High-pressure phase transitions in UP1-xSx

Abstract

High-pressure X-ray diffraction using synchrotron radiation has been performed on UP_1-x -S_x (X=0.1; 0.25; 0.4) up to 53 GPa UP_1-x S_x is a solid solution with a B1 (NaCl) structure. For all compositions a second order phase transition is observed around 10 GPa to a distorted B1 structure of rhombohedral symmetry. For UP_1-x S_x with x 0.25 a second phase transition is observed, which takes place in the region of 35 GPa This phase transition occurs when the nearest U-U distance reaches the Hill limit of 330–340 pm. The high-pressure phase seems to have orthorhombic or even monoclinic symmetry. It has some similarities to the high pressure phase of UP. UP^1-x S_x 4 shows only weak indications for an additional phase at 53 GPa. In conclusion, we observe that the second phase transition and the bulk modulus B, in UP shift to higher pressure, when phosphorus is replaced by sulfur.     

High pressure studies up to 50GPa of CuO

Abstract

Copper oxide has been studied at high pressure up to 50 GPa. A monoclinic structure was compatible with the measurements at all pressures, and no phase change was observed. A bulk modulus, B₀, = 98 GPa, and its pressure derivative B′₀ = 5.6 was obtained.     

Volume compression of thallium to 45 GPa

Abstract

High-pressure structural transition and volume compression for thallium were investigated to 45 GPa in a diamond anvil cell using the angular dispersive X-ray diffraction technique. Except for the known polymorphic transition at 3.7 GPa, no other structural change was observed in this pressure range. The equation of state of the high pressure phase has been obtained: its initial bulk modulus, B₀ = 33.1 GPa, is lower by 10% than that of the hexagonal phase at normal pressure.     

Pressure-induced phase transition in ThO2 and PuO2

Abstract

Thorium and plutonium dioxides were studied under pressure by the energy dispersive X-ray diffraction method. A double conical slit assembly was used to collect simultaneously the diffracted radiation at five and seven degrees.

ThO₂ undergoes a phase transformation at 40 GPa. The high-pressure phase remains stable up to 55 GPa, the highest pressure reached in the experiment. For PuO₂, a structural transformation occurs near 39 GPa. The observed high-pressure phases of ThO₂ and PuO₂ exhibit similar diffraction spectra. Like for some other fluorite type compounds, the ThO₂ and PuO₂ high-pressure phase has been indexed in the PbCl₂-type structure. The bulk modulus has been calculated as B₀= 262 GPa with a pressure derivative of B₀' = 6.7 for ThO₂ and as B₀ = 379 GPa with B₀' = 2.4 for PuO₂. The volume decrease at the transition is 12% for PuO₂ and 8% for ThO₂.     

Phase transitions in PbTe under quasi-hydrostatic pressure up to 50 GPa

PbTe has been investigated using synchrotron X-ray diffraction (XRD) in a diamond anvil cell under quasi-hydrostatic pressures up to 50 GPa. Upon compression to 6.6 GPa, the initial NaCl phase transforms to an intermediate phase, which is confirmed to be an orthorhombic structure with a space group Pnma. At 18.4 GPa, the intermediate Pnma phase undergoes a phase transition to the CsCl structure. The systemic analysis of the crystal structures between the NaCl and intermediate phases indicates that the structure of the Pnma phase could be derived from the distortion of the NaCl structure. The bulk modulus of the CsCl phase is B₀=52(2) GPa with V₀=60.8(4) ų and B′₀=4.0 (fixed), slightly larger than the NaCl phase (B₀=44(1) GPa) and the intermediate phase (B₀=49(3) GPa).     

TiN多型体高压相变的第一性原理计算

基于密度泛函理论框架下的赝势平面波方法,计算了B1(氯化钠结构)、B2(氯化铯结构)、B3(闪锌矿结构)、B_k(六方氮化硼结构)、B_h(碳化钨结构)和B8₁(砷化镍结构)6种TiN多型体的晶体结构、体积弹性模量和相对稳定性.计算指出,不存在B4(纤锌矿)结构的TiN.通过不同外压下的晶格弛豫得到每种结构的焓,发现外压 关键词： 氮化钛 赝势 高压相变 密度泛函理论     

Investigations of phase transition, elastic and thermodynamic properties of GaP by using the density functional theory

The phase transition of gallium phosphide (GaP) from zinc-blende (ZB) to a rocksalt (RS) structure is investigated by the plane-wave pseudopotential density functional theory (DFT). Lattice constant a₀, elastic constants c_ij, bulk modulus B₀ and the pressure derivative of bulk modulus B₀' are calculated. The results are in good agreement with numerous experimental and theoretical data. From the usual condition of equal enthalpies, the phase transition from the ZB to the RS structure occurs at 21.9 GPa, which is close to the experimental value of 22.0 GPa. The elastic properties of GaP with the ZB structure in a pressure range from 0 GPa to 21.9 GPa and those of the RS structure in a pressure range of pressures from 21.9 GPa to 40 GPa are obtained. According to the quasi-harmonic Debye model, in which the phononic effects are considered, the normalized volume V/V₀, the Debye temperature θ, the heat capacity C_v and the thermal expansion coefficient α are also discussed in a pressure range from 0 GPa to 40 GPa and a temperature range from 0 K to 1500 K.     

X-ray difffraction studies on samarium up to one megabar pressure

Abstract

High—pressure crystal structure studies have been performed on Sm up to 100 GPa using synchrotron x-radiation and a diamond anvil cell. The structural sequence Sm-dhcp-fcc-dist.fcc has been confirmed. There is no evidence of any volume collapse. The bulk modulus and its pressure derivative have been determined (B₀ = 30.7 GPa, B₀’ = 2.5).     

A high pressure Raman study of terbium molybdate

Abstract

Tb₂(MoO₄)₃ has been studied by Raman spectroscopy under hydrostatic pressure up to 9 GPa at room temperature. The measurements reveal two phase transitions, one at around 2 GPa and another one above 5 GPa. The first phase transition is associated with an increase in the coordination number of Mo while the second is probably a transition to an amorphous phase in which only a wide band originating from Mo-O vibrations remains. This behaviour is irreversible as the Raman spectrum of the initial structure is not recovered at atmospheric pressure.     

High pressure X-ray diffraction study of copper hydride at room temperature

Abstract

High pressure X-ray studies on CuH up to 23 GPa have been performed at room temperature using a gasketed diamond anvil cell. The experimental data on the molar volume of CuH as a function of pressure have been fitted to Murnaghan's equation of state giving a bulk modulus: B₀ = 72.5±2 GPa and B₀ = 2.7 ± 0.3. By comparison with the equation of state for pure copper the effective additive volume of hydrogen has been evaluated as a function of pressure. It decreases from 3.2 cm³/mol H, at ambient pressure reaching a flattening value of 1.7cm³hol H at about 60 GPa. This suggests a continuous transition of CuH from ionic or covalent character at normal pressure to metallic hydride behavior at high pressure     

The baddeleyite-type high pressure phase of Ca(OH)2

Abstract

A phase transition from Ca(OH)₂ I (portlandite) to Ca(OH)₂ II at high pressure and temperature has been confirmed, using in situ x-ray diffraction in a multianvil high pressure device (DIA). The structure was determined at 9.5 GPa and room temperature from data collected after heating the sample at 300°C at 7.2 GPa in a diamond anvil cell. Both the Le Bail fit and preliminary Rietveld refinement suggest that the new phase, which reverts to Ca(OH), I during pressure release, has a structure related to that of baddeleyite (ZrO₁); it is monoclinic (P2₁/c) with a= 4.887(2), b= 5.834(2), c = 5.587(2), β = 99.74(2)°. The coordination number of Ca increases from six to seven (5 + 2) across the transition. At 500°C, the phase boundary is bracketed at 5.7 ± 0.4 GPa by reversal experiments performed in the DIA.     

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.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

Cd1-xZnxTe在高压下的电学性质、状态方程与相变

 在金刚石压砧装置上,采用电阻和电容测量方法研究了Cd_1-xZn_xTe（x=0.04）在室温下、17 GPa内的电阻、电容与压力的关系。实验结果表明,它在3.1 GPa左右和5 GPa左右发生了两次电子结构相变,而在3.1 GPa以上和5.7 GPa左右发生了两次晶体结构相变。同时,还在活塞-圆筒测量装置上研究了Cd_1-xZn_xTe（x=0.04）在室温下、4.5 GPa内的p-V关系。实验结果表明它在3.8 GPa左右发生了相变。本工作还给出了它在相变前后的状态方程,以及它的Grüneisen参数γ₀、体弹模量B₀ 与B₀ 的压力导数B₀′。     

Compressibility of Fe1.087Te: a high pressure X-ray diffraction study

Fe_1.087Te exhibits three phases in the pressure range from ambient to 16.6?GPa and becomes amorphous at higher pressures. All three phases have tetragonal symmetry. The low pressure T-phase is stable in the pressure range 0≤P<4.1?GPa and is found to be relatively soft having zero pressure bulk modulus B ₀=36(1)?GPa. The intermediate cT-phase is less compressible with B ₀=88(5)?GPa and stable in the pressure range 4.1≤P<10?GPa while a more compressible phase was observed between 10 and 16.6?GPa.     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

Pressure-induced structural phase transition and electronic properties of RESb (RE = Ho,Er and Tm) compounds: ab initio calculations

The structural phase transition and electronic properties at ambient (B ₁-phase) and high pressure (B ₂-phase) of heavy rare earth monoantimonides (RESb; RE?=?Ho, Er, and Tm) have been studied theoretically using the self-consistent tight binding linear muffin tin orbital method. These compounds show metallic behavior under ambient condition and undergo a structural phase transition to the B ₂ phase at high pressure. We predict a structural phase transition from the B ₁ to B ₂ phase in the pressure range 30.0–35.0?GPa. Apart from this, the ground state properties, such as lattice parameter and bulk modulus are calculated and compared with the available theoretical and experimental results.     

The high-pressure structural transition in MnS: an ab initio study

The structural phase transition between B1 (α-MnS) and B3 (β-MnS) is investigated using a density functional theory method. The structural phase transition pressure Pt from α-MnS to β-MnS, which is determined on the basis of the third-order Birch–Murnaghan equation of states, is 30.75?GPa. Also, the lattice parameters a, the bulk modulus B and pressure derivative of bulk modulus B′, which are generally in good agreement with experiments and other theoretical values, are obtained under zero pressure. For further investigation of the structural phase transition pressure of MnS, the relative volumes V/V ₀, the bulk modulus B, first and second pressure derivatives (B′ and B″) of bulk modulus for the two structures of MnS have been calculated under various pressures.     

A structural study of promethium metal under pressure

Abstract

The structural behaviour of Pm metal has been investigated up to 60 GPa of pressure using a Diamond Anvil Cell (DAC) and the energy dispersive X-ray diffraction technique. The room temperature/pressure structural form of Pm is dhcp and it transforms to a fcc phase by 10 GPa. This cubic phase of the metal converts by 18 GPa to a third phase, which has frequently been referred to as representing a distorted fcc structure. This latter form of Pm was retained up to 60 GPa, the maximum pressure studied, but subtle changes in the X-ray spectra between 50 and 60 GPa hinted that an additional structural change could be forthcoming at higher pressures. From the experimental data a bulk modulus (B₀) of 38 GPa and a B₀′ constant of 1.5 were calculated using the Birch equation. This modulus for Pm is in accord with the moduli reported for the neighboring lanthanide metals.