The metal–insulator transition driven by electronic correlations is one of the most fundamental concepts in condensed matter. In mixed-valence compounds, this transition is often accompanied by charge ordering (CO), resulting in the emergence of complex phases and unusual behaviors. The famous example is the archetypal mixed-valence mineral magnetite, Fe3O4, exhibiting a complex charge-ordering below the Verwey transition, whose nature has been a subject of long-time debates. In our study, using high-resolution X-ray diffraction supplemented by resistance measurements and DFT+DMFT calculations, the electronic, magnetic, and structural properties of recently synthesized mixed-valence Fe4O5 are investigated under pressure to ∼100 GPa. Our calculations, consistent with experiment, reveal that at ambient conditions Fe4O5 is a narrow-gap insulator characterized by the original Verwey-type CO. Under pressure Fe4O5 undergoes a series of electronic and magnetic-state transitions with an unusual compressional behavior above ∼50 GPa. A site-dependent collapse of local magnetic moments is followed by the site-selective insulator-to-metal transition at ∼84 GPa, occurring at the octahedral Fe sites. This phase transition is accompanied by a 2+ to 3+ valence change of the prismatic Fe ions and collapse of CO. We provide a microscopic explanation of the complex charge ordering in Fe4O5 which “unifies” it with the behavior of two archetypal examples of charge- or bond-ordered materials, magnetite and rare-earth nickelates (RNiO3). We find that at low temperatures the Verwey-type CO competes with the “trimeron”/“dimeron” charge ordered states, allowing for pressure/temperature tuning of charge ordering. Summing up the available data, we present the pressure–temperature phase diagram of Fe4O5.

Perovskite-type nanomaterials of the compositions La1-yCayMn1-xB '' O-x(3 +/-delta) with B '' = Ni, Fe; x = 0.2, 0.5 and y = 0.4, 0.25 were prepared using two different preparation routes (synthesis by precipitation and the PVA/sucrose method) at 500 degrees C-700 degrees C. The calcined products of the syntheses were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and physisorption measurements. The materials from the PVA/sucrose method contain particles with diameters from 33 nm to 48 nm, generate specific surface areas up to 33m(2)/g and form pure compared to 45 nm-93 nm and up to 18m(2)/g from precipitation method which contain a significant amount of sodium ions. The agglomeration process was analyzed for one nanomaterial (B '' = Fe, x = 0.2, y = 0.4) from the PVA/sucrose method using temperature dependent XRD showing only a slight growth (4.3%) of nanoparticles at 600 degrees C. The materials from the PVA/sucrose method turned out to be more suitable as electrode materials in electrochemical applications (SOFC, sensors) because of smaller particle sizes, higher specific surface areas and purity. (c) 2018 Elsevier Masson SAS. All rights reserved.

Structural instability has a strong influence on the understanding of superconductivity in iron-containing 122 phases. Similar to the 122 iron-based high-temperature superconductors, the intermetallic compound BaNi2Ge2 undergoes an orthorhombic-to-tetragonal structural phase transition. The compound was prepared by arc-melting mixtures of the elements under an argon atmosphere. Single crystals were obtained by a special heat treatment in a welded tantalum ampule. The crystal structure of the compound was investigated by powder and single-crystal X-ray diffraction. Differential thermal analysis of BaNi2Ge2 showed a reversible phase transition at ca. 480 °C. In situ temperature-dependent synchrotron powder X-ray diffraction studies revealed that below 480 °C the crystal structure of BaNi2Ge2 is orthorhombic [own structure type, space group Pnma, a = 8.3852(4) Å, b = 11.3174(8) Å, and c = 4.2902(9) Å at 30 °C] and the high-temperature phase above 510 °C belongs to the tetragonal ThCr2Si2-type structure [space group I4/mmm, a = 4.2664(1) Å, and c = 11.2537(3) Å at 510 °C]. The reversible first-order low-temperature ↔ high-temperature phase transition around 480 °C is associated with distortion of the [Ni2Ge2] layer of low-temperature modification. The anisotropy of thermal expansion of the unit cell in BaNi2Ge2 was analyzed. The crystal chemistry and chemical bonding are discussed in terms of linear muffin-tin orbital band structure calculations and a topological analysis using the electron localization function. In related compounds, the level of distortion of the uncollapsed tetragonal ThCr2Si2-type structure depends on the valence electron count (VEC).

In situ high-pressure diffraction experiments on single-crystal α-quartz under quasi-hydrostatic conditions up to 19 GPa were performed with diamond-anvil cells. Isotropic pressures were calibrated through the ruby-luminescence technique. A 4:1 methanol–ethanol mixture and the densified noble gases helium and neon were used as pressure media. The compression data revealed no significant influence of the pressure medium at room temperature on the high-pressure behavior of α-quartz. In order to describe its compressibility for use as a pressure standard, a fourth-order Birch–Murnaghan equation of state (EoS) with parameters K T0 = 37.0 (3) GPa, K T0′ = 6.7 (2) and K T0′′ = −0.73 (8) GPa−1 was applied to fit the data set of 99 individual data points. The fit of the axial compressibilities yields M T0 = 104.5 (8) GPa, M T0′ = 13.7 (4), M T0′′ = −1.04 (11) GPa−1 (a axis) and M T0 = 141 (3) GPa, M T0′ = 21 (2), M T0′′ = 8.4 (6) GPa−1 (c axis), confirming the previously reported anisotropy. Assuming an estimated standard deviation of 0.0001% in the quartz volume, an uncertainty of 0.013 GPa can be expected using the new set of EoS parameters to determine the pressure.

At lower mantle conditions, subducted mid oceanic ridge basalts (MORB) will crystallize more than 20 vol% of an aluminum-rich phase, which is referred to generally as the new aluminum (NAL) phase. Given that a significant proportion of the lower mantle may be comprised of subducted crust, the NAL phase may contribute to the bulk elastic properties of the lower mantle. In this study we report for the first time the structure, Raman spectrum and elasticity of single crystals of Na-0.41[Na0.125Mg0.79Al0.085](2)[Al0.79Si0.21]O-12 NAL phase, synthesized at 2260 degrees C and 20 GPa. The single-crystal structure refinement of NAL, which is consistent with the space group P6(3)/m, reveals dynamic disorder of Na atoms along channels within the structure. The elastic tensor was experimentally determined at ambient conditions by Brillouin scattering spectroscopy. The elastic modulii obtained from the VoigtReuss-Hill approximation using the elastic constants determined in this study are K-S = 206 GPa and mu = 129 GPa, whereas the isotropic compressional and shear sound velocities are nu(P) = 9.9 km/s and nu(S) = 5.8 km/s. The NAL phase is elastically anisotropic, displaying 13.9% compressional and shear wave anisotropy. Elastic constants as well as Raman active modes of NAL have also been calculated using density-functional theory and density-functional perturbation theory.

Oxygen surface exchange and oxygen chemical diffusion coefficients of LaNi 0.4 Fe 0.6 O 3− δ ceramics are determined via conductivity relaxation method after stepwise change of temperature in the range of 700–950 °C in air and Ar/O 2 gas flow at oxygen partial pressures ( p_O_2 ) of 4 Pa, 18 Pa, 37 Pa, 47 Pa and 59 Pa. The highest conductivity (about 160 S·cm −1 ) is found at 950 °C in air. No oxygen exchange ( δ = 0) below 700 °C is observed in the investigated p_O_2 range. The oxygen exchange coefficients determined in reduction mode are higher than those determined in oxidation mode. This is explained by clusterization of oxygen vacancies on the surface of the sample investigated in oxidation mode. The opposite tendency is found for chemical diffusion coefficients. Unlike surface, the oxygen vacancies of the volume region are probably not clustered and have predetermined the higher oxygen diffusion mobility of the sample treated in oxidation mode.

We report crystal structure, electronic structure, and magnetism of manganese tetraboride, MnB4, synthesized under high-pressure high-temperature conditions. In contrast to superconducting FeB4 and metallic CrB4, which are both orthorhombic, MnB4 features a monoclinic crystal structure. Its lower symmetry originates from a Peierls distortion of the Mn chains. This distortion nearly opens the gap at the Fermi level, but despite the strong dimerization and the proximity of MnB4 to the insulating state, we find indications for a sizable paramagnetic effective moment of about 1.7 muB/f.u., ferromagnetic spin correlations and, even more surprisingly, a prominent electronic contribution to the specific heat. However, no magnetic order has been observed in standard thermodynamic measurements down to 2 K. Altogether, this renders MnB4 a structurally simple but microscopically enigmatic material; we argue that its properties may be influenced by electronic correlations.

Structural features of Li-extraction from the LiCrMnO4 cathode material in Li-electrochemical cells were investigated by in situ synchrotron powder diffraction and completed by ex situ neutron powder diffraction and magnetization measurements on delithiated products. The observed diffraction patterns reveal that both Li-extraction during the cell charge up to 5.15 V and the following Li-insertion upon the discharge down to 3.2 V vs. Li+/Li are one-Phase solid-solution processes. According to the Rietveld structure refinement Li atoms were completely removed from the tetrahedral 8a site during the cell charge with the C/10 rate. However, only about 40% Li atoms were inserted during the cell discharge with the same rate. The unit cell parameter a and the average metal-oxygen distance (Cr,Mn-O) show non-linear dependence on the lithium content upon extraction, with the slope changing at 4.9 V and manifesting a change in the mechanism of redox behavior. While the first step between 3 V and 4.9 V is presumably accompanied by the oxidation of Cr3+ to Cr4+, the charging above 4.9 V requires a further oxidation of Cr4+ toward Cr6+. The paramagnetic effective Moment of LixCrMnO4 compounds changes non-monotonously with decreasing Li-content. (C) 2013 The Electrochemical Society.

Structural and elastic properties of chalcopyrite-type CuInSe 2 are determined in almost full stability range of temperature from 11 to 1,073 K, by in situ X-ray diffraction, employing a synchrotron-radiation source. The studied polycrystalline sample was prepared from a stoichiometric single crystal. Phase analysis reveals the formation of a trace amount of indium oxide impurity phase at the highest temperatures studied. From the obtained smooth lattice-parameter dependencies on temperature, the temperature dependencies of thermal expansion coefficients are derived. These coefficients are found to follow the trends previously reported for narrow temperature intervals. The present results provide a clear experimental evidence that the linear expansion coefficient is slightly negative below 47 K in both, a and c , directions; this temperature limit is in between the previously reported theoretical value (35 K) and the experimental ones (60 and 80 K) of such limit.

Nominally hydrous high-pressure silicate phases such as the superhydrous phase B are of considerable importance for the understanding of the water-cycle between the surface and the interior of the Earth. This study tackles the controversial issue of hydrogen positions in superhydrous phase B, a phase believed to be potentially stable in cold subducting ultramafic slabs. To investigate the nature of hydrogen incorporation into the structure of superhydrous phase B, neutron powder diffraction experiments have been performed. A structural model based on Pnn2 symmetry has been used for the analysis of the data, which is consistent with earlier spectroscopic studies. Application of Fourier synthesis with subsequent analyses of difference nuclear density maps and Rietveld fits reveal two distinct positions for deuterium, at 4c (0.194, 0.052, 0.596) and at 4c (0.186, 0.119, 0.388). This unambiguously shows that deuterium lies within large channels, which are formed between the edgeshared octahedra and vertex-linked tetrahedra along the b-axis of the structure. These results contrast with recent polarized single-crystal infrared spectroscopy studies where the position of one of two H atoms was estimated to lie close to the octahedral edge of an MgO6 octahedron, thereby leaving the large structural channel empty.

AbstractThe title solid solution (0 ≤ x ≤ 1) is prepared from the elements (Ta bomb, 1323 K, 24 h) and characterized by XANES, and synchrotron and neutron powder diffraction.

We report results of a powder x-ray diffraction (XRD) study of vanadium sesquioxide, V2O3, under pressurization in a neon pressure-transmitting medium up to 57 GPa. We have established a bulk modulus value for corundum-type V2O3 of B0 = 150 GPa at B′ = 4. This bulk modulus value is the lowest among those known for the corundum-type-structured oxides, e.g. Al2O3, α-Fe2O3, Cr2O3, Ti2O3, and α-Ga2O3. We have proposed that this might be related to the difference in the electronic band structures: at room temperature V2O3 is metallic, but the above corundum-structured sesquioxides are semiconducting or insulating. Around ∼21–27 and ∼50 GPa we registered changes in the XRD patterns that might be addressed to phase transitions. These transitions were sluggish upon room-temperature compression, and hence we additionally facilitated them by the laser heating of one sample. We have refined the XRD patterns of only the first high-pressure phase in an orthorhombic lattice of a Rh2O3(II)-type. Our findings significantly extend the knowledge of the P–T phase diagram of V2O3 and advance the understanding of its properties. We speculate that the elastic properties of V2O3 can be closely linked to its electronic band structure and, consequently, we propose that slightly doped V2O3 (e.g. with Cr) could be a potential candidate for systems in which the bulk modulus value may be remarkably switched by moderate pressure or temperature.

The latest developments in single-crystal X-ray diffraction at high pressure and high temperature are described. Advances in diamond anvil cell designs and X-ray sources allow collecting single-crystal diffraction data at pressures up and above 100 GPa and at temperatures above 1000°C. The technical details of single-crystal X-ray diffraction at high pressure such as the choice of pressure-transmitting media or the different methods for measuring pressures and temperatures have been reviewed. Examples of structural solution of complex structures and new materials, structural refinements of high pressure polymorphs as well as accurate compressibility data are described in order to outline the several advantages of using single crystals instead of powdered samples in high pressure diffraction experiments.

The solid solution Yb(x)Ca(1-x)C2 (0 ≤ x ≤ 1) was synthesized by reaction of the elements at 1323 K. The crystal structures within this solid solution, as elucidated from synchrotron powder diffraction data, depend on x and exhibit some interesting features that point to a structure dependent valence state of Yb. Compounds with x ≥ 0.75 crystallize in the tetragonal CaC2 type structure (I4/mmm, Z = 2) and obey Vegard's law; for x ≤ 0.75 the monoclinic ThC2 type structure (C2/c, Z = 4) is found, which coexists with the monoclinic CaC2-III type structure (C2/m, Z = 4) for x ≤ 0.25. The monoclinic modifications show a strong deviation from Vegard's law. Their unit cell volumes are remarkably larger than expected for a typical Vegard system. HERFD-XANES spectroscopic investigations reveal that different Yb valence states are responsible for the observed volume anomalies. While all tetragonal compounds contain mixed-valent Yb with ∼75% Yb(3+) (similar to pure YbC2), all monoclinic modifications contain exclusively Yb(2+). Therefore, Yb(x)Ca(1-x)C2 is a very rare example of a Yb containing compound showing a strong structure dependence of the Yb valence state. Moreover, temperature dependent synchrotron powder diffraction, neutron TOF powder diffraction, and HERFD-XANES spectroscopy experiments reveal significant Yb valence changes in some compounds of the Yb(x)Ca(1-x)C2 series that are induced by temperature dependent phase transitions. Transitions from the tetragonal CaC2 type structure to the monoclinic ThC2 or the cubic CaC2-IV type structure (Fm3m, Z = 4) are accompanied by drastic changes of the mean Yb valence from ∼2.70 to 2.0 in compounds with x = 0.75 and x = 0.91. Finally, the determination of lattice strain arising inside the modifications with ordered dumbbells (ThC2 and CaC2 type structures) by DSC measurements corroborated our results concerning the close relationship between crystal structure and Yb valence in the solid solution Yb(x)Ca(1-x)C2.

We have performed a powder X-ray diffraction (XRD) study of vanadium sesquioxide, V2O3 under pressurization up to 57 GPa. For in-situ high-pressure XRD studies we employed a piston-cylindertype diamond anvil cell with a culet size of 250 m with a rhenium gasket. We loaded compacted powder platelet of V2O3 of ~ 15 m diameter into a hole of ~ 100 m diameter in the rhenium gasket preindented to ~ 40 m. We used neon as a pressure-transmitting medium, and loaded the cell using a gas-loading apparatus at BGI [1]. The XRD experiments were carried out at the PETRA III beamline P02.2 at DESY (Hamburg) using a wavelength of  = 0.29135 Å [2]. Pressure values were determined by the shift of the ruby luminescence line. Laser heating of the sample to high temperatures of 1500-2200 K was accomplished in-situ during 5-15 minutes using the doubleside off-axis laser-heating system at the beamline. The data were collected by a PerkinElmer XRD1621 detector and then the 2D X-ray images were integrated using Fit2D program. For the XRD experiments we selected briquette-like samples of V2O3 of 99.99% purity (Aldrich Company) and characterized them at ambient conditions using our in-house high-brilliance Rigaku diffractometer ( = 0.7108 Å).

Crystal structure,thermal expansion, oxygen non-stoichiometry, electrical conductivity and diffusion characteristics of two analogous LaFe0.7Ni0.3O3‑d and PrFe0.7Ni0.3O3‑d compositions were investigated depending on temperature (201000 °C) and oxygen partial pressure (0.6–21000 Pa). The found oxygen diffusion and oxygen exchange coefficients for the both compositions at similar conditions are near to each other and varied in the range of 110‑7110‑5 cm2s‑1 and 510‑6110‑4 cms‑1, respectively.

Primary pressure determinations involve the measurement of pressure without recourse to secondary standard materials. These measurements are essential for ensuring the accuracy of pressures measured in gasketed high-pressure devices. In this study, the wavelength of optical fluorescence bands and the density of single crystal Sm-doped yttrium aluminum garnet Y3Al5O12 (Sm:YAG) have been calibrated as a primary pressure scale up to 58GPa. Absolute pressures were obtained by integrating the bulk modulus determined via Brillouin spectroscopy with respect to volumes measured simultaneously by X-ray diffraction. A third-order Birch-Murnaghan equation of state of Sm:YAG yields V-0=1735.15(26)angstrom(3), K-T0=185(1.5)GPa, and K`=4.18(5). The accompanied pressure-induced shifts of the fluorescence lines Y1 and Y2 of Sm:YAG were calibrated to the primary pressure, thus creating a highly accurate fluorescence pressure scale. These shifts are described as P=(A/B)*{[1+(/(0))](B)-1} with A=2089.91(23.04), B=-4.43(1.07) for Y1, and A=2578.22(48.70), B=-15.38(1.62) for Y2 bands, here =-(0), and (0) are wavelengths in nanometer at pressure and ambient conditions. The sensitivity in the pressure determination of the Sm:YAG fluorescence shift is 0.32nm/GPa, which is identical to that of the ruby scale. Sm:YAG can be considered elastically isotropic up to 58GPa, implying insensitivity of the determined pressure to the crystallographic orientation under nonhydrostatic or quasi-hydrostatic conditions. The Sm:YAG fluorescence shift is apparently also independent of crystallographic orientation, in contrast to that of ruby. Since the Y fluorescence band of Sm:YAG is insensitive to temperature changes, this material is highly suitable for the measurement of pressure at elevated temperatures.