hybrid complexes or coordination polymers. FTIR absorbance peaks at ca. 1540 cm–1 for Pb(II) and ca. 1580 cm–1 for Zn(II) are consistent with the formation of carboxylate complexes. The complexes formed offer insight into the degradation processes observed in oil paint films, suggesting that soap formation is rapid when metal ions are solubilized and can occur with unsaturated fatty acids that are present in fresh oils. These complexes may account for the atypical luminescence observed in lead-containing cured oil paint films.

Determining a Structural Distortion and Anion Ordering in La2Si4N6C via Computation and Experiment.

A structural instability in the orthorhombic carbonitridosilicate La2Si4N6C arises when calculating the ab initio phonon dispersion curves. The presence of imaginary modes indicates the compound reported in space group Pnma is dynamically unstable with the eigenvectors showing a monoclinic distortion pathway leading to space group P21/c. Synthesizing La2Si4N6C using a high-temperature route and conducting a co-refinement with high-resolution synchrotron X-ray and neutron powder diffraction shows the predicted peak splitting confirming the predicted lower symmetry crystal structure. Further, the combination of ab initio computation, neutron diffraction, and a total scattering analysis based on a neutron pair distribution function analysis supports that the anions are fully ordered and that carbon is only found on the central position of a star-shaped C(SiN3)4 unit. These results illustrate the power of combining computation and experiment to unequivocally solve crystal structures from polycrystalline powders.

Carrier concentration dependence of structural disorder in thermoelectric Sn1-x Te.

SnTe is a promising thermoelectric and topological insulator material. Here, the presumably simple rock salt crystal structure of SnTe is studied comprehensively by means of high-resolution synchrotron single-crystal and powder X-ray diffraction from 20 to 800 K. Two samples with different carrier concentrations (sample A = high, sample B = low) have remarkably different atomic displacement parameters, especially at low temperatures. Both samples contain significant numbers of cation vacancies (1-2%) and ordering of Sn vacancies possibly occurs on warming, as corroborated by the appearance of multiple phases and strain above 400 K. The possible presence of disorder and anharmonicity is investigated in view of the low thermal conductivity of SnTe. Refinement of anharmonic Gram-Charlier parameters reveals marginal anharmonicity for sample A, whereas sample B exhibits anharmonic effects even at low temperature. For both samples, no indications are found of a low-temperature rhombohedral phase. Maximum entropy method (MEM) calculations are carried out, including nuclear-weighted X-ray MEM calculations (NXMEM). The atomic electron densities are spherical for sample A, whereas for sample B the Te electron density is elongated along the 〈100〉 direction, with the maximum being displaced from the lattice position at higher temperatures. Overall, the crystal structure of SnTe is found to be defective and sample-dependent, and therefore theoretical calculations of perfect rock salt structures are not expected to predict the properties of real materials.

Understanding the Anomalously High Capacity of Li- and Mn-Rich Cathodes for Li-Ion Batteries

The reaction mechanism of a high capacity lithium- and manganese-rich metal oxide has been investigated. High-resolution synchrotron X-ray powder diffraction (HRPD) and X-ray absorption spectroscopy (XAS) were used, respectively, to evaluate the electrochemical charge and discharge reactions in terms of local and bulk structural changes, and variations in the oxidation states of the transition metal ions. Ni K-edge XAS data indicate the participation of nickel in reversible redox reactions, whereas Mn K-edge absorption spectra show that the manganese ions do not participate in the electrochemical reactions. Rietveld refinements of the oxygen occupancy during charge and discharge provide evidence of reversible oxygen contribution by the host structure; this unique oxygen participation is likely the main reason for the anomalously high capacity of these electrodes. The HRPD data also show that during the early cycles, characteristic peaks of the Li2MnO3 component disappear when charged to 4.7 V, but reappear on discharge to 2.5 V, consistent with a reversible lithium and oxygen extraction process. The results provide new insights into the charge compensation mechanisms that occur when high capacity, lithium- and manganese-rich electrode materials are electrochemically cycled – a topic that is currently being hotly debated in the literature.

Crystal structure and phase transition of thermoelectric SnSe.

Tin selenide-based functional materials are extensively studied in the field of optoelectronic, photovoltaic and thermoelectric devices. Specifically, SnSe has been reported to have an ultrahigh thermoelectric figure of merit of 2.6 ± 0.3 in the high-temperature phase. Here we report the evolution of lattice constants, fractional coordinates, site occupancy factors and atomic displacement factors with temperature by means of high-resolution synchrotron powder X-ray diffraction measured from 100 to 855 K. The structure is shown to be cation defective with a Sn content of 0.982 (4). The anisotropy of the thermal parameters of Sn becomes more pronounced approaching the high-temperature phase transition (∼ 810 K). Anharmonic Gram-Charlier parameters have been refined, but data from single-crystal diffraction appear to be needed to firmly quantify anharmonic features. Based on modelling of the atomic displacement parameters the Debye temperature is found to be 175 (4) K. Conflicting reports concerning the different coordinate system settings in the low-temperature and high-temperature phases are discussed. It is also shown that the high-temperature Cmcm phase is not pseudo-tetragonal as commonly assumed.

Hybrid Gold Single Crystals Incorporating Amino Acids

Composite hybrid gold crystals are of profound interest in various research areas ranging from materials science to biology. Their importance is due to their unique properties and potential implementation, for example, in sensing or in bio-nanomedicine. Here we report on the formation of hybrid organic–metal composites via the incorporation of selected amino acids (histidine, aspartic acid, serine, glutamine, alanine, cysteine, and selenocystine) into the crystal lattice of single crystals of gold. We used electron microscopy, chemical analysis, and high-resolution synchrotron powder X-ray diffraction to examine these composites. Crystal shape, as well as atomic concentrations of occluded amino acids and their impact on the crystal structure of gold, was determined. Concentration of the incorporated amino acid was highest for cysteine, followed by serine and aspartic acid. Our results indicate that the incorporation process probably occurs through a complex interaction of their individual functional group...

Ab Initio Structure Determination and Photoluminescent Properties of an Efficient, Thermally Stable Blue Phosphor, Ba2Y5B5O17:Ce3+

The crystal structure of a novel barium yttrium borate, Ba2Y5B5O17, was determined using a combination of ab initio global optimization algorithms and density functional theory calculations along with Rietveld refinement of high-resolution synchrotron X-ray powder diffraction data. Synthesized using a high-temperature solid-state route, the structure consists of edge- and corner-sharing Y- and Ba-centered polyhedra along with BO3 trigonal planes. Ba and Y occupy four crystallographically independent sites with two fully occupied by Y and two having a statistical mixture of Y and Ba. Substituting Ce3+ into the structure for Y3+ yields blue photoluminescence (λem = 443 nm) upon excitation with UV (λem = 365 nm) light. The emission of this new compound is efficient with an external quantum yield of 70% and is stable as a function of temperature with a quenching temperature of ≈400 K.

Evidence of reversible oxygen participation in anomalously high capacity Li- and Mn-rich cathodes for Li-ion batteries

The reaction mechanism of a high capacity lithium- and manganese-rich metal oxide, 0.4Li2MnO3–0.6LiMn0.5Ni0.5O2, has been investigated at the atomic level. High-resolution synchrotron X-ray powder diffraction (HRPD) and X-ray absorption spectroscopy (XAS) were used, respectively, to evaluate the electrochemical charge and discharge reactions in terms of local and bulk structural changes, and variations in the oxidation states of the transition metal ions. Ni K-edge XAS data indicate the participation of nickel in reversible redox reactions, whereas Mn K-edge absorption spectra show that the manganese ions do not participate in the electrochemical reactions. Rietveld refinements of the oxygen occupancy during charge and discharge provide evidence of reversible oxygen release and re-accommodation by the host structure; this unique oxygen participation is likely the main reason for the anomalously high capacity of these electrodes. The HRPD data also show that during the early cycles, characteristic peaks of the Li2MnO3 component disappear when charged to 4.7V, but reappear on discharge to 2.5V, consistent with a reversible lithium and oxygen extraction process. The results provide new insights into the charge compensation mechanisms that occur when high capacity, lithium- and manganese-rich electrode materials are electrochemically cycled – a topic that is currently being hotly debated in the literature.

Structural and spectroscopic studies of alkali-metal exchanged stilbites

The structural, infrared (IR) and Raman spectroscopic properties of alkali-metal exchanged (Na+, K+, Rb+, Cs+) zeolites with the stilbite (STI) framework have been studied by Rietveld analysis of high-resolution synchrotron X-ray powder diffraction data, micro-Raman and synchrotron IR spectroscopy at ambient conditions. Ordered arrangements of the extra-framework cations (EFCs) and H2O molecules were found at the intersection of the 10- and 8-ring channels as well as framework distortions and deformations of the channel system caused by the EFCs distributions. The T(5)–T(5) distances along the chains of the structural building units range from 8.921(1) to 8.979(5) Å and scale with the size of EFCs in Na-, K-, Rb-, Cs-STI. Furthermore, our spectroscopic studies show a systematic increase of characteristic band frequencies for the 4- and 5-ring breathing vibration modes in the far-IR spectral region with the size of the EFCs. The frequencies of the ring vibrational bands in the IR and Raman spectra are very sensitive to the presence of specific EFCS, and framework distortion.

Structural disorder, anisotropic micro-strain and cation vacancies in thermo-electric lead chalcogenides

Thermoelectric materials can interconvert heat and electricity, and the extraordinary thermoelectric properties of lead chalcogenides (PbX, X = S, Se, Te) attract immense scientific interest. A key topic is the role of the cation in reaching a very low thermal conductivity necessary for efficient energy conversion. Here we present new structural insights about the deceptively simple rock-salt lead chalcogenides through a comparative multi-temperature synchrotron powder X-ray diffraction study. For the first time, the presence of anisotropic microstrain broadening as well as lead vacancies are quantified for all three compounds. The microstrain implies extended breakage of cubic symmetry as a sign of the incipient ferroelectric nature of PbX. The degree of microstrain is correlated to the transition pressure of a symmetry reducing phase transition, and this trend can be explained by anion mediated s-p hybridization on lead. The observed number of vacancies is greatest for PbS (4-8%), but two samples of PbS show different cation occupancy, and thus sample-dependent vacancies might be the property that unifies conflicting results reported for PbX. Gram-Charlier analysis identifies a local non-spherical distribution of Pb; however, model unbiased maximum entropy analysis indicates that any static displacement of Pb, if present, is less than 0.2 Å at 100 K.

Chabazite structures with Li+, Na+, Ag+, K+, NH4+, Rb+ and Cs+ as extra-framework cations

Chabazites with Li+, Na+, Ag+, K+, NH4+, Rb+, Cs+ as monovalent extra-framework cations have been prepared from natural chabazite (ORI-CHA, Ca1.6Na0.5Si8.4Al3.6O24·14.3H2O) and characterized using a combination of Rietveld analyses of high-resolution synchrotron X-ray powder diffraction data and synchrotron infrared (IR) spectroscopy. All monovalent cation-exchanged chabazites crystallize in the rhombohedral space group R3¯m; Li-CHA (Li2.9Si8.6Al3.4O24·13.2H2O), a = 9.4068(6), α = 94.9156(9); Na-CHA (Na3.4Si8.6Al3.4O24·11.4H2O), a = 9.4216(3), α = 94.0773(6); Ag-CHA (Ag3.5Si8.5Al3.5O24·15.9H2O), a = 9.4402(4), α = 94.1354(9); K-CHA (K3.2Si8.7Al3.3O24·10.7H2O), a = 9.4420(1), α = 94.2540(3); NH4-CHA ((NH4)2.1Si8.4Al3.6O24·11.6H2O), a = 9.4378(9), α = 94.6101(1); Rb-CHA (Rb4.1Si7.9Al4.1O24·6.5H2O), a = 9.4318(4), α = 94.533(1); Cs-CHA (Cs3.4Si8.6Al3.4O24·6.4H2O), a = 9.4447(4), α = 94.2604(8). We establish correlations between the unit-cell volume, extra-framework cation distribution, and cation selectivity in chabazites. We establish that the unit cell volume of monovalent-cation exchanged chabazites decreases in the order of decreasing ion selectivity, based on the standard free hydration energies of exchange i.e., Cs+ > K+ > Ag+ > Rb+ > Na+ > Li+ and not the cation size.

A new class of lithium ion conductors with tunable structures and compositions: Quaternary diamond-like thiogermanates

The new Li2CoGeS4 compound crystallizes in the Pn space group with the wurtz-kesterite structure, according to single crystal X-ray diffraction. The structure of Li2CoGeS4 and the high degree of phase-purity in which it is prepared are supported by high-resolution synchrotron X-ray powder diffraction. Varying the divalent ion in Li2-II-GeS4 materials yields three different structure types, all of which are derived from hexagonal diamond. These structural variations give rise to Li+-encompassing [II–GeS4]2− nets with different topologies that offer diversity in lithium ion diffusion pathways. In the first systematic study of the lithium ion conductivity in quaternary diamond-like materials, wurtz-kesterite-type Li2CoGeS4 and Li2FeGeS4 (Pn), lithium cobalt(II) silicate-type Li2MnGeS4 (Pna21), and wurtz-stannite-type Li2CdGeS4 (Pmn21) are presented as environmentally stable lithium ion conductors. These materials are comprised of cubic diamond-like [CoGeS4]2− and [FeGeS4]2− anionic frameworks, ABW-like [MnGeS4]2−, and square lattice-like [CdGeS4]2−. As assessed using impedance spectroscopy, Li2FeGeS4 exhibits the most promising Li+ ion conductivity of 1.8(3)×10−4S/cm at 100°C, while Li2CdGeS4 shows the lowest activation energy for lithium ion conduction, EA=0.74(2)eV.

Revisiting the Hydrogen Storage Behavior of the Na-O-H System

Solid-state reactions between sodium hydride and sodium hydroxide are unusual among hydride-hydroxide systems since hydrogen can be stored reversibly. In order to understand the relationship between hydrogen uptake/release properties and phase/structure evolution, the dehydrogenation and hydrogenation behavior of the Na-O-H system has been investigated in detail both ex- and in-situ. Simultaneous thermogravimetric-differential thermal analysis coupled to mass spectrometry (TG-DTA-MS) experiments of NaH-NaOH composites reveal two principal features: Firstly, an H2 desorption event occurring between 240 and 380 °C and secondly an additional endothermic process at around 170 °C with no associated weight change. In-situ high-resolution synchrotron powder X-ray diffraction showed that NaOH appears to form a solid solution with NaH yielding a new cubic complex hydride phase below 200 °C. The Na-H-OH phase persists up to the maximum temperature of the in-situ diffraction experiment shortly before dehydrogenation occurs. The present work suggests that not only is the inter-phase synergic interaction of protic hydrogen (in NaOH) and hydridic hydrogen (in NaH) important in the dehydrogenation mechanism, but that also an intra-phase Hδ+… Hδ– interaction may be a crucial step in the desorption process.

Metastable monoclinic and orthorhombic phases and electric field induced irreversible phase transformation at room temperature in the lead-free classical ferroelectricBaTiO3

For decades it has been a well-known fact that among the few ferroelectric compounds in the perovskite family, namely, ${\mathrm{BaTiO}}_{3}$, ${\mathrm{KNbO}}_{3}$, ${\mathrm{PbTiO}}_{3}$, and ${\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_{3}$, the dielectric and piezoelectric properties of ${\mathrm{BaTiO}}_{3}$ are considerably higher than the others in polycrystalline form at room temperature. Further, similar to ferroelectric alloys exhibiting morphotropic phase boundary, single crystals of ${\mathrm{BaTiO}}_{3}$ exhibit anomalously large piezoelectric response when poled away from the direction of spontaneous polarization at room temperature. These anomalous features in ${\mathrm{BaTiO}}_{3}$ remained unexplained so far from the structural standpoint. In this work, we have used high-resolution synchrotron x-ray powder diffraction, atomic resolution aberration-corrected transmission electron microscopy, in conjunction with a powder poling technique, to reveal that at 300 K (i) the equilibrium state of ${\mathrm{BaTiO}}_{3}$ is characterized by coexistence of metastable monoclinic Pm and orthorhombic (Amm2) phases along with the tetragonal phase, and (ii) strong electric field switches the polarization direction from the [001] direction towards the [101] direction. These results suggest that ${\mathrm{BaTiO}}_{3}$ at room temperature is within an instability regime, and that this instability is the fundamental factor responsible for the anomalous dielectric and piezoelectric properties of ${\mathrm{BaTiO}}_{3}$ as compared to the other homologous ferroelectric perovskite compounds at room temperature. Pure ${\mathrm{BaTiO}}_{3}$ at room temperature is therefore more akin to lead-based ferroelectric alloys close to the morphotropic phase boundary where polarization rotation and field induced ferroelectric-ferroelectric phase transformations play a fundamental role in influencing the dielectric and piezoelectric behavior.

Crystal chemistry of birefringent hydrogrossular

Crystal structure refinements of two fine-grained, massive, birefringent hydrogarnet samples from South Africa [1. green “jade” and 2. pink “jade”] were carried out with the Rietveld method, cubic space group $$ Ia\overline{3} d, $$ and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Electron-microprobe analysis (EMPA) gave bulk compositions as follows: (1) (Ca2.997Mg0.003)Σ3{Al1.794Fe 0.196 3+ Cr 0.004 3+ Mn 0.003 3+ Ti 0.002 4+ }Σ2[(SiO4)2.851(O4H4)0.151]Σ3 and (2) (Ca2.993Mg0.007)Σ3{Al1.977Fe 0.020 3+ Mn 0.003 3+ Cr 0.001 3+ }Σ2[(SiO4)2.272(O4H4)0.730]Σ3. Their crystal structure was modeled well as indicated by the Rietveld refinement statistical indicators where the reduced χ2 and overall R (F 2) values are 1.133 and 0.0467, respectively, for sample 1 and 1.308 and 0.0342 for sample 2. Two cubic phases are contained in each sample. For phase 1a in sample 1, the weight fraction (%), unit-cell parameter (A), and O–H bond distance (A) are as follows: 74.4(1), a = 11.88874(4), and O–H = 0.98(9); the corresponding data for phase 1b are 25.6(1), a = 11.9280(5), and O–H = 0.91(9). For phase 2a in sample 2, the corresponding data are 52.0(1), a = 12.0591(1), and O–H = 0.90(6); the corresponding data for phase 2b are 48.0(1), a = 11.9340(2), and O–H = 0.90(7). The anisotropic displacement ellipsoids for the O atoms show no unusual features and are not elongated along the “Si–O” bond direction, which is written as Z–O, because of the general formula, X3Y2Z3O12, for garnet. Phase 1a is near end-member grossular, ideally Ca3Al2Si3O12. The deficiencies of the site occupancy factors (sofs) for the Si (=Z) site indicate that there are significant [O4H4]4− replacing [SiO4]4−. The Z–O distance is large in phase 1b, phases 2a, and 2b compared to a typical Z–O distance in anhydrous grossular or phase 1a. The H atoms occur in different environments around the vacant Z site in the two samples, and they may also bond to the O atoms surrounding the X and Y sites, if they contain vacancies as indicated by the refinement sofs. Two cubic phases are intergrown in each sample and cause strain that arise from structural mismatch and give rise to strain-induced birefringence in hydrogrossular.

Optical anisotropy, zoning, and coexistence of two cubic phases in andradites from Quebec and New York

The crystal structure of two zoned birefringent andradite garnets, ideally Ca3Fe23+Si3O12, from (1) Black Lake, Quebec (QC) and (2) Willsboro, New York (NY), was refined with the Rietveld method, space group \( Ia\overline{3} d \), and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Electron probe microanalyzer results gave bulk or average compositions as follows: (1) Ca3[Fe1.923+Al0.07Mn0.013+Mg0.01]Σ2Si3O12, Adr96Grs3 and (2) {Ca2.99Mg0.01Mn0.012+}Σ3[Fe1.523+Al0.47Ti0.01]Σ2(Si2.98Al0.02)Σ3O12, Adr76Grs23. The HRPXRD data show the coexistence of two cubic phases as intergrowths in both samples. Phase-1a of the QC sample-1 has a composition of near-end-member andradite, and phase-1b is Adr91Grs9. For the NY sample-2, phase-2a is Adr76Grs23 and phase-2b is Adr59Grs39. The reduced χ2 and overall R (F2) Rietveld refinement values are 1.074 and 0.0268 for the QC sample, and 1.172 and 0.0333 for the NY sample. For the QC sample, the weight percentage and unit-cell parameter for phase-1a are 67.0(1) % and a = 12.06077(1) A, and those for phase-1b are 33.0(1) % and a = 12.0535(1) A with Δa = 0.007 A. For the NY sample, the corresponding values for phase-2a are 81.3(1) % and a = 12.00456(1) A, and those for phase-2b are 18.7(1) % and a = 11.96687(1) A with Δa = 0.038 A. The two cubic phases are intergrown and cause strain that arises from structural mismatch and gives rise to strain-induced birefringence. The above two samples are compared to andradite samples from Arizona and Magnet Cove.

Ti-RICH ANDRADITES: CHEMISTRY, STRUCTURE, MULTI-PHASES, OPTICAL ANISOTROPY, AND OSCILLATORY ZONING

Electron probe microanalyzer (EPMA) data were obtained for 11 Ti-rich andradite garnet, [8] X 3 [6] Y 2 [4] Z 3 [4] O 12 , samples from Magnet Cove, Arkansas, USA. The crystal structures of nine of these samples were refined with the Rietveld method, space group la¯ 3 d , and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. One sample (MC9) contains two distinct compositions corresponding to different phases and subtle compositional differences are observed in two other samples (MC8 and MC10). The EPMA data falls on a straight line representing solid solutions from the endmember andradite, Ca 3 Fe 3+ 2 Si 3 O 12 , to a composition, Ca 3 (Ti 4+ 1.5 Fe 2+ 0.5 )[Fe 3+ Si 2 ]O 12 , with the coupled substitution: Fe 3+ + Si 4+ ↔ Ti 4+ 1.5 + Fe 2+ 0.5 . The Y site contains Ti 4+ cations (up to ≈1.1 atom per formula unit, apfu ). A few samples show oscillatory zoning that arises mainly from the distribution of Ti atoms coupled with subtle variations in Fe, Al, and Mg atoms. The HRPXRD results show that seven samples contain two or three cubic phases intergrown together. These seven samples exhibit birefringence because of strain arising from structural mismatch. There are three Ti-rich schorlomite samples (MC1, MC5, and MC7) of which MC1 and MC7 are single cubic phases and are isotropic. The bond distances (average , Y–O, Z–O, and mean ) change linearly with increasing a unit-cell parameter. The Y–O distance is nearly constant. The Z–O distance increases most because of Fe 3+ (0.49 A) substituting for Si 4+ (0.26 A) cations at the Z site.

Long- and short-range structure studies of KBT-KBZ solid-solutions using synchrotron radiation.

The relaxor ferroelectric K(0.5)Bi(0.5)TiO(3) has been synthesised in a solid-solution series with K(0.5)Bi(0.5)ZrO(3), as K(0.5)Bi(0.5)Ti1-xZrxO(3). High-resolution synchrotron X-ray powder diffraction and X-ray absorption near edge structure spectroscopy were used to characterise the long-range average and local structural behaviour. Rietveld refinements against diffraction data show that a pseudocubic tetragonal region exists across the whole solid-solution series, with truly cubic symmetry only observed at x = 1 (pure KBZ). Variable-temperature diffraction data for x = 0 (pure KBT) showed a broad ferroelectric transition from tetragonal to cubic symmetry at approximately 683 K with a coexistence of both phases close to that temperature, accompanied by a marked volume contraction. Ti K-edge data showed that Zr doping has a minimal effect on Ti off-centering, and revealed no evidence for local clustering. Metal L-edges showed that Ti(4+) cations remain off-centered with increasing Zr content, while Zr(4+) cations approach a higher-symmetry coordination environment, most likely due to the increased size of the Zr atoms. Although there is a minimal effect on actual Ti-offsets, an effective dilution of these environments by Zr doping leads to a reduction in polar domains and a diminished ferroelectric response.

Hybrid inorganic-organic materials with an optoelectronically active aromatic cation: (C7H7)2SnI6 and C7H7PbI3.

Inorganic materials with organic constituents-hybrid materials-have shown incredible promise as chemically tunable functional materials with interesting optical and electronic properties. Here, the preparation and structure are reported of two hybrid materials containing the optoelectronically active tropylium ion within tin- and lead-iodide inorganic frameworks with distinct topologies. The crystal structures of tropylium tin iodide, (C7H7)2SnI6, and tropylium lead iodide, C7H7PbI3, were solved using high-resolution synchrotron powder X-ray diffraction informed by X-ray pair distribution function data and high-resolution time-of-flight neutron diffraction. Tropylium tin iodide contains isolated tin(IV)-iodide octahedra and crystallizes as a deep black solid, while tropylium lead iodide presents one-dimensional chains of face-sharing lead(II)-iodide octahedra and crystallizes as a bright red-orange powder. Experimental diffuse reflectance spectra are in good agreement with density functional calculations of the electronic structure. Calculations of the band decomposed charge densities suggest that the deep black color of tropylium tin iodide is attributed to iodide ligand to tin metal charge transfer, while the bright red-orange color of tropylium lead iodide arises from charge transfer between iodine and tropylium states. Understanding the origins of the observed optoelectronic properties of these two compounds, with respect to their distinct topologies and organic-inorganic interactions, provides insight into the design of tropylium-containing compounds for potential optical and electronic applications.