Synchrotron X-ray Diffraction Data Research Articles

High-Pressure Structural Evolution of Disordered Polymeric CS2.

Carbon disulfide is an archetypal double-bonded molecule belonging to the class of group IV-group VI, AB2 compounds. It is widely believed that, upon compression to several GPa at room temperature and above, a polymeric chain of type (-(C═S)-S-)n, named Bridgman's black polymer, will form. By combining optical spectroscopy and synchrotron X-ray diffraction data with ab initio simulations, we demonstrate that the structure of this polymer is different. Solid molecular CS2 polymerizes at ∼10-11 GPa. The polymer is disordered and consists of a mixture of 3-fold (C3) and 4-fold (C4) coordinated carbon atoms with some C═C double bonds. The C4/C3 ratio continuously increases upon further compression to 40 GPa. Upon decompression, structural changes are partially reverted, while the sample also undergoes partial disproportionation. Our work uncovers the nontrivial high-pressure structural evolution in one of the simplest molecular systems exhibiting molecular as well as polymeric phases.

Phase Analysis of Australian Uranium Ore Concentrates Determined by Variable Temperature Synchrotron Powder X-ray Diffraction.

The chemical speciation of uranium oxides is sensitive to the provenance of the samples and their storage conditions. Here, we use diffraction methods to characterize the phases found in three aged (>10 years) uranium ore concentrates of different origins as well as in situ analysis of the thermally induced structural transitions of these materials. The structures of the crystalline phases found in the three samples have been refined, using high-resolution synchrotron X-ray diffraction data. Rietveld analysis of the samples from the Olympic Dam and Ranger uranium mines has revealed the presence of crystalline α-UO2(OH)2, together with metaschoepite (UO2)4O(OH)6·5H2O, in the aged U3O8 samples, and it is speculated that this forms as a consequence of the corrosion of U3O8 in the presence of metaschoepite. The third sample, from the Beverley uranium mine, contains the peroxide [UO2(η2-O2)(H2O)2] (metastudtite) together with α-UO2(OH)2 and metaschoepite. A core-shell model is proposed to account for the broadening of the diffraction peaks of the U3O8 evident in the samples.

Xuite, Ca3Fe2[(Al,Fe)O3(OH)]3, a new mineral of the garnet group: Implications for the wide occurrence of nanominerals

Abstract Xuite, Ca3Fe2[(Al,Fe)O3(OH)]3, is a new member of the garnet supergroup discovered in basaltic scoria from Menan Volcanic Complex, Idaho, U.S.A. Oxidation of Fe-bearing pyroclasts at high temperatures led to the formation of xuite, together with luogufengite, valleyite, and hematite. The measured crystal size of xuite ranges from ~200 to 800 nm. The empirical chemical formula of xuite is (Ca0.92Mg0.08)3(Fe0.96Ti0.04)2[(AlO4H)0.44(FeO4H)0.33 (SiO4)0.05(☐O4H4)0.18]3. Xuite has a space group of Ia3d; its unit-cell parameter refined from high-resolution synchrotron X-ray diffraction (XRD) data is a = 12.5056(5) Å, with Z = 8 (calculated density = 3.53 g/cm3). Fourier-transform infrared spectroscopy spectrum of xuite shows absorption bands at 3682 and 3579 cm−1, indicating the presence of OH– in the hydrogarnet structure. In situ high-temperature synchrotron XRD combined with thermogravimetry and differential scanning calorimetry reveals that xuite undergoes dehydroxylation to form brownmillerite (Ca2FeAlO5) from ~236 to ~396 °C. Xuite occurs in the form of nanocrystals with a soft magnetic property, which provides important insights into the origin of basaltic scoria and associated paleomagnetism. Xuite was also found in Wyoming paralava, suggesting the possibility of its wide occurrence in various geological environments. The mineral was named after Huifang Xu and Hongwu Xu in honor of their sustained contributions to mineral sciences.

High-Pressure Synthesis and Magnetic States of Magnetoplumbite Cobaltates CaCo12O19 and BaCo12O19.

Novel cobalt oxides, CaCo12O19 and BaCo12O19, have been synthesized under high-pressure and high-temperature conditions of 7 GPa and 1373 K, respectively. Rietveld refinement using synchrotron X-ray diffraction data indicates that the CaCo12O19 and BaCo12O19 crystallize in a magnetoplumbite structure with a hexagonal space group of P63/mmc (No. 194) as well as SrCo12O19. The magnetic study demonstrates that itinerant and localized 3d electrons coexist in all ACo12O19 (A = Ca, Sr, Ba) and the magnetic ground state transforms from antiferromagnetic (A = Ca) to ferrimagnetic (A = Sr) to antiferromagnetic (A = Ba), which is in stark contrast to the systematic change in the magnetoplumbite-related cobalt oxides of ACo6O11 from antiferromagnet (A = Ca) to ferrimagnet (A = Sr) to ferromagnet (A = Ba). The nonmonotonic magnetic evolution with isoelectronic A-site substitution in ACo12O19 is probably attributed to changes in the interactions between two magnetic sublattices of localized 3d electrons at trigonal-bipyramidal and tetrahedral sites for ACo12O19. This finding proposes the complex magnetic properties in the layered cobalt oxides with multiple magnetic sublattices in the coexistence system of itinerant and localized electrons.

Near room-temperature formation of Cu3Sn: In-situ synchrotron X-ray diffraction characterization and thermodynamic assessments of its nucleation

The fabrication of Cu3Sn at near room temperatures and atmospheric pressure remains out of reach. We propose a novel concept to address this challenge by exploiting both the classical nano-island model for dealloying and the nanocurvature effect on nucleation, in conjunction with bespoke precursor alloy design. In-situ synchrotron X-ray diffraction (XRD) confirmed significant formation of Cu3Sn at both 70°C and 55°C in 180 min via a dealloying-realloying process in the designed precursor alloy Al67Cu18Sn15, compared with much slower formation at 125°C by conventional approaches. Furthermore, Cu3Sn formed just 10 min after Cu6Sn5 emerged at 70°C, being two orders of magnitude faster than in Cu/Cu6Sn5 diffusion couples at 125°C. The energy barrier to the formation of a lenticular Cu3Sn nucleus on Cu6Sn5 nanocaps was assessed systematically using the CALPHAD approach through the initial driving force (DF) concept and DF-plateau method. The predictions were benchmarked against diffusion couple studies, which fully supported the experimental results. In addition, the in-situ synchrotron XRD data on phase evolution was analysed in detail using the well-established Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, which provided further fundamental support to our conceptual design. The concept and methodology demonstrated are expected to be applicable to the fabrication of other challenging materials.

Distorted octahedra induced anisotropic strain and local disorder in delafossite CuCrO2

Delafossites (ABO2) are of great interest since they exhibit interesting structural and magnetic transitions. Edge shared geometry of octahedra in delafossites gives rise to distortion in octahedra. These distortions can influence the structural and physical properties of these materials. In the present study, the temperature dependence of structural changes related to CrO6 octahedra in CuCrO2 (a member of the delafossite family) has been investigated. Rietveld analysis of room temperature XRD data indicates that anisotropic strain, along with preferred orientation, is present in the sample. At room temperature, the CrO6 octahedra were found distorted. Careful analysis of temperature-dependent (300–610K) synchrotron X-ray diffraction data indicates that the degree of distortion of octahedra decreases with increasing temperature. Bond length Cu–O shows negative thermal expansion, which was driven by the reduction in the distortion of octahedra with increasing temperature. The existence of distorted octahedra has been proposed to be the reason for breaking the local symmetry, which is anticipated to be responsible for the presence of experimentally observed forbidden modes in the Raman spectra.

Microstructural evolution of proton irradiated Fe-2.25Cr–1Mo characterized using synchrotron XRD (SXRD)

Synchrotron X-ray diffraction data (SXRD) has been used to characterize the microstructural changes in 3.5 MeV proton irradiated 2.25Cr–1Mo (T22) steel samples as a function of dose through different techniques of X-ray diffraction line profile analysis (XRDLPA). Williamson-Hall (W–H) method, modified Rietveld method and convolutional multiple whole profile (CMWP) fitting technique have been used to evaluate the coherent domain size, r.m.s microstrain, dislocation density and dislocation character from SXRD data. It is demonstrated that XRDLPA from SXRD data is a powerful tool for extracting the microstructural parameters of irradiated samples in a reliable manner. Transmission Electron Microscopy (TEM) investigation has been carried out on unirradiated and irradiated samples to corroborate the results of XRDLPA. It is observed that dislocation density increases with dose and forms dislocation loops at highest dose. Vickers microhardness measurement has also been carried out to get the information on the mechanical property changes with dose. The variation in Vickers microhardness as a function of dose has been explained using the results from XRDLPA and TEM analysis.

Line profile analysis of synchrotron X-ray diffraction data of iron powder with bimodal microstructural profile parameters

Room-temperature synchrotron X-ray diffraction and subsequent detailed line profile analysis of Fe powder were performed for microstructural characterization. The peak shapes of the diffraction pattern of Fe were found to be super-Lorentzian in nature and the peak widths were anisotropically broadened. These peak profile features of the diffraction pattern are related to the microstructural parameters of the material. In order to elucidate these features of the diffraction pattern, detailed line (peak) profile analyses were performed using the Rietveld method, modified Williamson–Hall plots and whole powder pattern modelling (WPPM), and related microstructural parameters were determined. Profile fitting using the Rietveld and WPPM methods with a single microstructural (unimodal) model shows systematic deviation from the experimentally observed diffraction pattern. On the basis of Rietveld analysis and microstructural modelling it is revealed that the microstructure of Fe consists of two components (bimodal profile). The microstructural parameters of crystallite/domain size distribution, dislocation density, nature of dislocations and phase fraction were evaluated for both components. The results obtained using different methods are compared, and it is shown that diffraction peak profile analysis is capable of modelling such inhomogeneous bimodal microstructures.

Crystal Structure of Moganite and Its Anisotropic Atomic Displacement Parameters Determined by Synchrotron X-ray Diffraction and X-ray/Neutron Pair Distribution Function Analyses

The crystal structure of moganite from the Mogán formation on Gran Canaria has been re-investigated using high-resolution synchrotron X-ray diffraction (XRD) and X-ray/neutron pair distribution function (PDF) analyses. Our study for the first time reports the anisotropic atomic displacement parameters (ADPs) of a natural moganite. Rietveld analysis of synchrotron XRD data determined the crystal structure of moganite with the space group I2/a. The refined unit-cell parameters are a = 8.7363(8), b = 4.8688(5), c = 10.7203(9) Å, and β = 90.212(4)°. The ADPs of Si and O in moganite were obtained from X-ray and neutron PDF analyses. The shapes and orientations of the anisotropic ellipsoids determined from X-ray and neutron measurements are similar. The anisotropic ellipsoids for O extend along planes perpendicular to the Si-Si axis of corner-sharing SiO4 tetrahedra, suggesting precession-like movement. Neutron PDF result confirms the occurrence of OH over some of the tetrahedral sites. We postulate that moganite nanomineral is stable with respect to quartz in hypersaline water. The ADPs of moganite show a similar trend as those of quartz determined by single-crystal XRD. In short, the combined methods can provide high-quality structural parameters of moganite nanomineral, including its ADPs and extra OH position at the surface. This approach can be used as an alternative means for solving the structures of crystals that are not large enough for single-crystal XRD measurements, such as fine-grained and nanocrystalline minerals formed in various geological environments.

Strongly reduced lattice thermal conductivity in Sn-doped rare-earth (M) filled skutterudites M x Co4Sb12-y Sn y , promoted by Sb-Sn disordering and phase segregation.

CoSb3 thermoelectric skutterudite has been filled with rare-earth metals (M = La, Ce, Yb) and partially doped with Sn in specimens of MxCo4Sb12−ySny stoichiometry. This has been achieved under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. A structural investigation using synchrotron X-ray diffraction data reveals a phase segregation in twin skutterudite phases with filling fraction fluctuation and different unit-cell sizes. As a result of three effects acting as phonon scatterers, namely the rattling effect of M at the wide 8a cages of the cubic Im3̄ structure, the phase segregation, and the intrinsic disorder introduced by Sn substitution at the Sb sublattice, the total thermal conductivity (κ) dramatically falls to reach minimum values under 2 W m−1 K−1, well below those typically exhibited by other thermoelectric materials based upon single-filled skutterudites. The power factor is substantially enhanced to 1.11 mW m−1 K−2 in Yb0.5Co4Sb11.6Sn0.4 with respect to the unfilled composition, as a result of the charge transfer promoted by the filler.

Impact of Nb Substitution on Crystal Structures and Electrode Reversibility of LiNiO2

Introduction Lithium-ion batteries with high energy density have been currently used for transportation applications such as electric vehicles. As a positive electrode material, layered LiNiO2 shows high reversible capacity, good rate characteristics, and low material cost when compared to LiCoO2. However, a complicated phase transitions during charge and discharge proceed for LiNiO2, which influences long-term cyclability.[1] In this study, impact of Nb substitution on crystal structures and electrode reversibility of LiNiO2 is examined, and its origin of improved reversibility as electrode materials is discussed in detail. Experimental Nb-substituted LiNiO2 samples were synthesized from LiOH・H2O, Ni (OH)2, and Nb2O5 used as starting materials, and mixtures were calcined at 650 oC. Crystal structures of the samples were analyzed by synchrotron X-ray diffraction (XRD), and the particle morphology was observed by using a scanning electron microscope (SEM). Results and Discussion Synchrotron XRD data were collected for both LiNiO2 with or without Nb substitution. as shown in Figure 1. Although Nb substitution (5 mol% of Nb) for LiNiO2 clearly results in lowered crystallinity, successful synthesis of a rocksalt layered oxide, which is isostructural with LiNiO2, is noted from synchrotron XRD data.Figure 2 shows results of electrochemical properties of LiNiO2 with or without Nb substitution. Although LiNiO2 without Nb ions delivers an initial discharge capacity of over 200 mA h g-1, the increase in polarization and reduction of reversible capacities are observed on continuous electrochemical cycles. On the other hand, the increase in polarization was effectively suppressed for the Nb-substituted sample, leading to good reversibility.Based on these results, we will discuss factors affecting electrode reversibility of LiNiO2 and the possibility of cobalt-free high-capacity and long-cycle life positive electrode materials. Reference [1] U.-H.Kim et.al., and Y.-K.Sun, Energy Environ. Sci., 11, 1271 (2018). Figure 1

ϒ-Li3PO4-Type Solid Electrolytes in Li4GeO4-Li4SiO4-Li3VO4 Quasi-Ternary System with High Lithium Ionic Conductivity

Lithium superionic conductors (LISICONs) are promising materials to realize high safety, high energy all-solid-state lithium-ion batteries. Through a dual-cation-substitution strategy, we designed ternary solid solutions of Ge-Si-V derivatives with a γ-Li3PO4-type structure in the Li4GeO4-Li4SiO4-Li3VO4 quasi-ternary system, and the relationship among their compositions, phase formation, and ionic conductivity was analyzed. The samples were fabricated through a solid-state reaction at 1173 K. The crystalline phase of the samples was identified through X-ray diffraction (XRD). The crystal structure was elucidated with Rietveld refinement of Synchrotron X-ray diffraction data. The morphologies and elemental distributions of the synthesized samples were examined through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The ionic conductivities of the synthesized samples at various temperatures were determined by AC impedance spectroscopy. An all-solid-state battery using the discovered LISICON material was fabricated in an Ar-filled glovebox. The positive and negative electrodes were composed of a mixture of LiNbO3-coated LiCoO2 with Li10GeP2S12 and Li-In metal, respectively. A charge-discharge test was conducted between 1.9 and 3.6 V at an applied current of 0.03 mA cm−1 (0.05 C rate) at 373 K. This study determined the phase-formation regions in the Li4GeO4-Li4SiO4-Li3VO4 quasi-ternary system and demonstrated relatively high ionic conductivity of the γ-Li3PO4-type phase structures.A high conductivity with low activation energy was achieved in ternary compositions compared to that in any binary system. The bulk ionic conductivity of 5.8 × 10−5 S cm−1 at 298 K was obtained for Li3.55(Ge0.45Si0.10V0.45)O4 with a low activation energy of 0.37 eV. Li3.55(Ge0.45Si0.10V0.45)O4 maintained high ionic conduction property (~10−5 S cm−1) even for the total conductivity (sum of bulk and grain-boundary contributions). Ionic conductivity of the Li4GeO4-Li4SiO4-Li3VO4 system was visualized along with the phase-identification results, as shown in the provided figure. In this color contour map (Fig. 1), relatively high ionic conductivity is confirmed around the γ-Li3PO4-type phase area (the region around sample #9); Li3VO4, Li4SiO4, and Li4GeO4 solid solutions or their mixture exhibited low ionic conductivity. Compared to any binary composition system (i.e., Li-X-Y-O), the existing tie line between the corner of this triangular ternary composition system (i.e., Li-X-Y-Z-O) has a higher ionic conductivity. Particularly, the ionic conductivity of Li3.55(Ge0.45Si0.10V0.45)O4 at #9 is slightly higher than that of Li3.6(Ge0.6V0.4)O4, which is the composition showing the highest ionic conductivity reported to date of LISICONs in the binary systems. Besides, the activation energy of Li3.55(Ge0.45Si0.10V0.45)O4 was much lower than that of Li3.6(Ge0.6V0.4)O4. The activation energy decreased by ~17.8% compared with that of the Li3.6(Ge0.6V0.4)O4 (0.45 eV). Therefore, the novel material-search concept (more complex composition) is one of the positive directions for developing the LISICON-type ionic conductors. The absolute value of 5.8 × 10−5 S cm−1 exceeded that of the reported ternary material in the Li4GeO4-Li4PO4-Li3VO4 system (5.1 × 10−5 S cm−1) (ACS Appl. Energy Mater. 2 (2019) 6608–6615). In other words, the ionic conductivity of the discovered material is higher than that of all the practically existing LISICONs, including the ternary systems. Further optimization in the ternary system or a more complex composition system could be a promising way for the subsequent material search.The high ionic conductivity property (σ) was achieved by reducing the activation energy (E a) and increasing the pre-exponential factor (σ0) through dual doping, which determines the σ value as described in the Arrhenius equation: σT = σ0exp(−E a/kBT). Li3.55(Ge0.45Si0.10V0.45)O4 functioned as a solid electrolyte in an all-solid-state cell, indicating the features of a pure lithium ionic conductor without electronic conduction. The LISICON oxide prepared via a cold-press functioned as a solid-electrolyte separator similar to sulfide solid electrolytes in all-solid-state batteries without high-temperature sintering. This demonstration could be the first step to developing room-temperature-LISICONs-based all-solid-state lithium ionic batteries. This work verified that the dual-cation-substitution strategy is beneficial for compositional and structural optimization, affording enhanced ion-conducting properties. Additionally, the substitution of more kinds of cations with larger radii into LISICONs is proposed; further enhancement in ionic conductivity could be expected in the case of the LISICON with a larger lattice size. Ternary LISICON systems using cations with larger ionic radii (e.g., Al3+, Ga3+) are an attractive candidate for future research. Figure 1

Influence of Cr and Fe doping on the structure, magnetic and optical properties of nano CuCo2O4

Nano CuCo2-xMxO4 (x = 0, 0.1, 0.2, M = Cr or Fe) samples were synthesized by hydrothermal method. Synchrotron x-ray diffraction data obtained for the samples were subjected to phase analysis and manifested a single-phase cubic spinel structure for Cr-doped samples, while for Fe-doped samples two phases were identified. Cation distribution and cell parameter (a) were obtained from Rietveld X-ray diffraction analysis. FTIR analysis affirmed the formation of the cubic spinel and the cation distribution obtained. The nano nature of the samples and the particle morphology were examined by high-resolution transmission electron microscope (HRTEM) with selected area electron diffraction (SAED). UV-diffuse reflectance revealed that all samples have two optical energy gaps. For all Fe doped samples, the optical band gaps decreased, while for Cr-content x = 0.1 the bandgaps increased then reduced for x = 0.2. Doped samples exhibited a blue or red shift depending on the kind and amount of the dopant ions. The PL intensity and the emitted colors depended on the kind and amount of the dopant ions. Magnetic measurements disclosed the paramagnetic nature of CuCo2O4, while a weak ferromagnetic is revealed for CuCo2-xCrxO4 and a ferromagnetic nature for CuCo2-xFexO4. Lowering the bandgap upon doping could make better mobility of lattice oxygen and enhancing the catalyst reducibility. Thus, the Cr and Fe-doped samples are expected to have better catalytic activity than the pristine one.

High Pressure Effect on Structural and Electrochemical Properties of Anionic Redox-Based Lithium Transition Metal Oxides

Summary The richness of anionic redox chemistry in the solid state offers new opportunities and a possible paradigm shift in energy storage. The excess capacity that goes beyond conventional theoretical values is attributed to the anionic redox in Li-rich transition metal oxide cathodes for Li-ion batteries. Their electrochemical behavior is thermodynamically determined by structural evolution. To better understand the electrochemical dependence on structural factors, we have induced structural modifications in pristine and electrochemically activated Li1.144Ni0.136Co0.136Mn0.544O2 through high-pressure treatment. A unique cyclical change of structural reordering is observed in the anionic redox-activated material during operando pressure sweep, characterized by a periodic evolution of superlattice peak intensity in synchrotron X-ray diffraction patterns. During the structural reordering period, the bulk compressibility of the material decreases, even becoming negative. These insights elucidate the structural flexibility and metastability of anionic redox-based materials, which can undergo large compressions and structural modifications while delivering good electrochemical properties.

Charge density of difluorides from synchrotron diffraction data and investigation of bonding in low valent binary fluorides

The experimental charge density analysis of two isostructural transition metal difluorides, CoF2 and NiF2 has been performed using single crystal synchrotron X-ray diffraction data. The multipole and MEM models of electron density and the thermal vibration parameters suggest that the difluoride of nickel is a hard molecule with more shared shell interaction nature than the difluoride of cobalt. In the course of study, a comparative study on fluorine bonding in low valent binary fluorides with metal cations of oxidation states, ranging from +1 to +3 have been accomplished and thoroughly analysed. The explored topological features derived from the Bader's theory of Atoms In Molecules (AIM) approach reveals the existence of ionic nature of bonding at +1 valence state of metal, as viewed in LiF and NaF and as the oxidation on metals ascends, intermediate interaction with increase in covalent nature is evidenced.

Structure, thermal stability, optoelectronic and electrophysical properties of Mg- and Na-codoped bismuth niobate pyrochlores: Experimental and theoretical study

Novel bismuth niobates Bi1.5Mg1–xNaxNb1.5O7–δ and Bi1.5Mg0.9–xNaxNb1.5O7–δ (0 ≤ x ≤ 0.50) with the pyrochlore structure have been investigated experimentally and theoretically. The compounds were synthesized by an organic-inorganic precursors combustion method. The pyrochlore structure was formed at x = 0.10; 0.25 for cation completed and 0.25; 0.40 for cation deficient compositions, respectively. From differential scanning calorimetry (DSC) and thermogravimetry (TG), the thermal stability of the compounds up to their melting points (1250–1270 °C) was concluded. The single-crystal forms of the pyrochlores can be easily obtained from the melts. Structural characterization was performed using a Rietveld refinement against X-ray diffraction (XRD) as well as the simultaneous refinement using high resolution neutron powder diffraction (NPD) and synchrotron X-ray diffraction (SXRD) data. According to the results, the displacements of A (from 16c to 96g) and O′ (from 8a to 32e or 96g) atoms from ideal pyrochlore sites as well as the Na atoms distribution in the bismuth sub-structure, and the Mg atoms in the niobium sub-structure were determined. The same distribution of dopants in the structure was proven by DFT-PBE simulations of crystal structure and thermodynamic properties. The most preferable (Bi1.5Mg0.5)(Nb1.5Mg0.5)O7 and (Bi1.5Na0.5)(Nb1.5Mg0.5)O7 models were predicted. Both calculated/experimental direct band gap values were found to decrease from Mg-doping (3.20/3.21 eV) to Mg, Na-codoping (3.16/3.15 eV). The electrical properties were investigated by impedance spectroscopy method in air, in oxygen, and humid atmospheres in the temperature range of 25–750 °C. The typical dielectric behavior of the compounds up to 200 °C was detected. With the Na content rising the increasing of ε′ values from 86 (x = 0) to 145 (x = 0.4) was revealed at the same tanδ ≈ 0.0025 (1 MHz, 25 °C). Calculated temperature coefficients of capacitance (TCC) values varied from −594 to −724 ppm/°C (25–280 °C). Besides, contribution from electronic conductivity (T < 360 °C), proton transport below 360–400 °C as well as the ionic (oxygen) conductivity at T > 400 °C were observed.

Crystal structure of monoclinic hafnia (HfO2) revisited with synchrotron X-ray, neutron diffraction and first-principles calculations.

A study on the crystal structure of monoclinic HfO2 has been performed using synchrotron X-ray and neutron diffraction data separately, as well as a combination of both. The precision of the structural parameters increases significantly due to application of the neutron diffraction technique. The experimental oxygen positions in HfO2, derived precisely, are visualized only by semi-local density functional calculations in terms of the calculated electronic band gap, but are not captured as accurately by using hybrid functionals.

Quantification of the Magnetic Anisotropy of a Single-Molecule Magnet from the Experimental Electron Density.

Reported here is an entirely new application of experimental electron density (EED) in the study of magnetic anisotropy of single-molecule magnets (SMMs). Among those SMMs based on one single transition metal, tetrahedral CoII-complexes are prominent, and their large zero-field splitting arises exclusively from coupling between the d and dxy orbitals. Using very low temperature single-crystal synchrotron X-ray diffraction data, an accurate electron density (ED) was obtained for a prototypical SMM, and the experimental d-orbital populations were used to quantify the dxy-d coupling, which simultaneously provides the composition of the ground-state Kramers doublet wave function. Based on this experimentally determined wave function, an energy barrier for magnetic relaxation in the range 193-268 cm-1 was calculated, and is in full accordance with the previously published value of 230 cm-1 obtained from near-infrared spectroscopy. These results provide the first clear and direct link between ED and molecular magnetic properties.

Detecting the Guttman effect with the help of ordinal correspondence analysis in synchrotron X-ray diffraction data analysis

ABSTRACT We propose a method for detecting a Guttman effect in a complete disjunctive table U with Q questions. Since such an investigation is a nonsense when the Q variables are independent, we reuse a previous unpublished work about the chi-squared independence test for Burt's tables. Then, we introduce a two-steps method consisting in plugging the first singular vector from a preliminary Correspondence Analysis (CA) of U as a score x into a subsequent singly-ordered Ordinal Correspondence Analysis (OCA) of U . OCA mainly consists in completing x by a sequence of orthogonal polynomials superseding the classical factors of CA. As a consequence, in presence of a pure Guttman effect, we should in principle have that the second singular vector coincide with the polynomial of degree 2, etc. The hybrid decomposition of the Pearson chi-squared statistics (resulting from OCA) used in association with permutation tests makes possible to reveal such relationships, i.e. the presence of a Guttman effect in the structure of U , and to determine its degree - with an accuracy depending on the signal to noise ratio. The proposed method is successively tested on artificial data (more or less noisy), a well-known benchmark, and synchrotron X-ray diffraction data of soil samples.

Experimental and theoretical study of dense YBO3 and the influence of non-hydrostaticity

YBO3 is used in photonics applications as a host for red phosphors due to its desirable chemical stability, high quantum efficiency and luminescence intensity. Despite its fundamental thermodynamic nature, the isothermal bulk modulus of YBO3 has remained a contentious issue due to a lack of comprehensive experimental and theoretical data and its vibrational modes are far from being understood. Here, we present an experimental-theoretical structural and vibrational study of YBO3. From structural data obtained from synchrotron X-ray diffraction data and ab initio calculations, we have determined the YBO3 bulk modulus, isothermal compressibility tensor and pressure-volume (P-V) equation of state (EoS). The isothermal compressibility tensor reveals that the compressibility of YBO3 is highly anisotropic, with the principal compression axis lying perpendicular to the ab-plane being approximately twice as stiff as the two axes perpendicular to it. From the vibrational data obtained from Raman scattering measurements and ab initio calculations, the experimental and calculated pressure response of the YBO3 Raman modes is also determined with the corresponding Grüneisen parameters and the symmetry of the experimental modes has been tentatively assigned and discussed. No evidence for a pressure-induced phase transition in YBO3 is observed up to 27 GPa, however we note that an apparent discontinuity in the compressibility at 8 GPa, likely due to the onset of non-hydrostaticity, could lead to the misinterpretation of an atypically high bulk modulus.