High-pressure in-situ X-ray diffraction and Raman spectroscopy of Ca2AlFeO5 brownmillerite

The behavior of Ca₂AlFeO₅ brownmillerite was studied by in situ synchrotron X-ray diffraction and Raman spectroscopy at 300?K with pressures up to 26.5 and 32.1 GPa, respectively. A reversible structural phase transition was observed. The P–V data were fitted by a third-order Birch–Murnaghan equation of state, and the isothermal bulk modulus was obtained as K₀?=?181.9(76) GPa with K₀′_?=?4.4(17). If K₀′ was fixed to 4, K₀ was obtained as 183.8(20) GPa. Ca₂AlFeO₅ brownmillerite shows an axial elastic anisotropy since the b-axis is more compressible than a- and c-axis. Combined with previous results, the isothermal bulk modulus and axial compressibility of Ca₂AlFeO₅ brownmillerite increase with more Al incorporated in the structure. The Raman spectra of Ca₂AlFeO₅ brownmillerite were analyzed and the pressure coefficients vary from 2.23 to 4.90?cm^?1/GPa. The isothermal mode Grüneisen parameters range from 0.83 to 1.77 and the thermal Grüneisen parameter is determined as 1.08(11).     

High pressure X-ray diffraction study on BaWO4-II

BaWO₄-II has been synthesized at 5 GPa and 610°C. Its high pressure behavior was studied by in situ synchrotron X-ray diffraction measurements at room temperature up to 17 GPa. BaWO₄-II retains its monoclinic structure. Bulk and axial moduli determined by fitting a third-order Birch–Murnaghan equation of state to lattice parameters are: K ₀=86.2±1.9 GPa, K ₀(a)=56.0±0.9 GPa, K ₀(b)=85.3±2.4 GPa, and K ₀(c)=146.1±3.2 GPa with a fixed K′=4. Analysis of axial compressible modulus shows that the a-axis is 2.61 times more compressible than the c-axis and 1.71 times more compressible than the b-axis. The beta angle decreases smoothly between room pressure and 17 GPa from 93.78° to 90.90°.     

In-situ X-ray diffraction study on β-CrOOH at high pressure and high-temperature

ABSTRACT

High pressure hydrous phases with distorted rutile-type structure have attracted much interest as potential water reservoirs in the Earth’s mantle. An in-situ X-ray diffraction study of β-CrOOH was performed at high pressures of up to 6.2?GPa and high-temperatures of up to 700?K in order to clarify the temperature effect on compression behaviors of β-CrOOH. The P-V-T data fitted to a Birch–Murnaghan equation of state yielded the following results: isothermal bulk modulus K_T0?=?191(4)?GPa, temperature derivative (?K_T/?T)_P?=??0.04(2)?GPa?K^?1, and volumetric thermal expansion coefficient α?=?3.3(2)?×?10^?5?K^?1. In this study, at 300?K, the a-axis became less compressible at pressures above 1–2?GPa. We found that the pressure where the slopes of a/b and a/c ratios turned positive increased with temperature. This is the first experimental study indicating the temperature dependence of the change in the axial compressibility in distorted rutile-type M³⁺OOH.     

Studies on Im-3-type KSbO3 using high pressure X-ray diffraction and Raman spectroscopy

In situ X-ray diffraction and Raman scattering experiments using a diamond anvil cell revealed that Im-3-type KSbO₃ remains stable up to 40.5?GPa with a bulk modulus K₀?=?101.6 (7)?GPa. Rietveld structure refinements and mode Grüneisen parameters suggested that the stability mechanism of this three-dimensional cubic tunnel structure was attributed to the isotropic compression for all types of Sb–O bonding in the unit of SbO₆ octahedron. Isotropic structure adjustment with external pressure reflected the nature that Im-3-type KSbO₃ model structure has a high ionic tolerance with a change in the chemical pressure in the isomorphous substitutions.     

High-pressure effect on inverse spinel LiCuVO4:X-ray diffraction and Raman scattering

The effect of external quasi-hydrostatic pressure on the inverse spinel structure of LiCuVO 4 was studied in this paper. High-pressure synchrotron X-ray diffraction and Raman spectroscopy measurements were carried out at room temperature up to 35.7 and 40.3 GPa, respectively. At a pressure of about 20 GPa, both Raman spectra and X-ray diffraction results indicate that LiCuVO4 was transformed into a monoclinic phase, which remained stable up to at least 35.7 GPa. Upon release of pressure, the high-pressure phase returned to the initial phase. The pressure dependence of the volume of low pressure orthorhombic phase and high-pressure monoclinic phase were described by a second-order Birch-Murnaghan equation of state, which yielded bulk modulus values of B 0 = 197(5) and 232(8) GPa, respectively. The results support the empirical suggestion that the oxide spinels have similar bulk modulus around 200 GPa.     

Thermal equation-of-state of osmium: a synchrotron X-ray diffraction study

The pressure-volume-temperature behavior of osmium was studied at pressures and temperatures up to 15 GPa and 1273 K. In situ measurements were conducted using energy-dispersive synchrotron X-ray diffraction in a T-cup 6-8 high pressure apparatus. A fit of room-temperature data by the third-order Birch-Murnaghan equation-of-state yielded isothermal bulk modulus K₀=435(19) GPa and its pressure derivative K′₀=3.5(0.8) GPa. High-temperature data were analyzed using Birch-Murnaghan equation of state and thermal pressure approach. The temperature derivative of bulk modulus was found to be −0.061(9) GPa K⁻¹. Significant anisotropy of osmium compressibility was observed.     

First-principles study of structural,elastic, electronic and thermodynamic properties of topological insulator Bi2Se3 under pressure

The structural, elastic, electronic and thermodynamic properties of the rhombohedral topological insulator Bi₂Se₃ are investigated by the generalized gradient approximation (GGA) with the Wu–Cohen (WC) exchange-correlation functional. The calculated lattice constants agree well with the available experimental and other theoretical data. Our GGA calculations indicate that Bi₂Se₃ is a 3D topological insulator with a band gap of 0.287 eV, which are well consistent with the experimental value of 0.3 eV. The pressure dependence of the elastic constants C_ij, bulk modulus B, shear modulus G, Young’s modulus E, and Poisson’s ratio σ of Bi₂Se₃ are also obtained successfully. The bulk modulus obtained from elastic constants is 53.5 GPa, which agrees well with the experimental value of 53 GPa. We also investigate the shear sound velocity V_S, longitudinal sound velocity V_L, and Debye temperature Θ_E from our elastic constants, as well as the thermodynamic properties from quasi-harmonic Debye model. We obtain that the heat capacity C_v and the thermal expansion coefficient α at 0 GPa and 300 K are 120.78 J mol^?1 K^?1 and 4.70 × 10^?5 K^?1, respectively.     

Pressure-induced phase transformation of CsPbI3 by X-ray diffraction and Raman spectroscopy

The structural transformation of cesium lead iodine (CsPbI₃) has been investigated in diamond anvil cells up to ～15 GPa at room temperature by employing synchrotron radiation X-ray diffraction and Raman spectroscopy. One reversible transformation from orthorhombic (Pnma) to monoclinic (P2₁/m) phase has been observed at 3.9 GPa. Isothermal pressure–volume relationship of orthorhombic CsPbI₃ is well fitted by the third-order Birch–Murnaghan equation of state with K₀ = 14(3) GPa, K′₀ = 6(2) and V₀ = 891(7) ų. The ultralow value of bulk modulus K₀ demonstrates the high compressible nature of CsPbI₃, similar to those of organic–inorganic metal halide perovskites. The present results provide essential information on the intrinsic properties and stability of CsPbI₃, which may be applied in photovoltaic devices.     

First-principles study of elastic and electronic properties of layered ternary nitride SrZrN2 under pressure

The structural, elastic, and electronic properties of SrZrN₂ under pressure up to 100?GPa have been carried out with first-principles calculations based on density functional theory. The calculated lattice parameters at 0?GPa and 0?K by using the GGA-PW91-ultrasoft method are in good agreement with the available experimental data and other previous theoretical calculations. The pressure dependence of the elastic constants and the elastic-dependent properties of SrZrN₂, such as bulk modulus B, shear modulus G, Young's modulus E, Debye temperature Θ, shear and longitudinal wave velocity V_S and V_L, are also successfully obtained. It is found that all elastic constants increase monotonically with pressure. When the pressure increases up to 140?GPa, the obtained elastic constants do not satisfy the mechanical stability criteria and a phase transition might has occurred. Moreover, the anisotropy of the directional-dependent Young's modulus and the linear compressibility under different pressures are analysed for the first time. Finally, the pressure dependence of the total and partial densities of states and the bonding property of SrZrN₂ are also investigated.     

An in situ high-pressure X-ray diffraction experiment of hydroxyapophyllite

The compression behavior of a natural hydroxyapophyllite is investigated up to about 10.01 GPa at 300 K using in situ angle-dispersive X-ray diffraction and a diamond anvil cell at High Pressure Experiment Station, Beijing Synchrotron Radiation Facility (BSRF). Over this pressure range, no phase change or disproportionation is observed. The isothermal equation of state is determined for the first time. The values of zero-pressure volume V₀, isothermal bulk modulus K₀, and K₀' refined with a third-order Birch-Murnaghan equation of state are V₀=1276.3±0.9 ų, K₀=71±3 GPa, and K₀'=8±1. Furthermore, we confirm that the values of linear compressibility β along a and c directions of hydroxyapophyllite are elastically anisotropic.     

High-pressure Raman and X-ray studies of barite,BaSO4

High-pressure Raman spectroscopic and X-ray diffraction experiments of barite, BaSO₄, were carried out in a diamond anvil cell up to 25?GPa at room temperature. On the basis of the changes in the diffraction patterns and the variation of lattice parameters with pressure, it is inferred that barite undergoes a phase transformation at 10?GPa. The phase transition accompanies the change in the force constant of vibrational modes in barite. Further compression beyond the phase transition causes the distortion of SO₄ tetrahedron as indicated by the splitting in the SO₄ stretching modes. Both X-ray and Raman data support that the phase transition in BaSO₄ is reversible. The compression data yield a bulk modulus of 63?±?2?GPa for barite. Barite shows anisotropic compressibility along three crystallographic axes with c being the most compressible axis.     

Dual mode ultrasonic interferometry in multi-anvil high pressure apparatus using single-crystal olivine as the pressure standard

Acoustic measurements of compressional (P) and shear (S) wave travel times were performed in a 1000-ton uniaxial split-cylinder apparatus (USCA-1000) of the Kawai-type up to 10?GPa at room temperature, using dual mode lithium niobate transducers and ultrasonic interferometry. The cell pressures were calibrated continuously by in-situ measurements of the travel times in a single crystal San-Carlos olivine buffer rod inside the cell assembly. Elastic compressional and shear wave velocities in a dense, fine-grained polycrystalline Al₂O₃ were measured simultaneously to 10?GPa; from these data, the elastic moduli and their pressure derivatives were obtained for the longitudinal modulus {L ₀?=?461(3)?GPa, L ₀′?=?7.0(1)}, the shear modulus {G ₀?=?162(2)?GPa, G ₀′?=?1.9(1)} and the bulk modulus {K ₀?=?245(3)?GPa, K ₀′?=?4.5(1)}.     

In situ high-pressure X-ray diffraction experiments and ab initio calculations of Co2P

In situ high-pressure experiments of Co2P are carried out by means of angle dispersive X-ray diffraction with diamond anvil cell technique. No phase transition is observed in the present pressure range up to 15 GPa at room temperature, even at high temperature and 15 GPa. Results of compression for Co2P are well presented by the second-order Birch-Murnaghan equation of state with V0 = 130.99(2)3 (1=0.1 nm) and K0 = 160(3) GPa. Axial compressibilities are described by compressional modulus of the axis: Ka = 123(2) GPa, Kb = 167(8) GPa and Kc = 220(7) GPa. Theoretical calculations further support the experimental results and indicate that C23-type Co2P is stable at high pressure compared with the C22-type phase.     

Structural and thermodynamic properties of WB at high pressure and high temperature

The structure parameters and electronic structures of tungsten boride (WB) have been investigated by using the density functional theory (DFT). Our calculating results display the bulk modulus of WB are 352±2 GPa (K′₀=4.29) and 322±3 GPa (K′₀=4.21) by LDA and GGA methods, respectively. We have analyzed the probable reason of the discrepancy from the bulk modulus between theoretical and experimental results. The compression behavior of the unit cell axes is anisotropic, with the c-axis being more compressible than the a-axis. By analyzing the bond lengths information, it also demonstrated that WB has a lower compressibility at high pressure. From the partial densities of states (PDOS) of WB, we found that the Fermi lever is mostly contributed by the d states of W atom and p states of B atom and that the contributions from the s, p states of W atom and s states of B atom are small. Moreover, using the Gibbs 2 program, the thermodynamic properties of WB are obtained in a wide temperature range at high pressure for the first time in this work.     

Thermal equation of state of goethite (α-FeOOH)

ABSTRACT

The pressure–volume–temperature (P–V–T) equation of state (EoS) of natural goethite (α-FeOOH) has been determined by an X-ray diffraction study using synchrotron radiation. Fitting the volume data to the third-order Birch–Murnaghan EoS yielded an isothermal bulk modulus, B₀ of 85.9(15)?GPa, and a pressure derivative of the bulk modulus, B′, of 12.6(8). The temperature derivative of the bulk modulus, (?B/?T)_P, was –0.022(9)?GPa?K^?1. The thermal expansion coefficient α₀ was determined to be 4.0(5)?×?10^?5?K^?1.     

High pressure X-ray diffraction study of Fe2B

We report the results of a synchrotron based X-ray diffraction study of bct-Fe₂B under quasi-hydrostatic conditions from 0 to 50 GPa. Over this pressure range, no phase change or disproportionation has been observed. A weighted fit of the data to the Birch-Murnaghan equation of state yields a value of the bulk modulus, K, of 164±14 GPa and the first pressure derivative of the bulk modulus, K′, of 4.4±0.5. The compression is found to be anisotropic, with the a-axis being more incompressible than the c-axis.     

C 60 and the icosahedral group Ih: Simplification methods and group-theoretical applications

Group-theoretical methods are used to induce “global” distortions of C ₆₀ from basis function(s) of a single “local” distortion in two cases: (1) The 60 vibrational modes arising from radial displacements of carbon atoms are found in terms of basis functions of A′, the one-dimensional symmetric representation of C_s; (2) The 24 vibrational modes arising from tangential displacements of hypothetical atoms placed at pentagon centers are found in terms of basis functions of E ₁, the vector irreducible representation of C_5v . The induction process is simplified by an icon notation (v) or (vμ) which uniquely labels the operations in I_h if μ,v = 0, 1, 2, 3, 4 and under/over lining of a digit is included. These icons describe the transformations IUS^vTS^μ of Felix Kline referred to a five-fold z-axis and two-fold x-axis and serve to label and “distinguish” symmetrically equivalent points of the truncated icosahedron.     

Raman spectroscopy of melamine at high pressures up to 60 GPa

In this work, the Raman scattering of melamine was studied under high pressure up to 60 GPa. The behavior of the most intensive peaks of the Raman spectrum of melamine, 677 cm^?1 and 985 cm^?1 modes, and their line widths do not show any phase transition or indication of formation of sp ³ bonds. Comparing the behavior of the line width of the Raman peaks of graphite under pressure and that of melamine leads us to conclude that the s-triasine (C–N) ring is more rigid than the C–C graphite ring. High pressure results with melamine suggest that the direct phase transition g-C₃N₄ to dense C₃N₄ phase should occur above 60 GPa.     

Compression behavior of NiO in a diamond cell

Abstract

The compressibility of synthetic polycrystalline NiO was studied in a diamond anvil cell at room temperature utilizing two different X-ray sources. A standard film with a conventional X-ray source and the energy dispersive X-ray diffraction (EDXRD) method with synchrotron radiation were used for data acquisition. In the film method, the sample was compressed in a 4:1 methanol to ethanol solution up to 7 GPa with ruby fluorescence as a pressure calibrant. In the energy dispersive method, NiO powder was mixed with gold and compressed in two different conditions: gasketed and ungasketed up to 30 GPa. In the gasketed run, water was used as the pressure transmitting medium while gold was used as pressure calibrant in both runs.

Hydrostatic compression of NiO in both diffraction methods yields a bulk modulus (K _o) of 187 ± 7 GPa assuming K′ = 4. The compression of gasketed NiO of the synchrotron experiment, however, showed an obvious break at pressure exceeding 4 GPa due to the loss of hydrostaticity. NiO in a nonhydrostatic condition behaves with less compressibility than the hydrostatic results with a nominal K _o of 238 ± 10 GPa. The lower compressibility of NiO in synchrotron runs is attributed to the uniaxial loading effect which was more easily detected by the EDXRD geometry. The discrepancy in the bulk modulus can be attributed to the contrast in the shear strength between the sample and pressure medium and the Poisson effect of the sample under uniaxial loading.