High-pressure crystal structure of thorium disulfide and diselenide and uranium disulfide

Abstract

The crystal structure of ThS₂, ThSe₂ and US₂ has been investigated for pressure up to 60GPa using x-ray powder diffraction. The bulk moduli are 175(10), 155(10) and 155(20) GPa, respectively. A pressure-induced phase transformation occurs at about 40 GPa for ThS₂, 30 GPa for ThSe₂ and 15 GPa for US₂. The results for ThSe₂ indicate that its high-pressure phase has a monoclinic structure. The same structure is compatible with the observed high-pressure spectra of ThS₂ and US₂. However, the crystal system assignment is less certain for these compounds.     

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     

High-Pressure Behavior of Nano Titanium Dioxide

Nanocrystalline rutile Titanium dioxide has been studied by X-ray diffraction at ambient temperature up to 47.4 GPa. The material is found to transform to the monoclinic baddeleyite structure between 20 and 30 GPa, which is higher than the corresponding pressure range for bulk material. Upon decompression, the baddeleyite phase transforms to the f -PbO 2 phase at about 4-2 GPa. The experimental bulk moduli are 211(7) GPa for the rutile phase, 235(16) for the baddeleyite type and 212(25) GPa for the f -PbO 2 type phase. The results are compared with previous measurements of bulk rutile Titanium dioxide.     

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.     

The pressure response of Raman active phonon modes of Tm3Al5O12

Abstract

We present the Raman spectrum of Tm₃Al₅O₁₂ single crystal and its pressure dependence for hydrostatic pressure up to 11GPa and room temperature. Tm₃Al₅O₁₂ belongs to the crystal family of rare earth garnets (Re₃A1₂(AlO₄)₃, Re: Gd, Tb, Dy, Er,…), which crystallize in the body-centered cubic lattice and contains eight molecular units in the conventional unit cell, Group theory predicts 25 Raman active modes for these compounds, while experimentally are observed 15 modes. As crystal volume decreases all Raman peaks exhibit pressure coefficients varying from 0.7 to 5.6cm^?1/ GPa. A large part of the vibrational spectra of these compounds could be explained taking into account the vibrational properties of molecular subunits, namely AlO₄.     

High-pressure X-ray diffraction study of the δ-phase in the uranium-neptunium binary system

Abstract

High pressure X-ray diffraction studies were performed at room temperature on a uranium-neptunium binary alloy (U_{0, 40} Np_0.60) using a diamond anvil cell in an energy dispersive facility. The sample maintained its simple cubic phase up to 62 GPa (highest pressure reached in This experiment). The bulk modulus and its pressure derivative were determined to be B₀ = 82 (2) GPa and B₀′ = 9.4 (1.3), from the experimental data in the pressure range 0–20 GPa. The present results are compared with those obtained by the same techniques used for uranium and neptunium.     

High-pressure phases of plutonium monoselenide studied by X-ray diffraction

Abstract

Plutonium monoselenide was studied under high pressure up to 47 GPa, at room temperature, using a diamond anvil cell in an energy dispersive X-ray diffraction facility. At ambient pressure, PuSe has the NaC1-type (B1) structure. The compound has been found to undergo a second-order crystallographic phase transition at around 20 GPa. This phase can be described as a distorted B1 structure, with a rhombohedral symmetry. PuSe transforms to a new phase at around 35 GPa, which can be indexed in the cubic CsCl-type (B2). The volume collapse at this phase transition is 11%. When releasing pressure, we observed a strong hysteresis to the inverse transformation down to 5 GPa. From the pressure-volume relationship, the bulk modulus has been determined to B ₀ = 98 GPa and its pressure derivative as B ₀ = 2.6. These results are compared to those obtained with other actinide monmictides and monochalcogenides.     

Structural,elastic and electronic properties of ACTi3 (A=Al,In and Tl) antiperovskite

First principle calculations were performed to investigate the structural, elastic and electronic properties of unexplored antiperovskite ACTi₃, with A=Al, In and Tl. The calculated structural parameters were found to be in good agreement with the available experimental data, with deviations being less than 2.7%. The bulk modulus was found to be equal to 155 GPa for AlCTi₃ and to a value 5% lower, 147 GPa, for TlCTi₃. For values of applied pressures up to 40 GPa, elastic moduli were calculated and the mechanical stability criteria were verified. The band structure of these compounds has been found to display a metallic character, with strong ionic–covalent bonds between Ti and C atoms, and ionic bonds along A and Ti atoms. The overlap population analysis showed that the stiffness decreases with an increase in the antibonding state between Ti and A atoms.     

Elastic properties of high-temperature superconductors Pb2Sr2ACu3O8 (A=Ho,Y,Y0.5Ca0.5) derived from high-pressure X-ray diffraction

The crystal structure of the superconducting compounds Pb₂Sr₂ACu₃O₈, where A=Ho, Y and Y_0.5Ca_0.5, has been studied at room temperature in the pressure range up to 50 GPa. A transition from orthorhombic to tetragonal structure is observed at about 10 GPa for all these components. The transition is reversible and without any discontinuous volume change. The value of the bulk modulus for the two compounds with A=Ho and Y is in the range 160–164 GPa, whereas for A=Y_0.5 Ca_0.5 a larger value of 195 GPa has been measured.     

A high-pressure study of Th3P4 and some U3X4 compounds

Abstract

The high-pressure crystal structures of Th₃P₄ and U₃X₄, where X = P, As and Sb, have been studied by means of synchrotron x-ray diffraction in the pressure range up to 50 GPa. The cubic phase of these compounds is retained in the whole pressure range. The bulk modulus B₀ and its pressure derivative B₀’ have been determined for each compound. A log-log plot of B₀ versus unit-cell volume gives a straight line for the uranium pnictides, with a slope about -5/3.     

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.     

Structural,mechanical and thermal properties of some Holmium Pnictides under pressure: A theoretical approach

The high pressure structural, elastic and thermal properties of holmium pnictides HoX (X=N, P, As and Bi) were investigated theoretically by using an inter-ionic potential theory with modified ionic charge parameter. We have predicted a structural phase transition from NaCl (B₁) to CsCl (B₂)-type structure at pressure of 139 GPa for HoN, 52 GPa for HoP, 44 GPa for HoAs and 26 GPa for HoBi. Other properties, such as lattice constant, bulk modulus, cohesive energy, second and third-order elastic constants were calculated and compared with the available experimental and theoretical data. In order to gain further information the brittle behaviour of these compounds was observed. Some other properties like Shear modulus (G), Young's modulus (E), Poisson's ratio (ν), anisotropy factor (A), sound velocities, Debye temperature (θ_D) were calculated. The variation of elastic constants (C₁₁ and C₄₄) and Debye temperature (θ_D) with pressure was also presented.     

Bulk moduli and high-pressure phases of the uranium rocksalt structure compounds: II. The monopnictides

Abstract

The high-pressure crystal structures of the compounds UX, where X = N, P, As and Sb, have been studied using X-ray diffraction in the pressure range up to about 60 GPa Rhornbohedral distortions are observed for UN and Up above 29 GPa and lO GPa, respectively. In Up a further transformation to an orthorhombic phase occurs at 28 GPa. UAs and USb transform to the CsCl structure at 20 GPa and 9 GPa, respectively. The latter transformations show a considerable hysteresis when the pressure is released. The scaling behaviour of the bulk modulus has been studied. It is confirmed that a log-log plot of bulk modulus versus specific volume for the cubic phases gives a straight line with a slope near ? 5/3.     

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 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.     

The effects of magnetic moment collapse under high pressure,on physical properties in mono-borides TMB (TM = Mn,Fe): a first-principles

In this paper, spin polarization and pressure effects on the structure, magnetic and anisotropic elastic properties of the 3d transition-metal mono-borides TMB (TM = Mn, Fe) have been investigated by using generalized gradient approximation within the framework of density functional theory. It seems that manganese in MnB carries a higher magnetic moment (1.83 μ_B) than iron in FeB (1.12 μ_B). Applied pressure ranges from 0 to 150 GPa, these ferromagnetic compounds show at a certain pressure (143 GPa for MnB and 77 GPa for FeB) a pronounced abrupt collapse of the magnetic moment (first-order quantum phase transitions). Furthermore, elastic properties, including bulk, shear and Young moduli as well as the Poisson ratio are obtained by Voigt–Reuss–Hill approximation. By the elastic stability criteria, it is predicted that MnB and FeB are stable up to the selected pressures. In both cases, mechanical anisotropies are discussed by calculating different anisotropic indexes and factors. The three-dimensional surfaces and planar contours of Young, and bulk moduli of compounds are plotted, at several crystallographic planes ((100), (010) and (001)) to reveal their elastic anisotropy.     

High pressure X-ray diffraction study on icosahedral boron arsenide (B12As2)

The high pressure properties of icosahedral boron arsenide (B₁₂As₂) were studied by in situ X-ray diffraction measurements at pressures up to 25.5 GPa at room temperature. B₁₂As₂ retains its rhombohedral structure; no phase transition was observed in the pressure range. The bulk modulus was determined to be 216 GPa with the pressure derivative 2.2. Anisotropy was observed in the compressibility of B₁₂As₂—c-axis was 16.2% more compressible than a-axis. The boron icosahedron plays a dominant role in the compressibility of boron-rich compounds.     

流体静高压下的锂离子导电性

本文改进实验方法,在0.0001—1.23GPa流体静高压下测量了整片非晶锂离子导体B₂O₃-0.7Li₂O-0.7LiCl-xAl₂O₃(x=0.05;0.15)及其粉末压片的离子电导率及激活体积。发现粉末压片电导率峰值是由非晶微粒间的接触电导及非晶微粒体电导两者叠加;对整片非晶电导率的压力效应用离子迁移通道的物理图象给出初步的微观解释。此外,还观测到氧化铝组分减少使电导率的压力转变点明显降低;测量出不同温度热处理以及300℃等温热处理4—20h后离子电导率-压力曲线的变化规律,仍可归因于非晶态相分离及两种非晶相的先后晶化。 关键词：     

High pressure X-ray diffraction experiments on NpS and PUS up to 60 GPa

Abstract

Neptunium and plutonium monosulfides were studied under high pressure up to ～60 GPa using a diamond anvil cell in an energy dispersive X-ray diffraction facility. The compounds, of cubic rock salt structure type at ambient pressure, do not show any crystallographic phase transition in the domain of investigation. From the pressure-volume relationship, we determined bulk moduli of 92 and 120 GPa with pressure derivatives of 4.6 and 4.1 for NpS and PUS respectively.     

High-precision measurements of the compressibility and the electrical resistivity of bulk <Emphasis Type="Italic">g</Emphasis>-As<Subscript>2</Subscript>Te<Subscript>3</Subscript> glasses at a hydrostatic pressure up to 8.5 GPa

High-precision studies of the volume and the electrical resistivity of g-As₂Te₃ glasses at a high hydrostatic pressure up to 8.5 GPa at room temperature are performed. The glasses exhibit elastic behavior in compression only at a pressure up to 1 GPa, and a diffuse structural transformation and inelastic density relaxation (logarithmic in time) begin at higher pressures. When the pressure increases further, the relaxation rate passes through a sharp maximum at 2.5 GPa, which is accompanied by softening the relaxing bulk modulus, and then decreases, being noticeable up to the maximum pressure. When pressure is relieved, an unusual inflection point is observed in the baric dependence of the bulk modulus near 4 GPa. The polyamorphic transformation is only partly reversible and the residual densification after pressure release is 2%. In compression, the electrical resistivity of the g-As₂Te₃ glasses decreases exponentially with increasing pressure (at a pressure up to 2 GPa); then, it decreases faster by almost three orders of magnitude in the pressure range 2–3.5 GPa. At a pressure of 5 GPa, the electrical resistivity reaches 10^–3 Ω cm, which is characteristic of a metallic state; this resistivity continues to decrease with increasing pressure and reaches 1.7 × 10^–4 Ω cm at 8.1 GPa. The reverse metal–semiconductor transition occurs at a pressure of 3 GPa when pressure is relieved. When the pressure is decreased to atmospheric pressure, the electrical resistivity of the glasses is below the initial pressure by two–three orders of magnitude. Under normal conditions, both the volume and the electrical resistivity relax to quasi-equilibrium values in several months. Comparative structural and Raman spectroscopy investigations demonstrate that the glasses subjected to high pressure have the maximum chemical order. The glasses with a higher order have a lower electrical resistivity. The polyamorphism in the As₂Te₃ glasses is caused by both structural changes and chemical ordering. The g-As₂Te₃ compound is the first example of glasses, where the reversible metallization under pressure has been studied under hydrostatic conditions.