Studies of crystallization process of silica and germania glasses at high pressure

Abstract

The process of crystallization of SiO₂ and GeO₂ glasses was studied using samples retrieved after heat treatment at high pressures up to 12GPa. Two different samples of fused quartz and silica gel were studied in order to compare SiO₂ glass structure. Upon heating beyond 400°C at fixed pressures under which stishovite, rutile-type phase, is thermodynamically stable, the SiO₂ glasses underwent crystallization into coesite and stishovite, without quartz. In the course of heating GeO₂ glass in the stability field of rutile-type phase, low-quartz-type phase appeared as an intermediate at pressures below 7GPa, whereas only rutile-type phase was observed at 12GPa. The crystallization sequences are discussed in terms of coordination numbers in the glass and crystalline states.     

The under-pressure behaviour of mechanical,electronic and optical properties of calcium titanate and its ground state thermoelectric response

Abstract

In this study, the elastic, electronic, optical and thermoelectric properties of CaTiO₃ perovskite oxide have been investigated using first-principles calculations. The generalised gradient approximation (GGA) has been employed for evaluating structural and elastic properties, while the modified Becke Johnson functional is used for studying the optical response of this compound. In addition to ground state physical properties, we also investigate the effects of pressure (0, 30, 60, 90 and 120 GPa) on the electronic structure of CaTiO₃. The application of pressure from 0 to 90 GPa shows that the indirect band gap (Γ-M) of CaTiO₃ increases with increasing pressure and at 120 GPa it spontaneously decreases transforming cubic CaTiO₃ to a direct (Γ-Γ) band gap material. The complex dielectric function and some optical parameters are also investigated under the application of pressures. All the calculated optical properties have been found to exhibit a shift to the higher energies with the increase of applied pressure suggesting potential optoelectronic device applications of CaTiO₃. The thermoelectric properties of CaTiO₃ have been computed at 0 GPa in terms of electrical conductivity, thermal conductivity and Seebeck coefficient.     

First principle calculation of elastic and thermodynamic properties of stishovite

Using ab initio plane-wave pseudo-potential density functional theory method,the elastic constants and band structures of stishovite were calculated.The calculated elastic constants under ambient conditions agree well with previous experimental and theoretical data.C13,C33,C44,and C66 increase nearly linearly with pressure while C 11 and C 12 show irregularly changes with pressure over 20 GPa.The shear modulus(C11-C12)/2 was observed to decrease drastically between 40 GPa and 50 GPa,indicating acoustic mode softening in consistency with the phase transition to CaCl 2-type structure around 50 GPa.The calculated band structures show no obvious difference at 0 and 80 GPa,being consistent with the high incompressibility of stishovite.With a quasi-harmonic Debye model,thermodynamic properties of stishovite were also calculated and the results are in good agreement with available experimental data.     

In situ X-ray observation of decomposition of hydrous aluminum silicate AlSiO3OH and aluminum oxide hydroxide d-AlOOH at high pressure and temperature

We clarified the stability limit of phase Egg, AlSiO₃OH, a candidate for water reservoir in the siliceous sediment of slabs in the transition zone conditions by in situ X-ray observation using high energy X-ray from synchrotron radiation source of SPring-8. Phase Egg is stable at least up to 1625 °C at 17 GPa. We observed decomposition of phase Egg into δ-AlOOH and stishovite at pressures greater than 23 GPa at temperatures below 1200 °C. No water release occurs associated with the decomposition. At temperatures above 1200 °C at 23 GPa, we observed decomposition of phase Egg into corundum+stishovite+fluid. We also determined the phase boundary of the decomposition reaction of δ-AlOOH to corumdum+fluid based on the in situ X-ray diffraction at high pressure and temperature.     

Pressure-induced spin transitions of iron in MgSiO3 perovskite: a GGA+U study

First-principles calculations using the GGA+U method have been made to investigate the spin transitions of iron in MgSiO₃ perovskite up to 120 GPa, with Fe²⁺ and Fe³⁺ at the A and B sites involving different substitution mechanisms and various charge compensation configurations. Our results, unlike those from previous local density approximation and generalized gradient approximation calculations, are now consistent with available experimental data for the spin states of Fe³⁺ in perovskite. In particular, our calculations show that both Fe²⁺ and Fe³⁺ at the A site do not exhibit any spin transition at the lower mantle conditions. However, Fe³⁺ at the B site in both (Mg_0.9375Al_0.0625)(Si_0.9375Fe_0.0625)O₃ and (Mg_0.9375Fe_0.0625)(Si_0.9375Fe_0.0625)O₃ undergoes a high-spin to low-spin transition at ～ 35 GPa, and this spin-transition pressure is largely independent on substitution mechanisms and charge compensation configurations.     

Theoretical investigations of the physical properties of tetragonal CaSiO3 perovskite

The first principles study of the physical properties of tetragonal CaSiO₃ perovskite is reported using the pseudopotential plane wave method within the local density approximation. The calculated equilibrium volume is in good agreement with the available experimental data. A complete elastic tensor of the tetragonal CaSiO₃ perovskite is determined in the wide pressure range. The geologically important quantities: sound velocity, Young’s modulus, Poisson’s ratio, and crystal anisotropy, are derived from the calculated data. A systematic decrease of crystal’s anisotropy with pressure is noticed except symmetry plane (001). Finally, by using a quasiharmonic Debye model, the Debye temperature, the heat capacity, the coefficient of thermal expansion, and the Grüneisen parameter are also obtained in the present work.     

Phase transformations in the TiO2–SiO2 system at high pressures and temperatures

In situ X-ray diffraction study of a sol–gel-produced SiO₂–TiO₂ glass and intimately mixed ultrafine powders of SiO₂ and TiO₂ was used to investigate the effect of TiO₂ on the high-pressure phase transformations of SiO₂ and specifically on the change in the p,T-conditions of the formation of coesite and stishovite. Our findings have shown that at pressures to 7.5?GPa and temperatures up to 1300?K the presence of TiO₂ does not favor the formation of stishovite.     

High pressure investigation on double perovskite Ba2MgWO6

The results of high-pressure angle dispersive X-ray diffraction measurements up to 34.3 GPa on the double perovskite Ba₂MgWO₆ are presented. The ambient rock salt phase (SG: Fm-3m) is found to be stable up to the highest pressure of the present measurements. The third order Birch-Murnaghan equation of state when fitted to pressure-volume data, yielded a zero pressure bulk modulus (B₀),and its first and second pressure derivatives as 137.0(81) GPa, and 3.9(5) and −0.03 GPa⁻¹, respectively.     

High pressure synthesis at 10 GPa and 1400 K using a small cubic anvil apparatus with a multi-anvil 6-6 system

In the present study, high pressure synthesis up to 10 GPa was done using a small cubic anvil apparatus (W45×D52×H92 cm³, load capacity of 1.80 MN) with a multi-anvil 6-6 system. Its performance was demonstrated by synthesizing a ferromagnetic perovskite oxide, CaCu₃Fe₄O₁₂, at pressure–temperature conditions of 10 GPa and 1400 K. The synthesized CaCu₃Fe₄O₁₂ perovskite was ～1 mm in diameter and ～2 mm in height and its size was large enough for performing magnetic susceptibility measurements at 5–300 K using a superconducting quantum interference device magnetometer and phase identification by X-ray diffraction. The experimental system developed in the present study has many advantages when used in high pressure synthesis experiments, and the technical development of a small cubic anvil apparatus will greatly contribute to the advancement of high pressure synthesis of novel materials.     

高压下钙钛矿结构MgSiO3的分子动力学研究

利用分子动力学方法,研究了高温高压下钙钛矿结构MgSiO3的状态方程.研究表明,分子动力学模拟结果很好地再现了广泛温度和压强范围内钙钛矿结构MgSiO3的摩尔体积.温度300 K压强上升到120 GPa模拟的钙钛矿结构MgSiO3状态方程和有效的实验结果基本一致.在更高温度和更高压强下模拟的钙钛矿结构MgSiO3状态方程和他人的计算值吻合的很好.另外,还分别计算了温度300 K,900 K,1500 K和2500 K压强上升到120 GPa时MgSiO3的体积压缩率.     

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.     

Compression of volume and lattice parameters of 2H-CsCdBr3 under high pressure

Abstract

Powder x-ray diffraction experiments have been performed on 2H-CsCdBr₃. at room temperature up to 25 GPa. At normal pressure this compound shows unidimensional electronic properties. Such unidimensional behaviour is not evident in terms of elastic bulk properties under pressure. No phase transformation occurs in this pressure range. The a and c lattice parameters steadily decrease with pressure; their ratio lowers by only 2% up to 25 GPa. The bulk modulus is low, 21.2 GPa, and is in very good agreement with the bulk modulus-volume systematics for ionic compounds. The value of the first pressure derivative is also typical of ionic compounds.     

First-principles investigation on high-pressure structural evolution of MnTiO3

First-principle calculations using density-functional theory with linearized augmented plane wave method and projector-augmented method have been performed for the high-pressure MnTiO₃ polymorphs and their possible dissociation products. Theoretical results demonstrate that ilmenite-type MnTiO₃ transforms into perovskite phase at 27 GPa and 0 K. The lithium niobate phase of MnTiO₃ is confirmed to be metastable according to its higher Gibbs free energy compared with that of ilmenite at ambient conditions. In ilmenite and lithium niobate phases, MnO₆ octahedra become more distorted while TiO₆ octahedra become more regular with increasing pressure. In orthorhombic perovskite phase, the structural distortion deviated from the ideal cubic perovskite is enhanced at higher pressure. Based on the non-spin-polarized calculations, perovskite phase MnTiO₃ is predicted to dissociate into Fm3?m-MnO+P2₁/c-MnTi₂O₅ at 29 GPa.     

High-pressure thermoelectric characteristics of Bi2Te3 semiconductor with different charge carrier densities

The measurements of the absolute values of the thermopower and of the relative electrical resistance have been performed for n type Bi₂Te₃ under hydrostatic pressure up to 9 GPa at room temperature. Under pressures exceeding 5 GPa and up to the phase transition (at 7 GPa), the samples with the charge carrier density below 10^?19 cm^?3 exhibit an anomalous growth of the thermopower. For the purest sample (n = 10^?18 cm^?3), the thermopower is as high as +150 μV/K. The pressure dependence of the electrical resistance for n-Bi₂Te₃ does not exhibit any anomalies up to the pressure corresponding to the phase transition (7 GPa). Thus, the state with the giant thermoelectric efficiency is found in Bi2Te3 under pressure before the phase transition.     

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.     

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 melting curve of CaSiO3 perovskite under lower mantle pressures

The high pressure melting curve of CaSiO₃ perovskite is simulated by using the constant temperature and pressure molecular dynamics method combined with effective pair potentials for the first time. The simulated results for the partial radial distribution function all compare well with experiment. The calculated equation of state is very successful in accurately reproducing the recent experimental data over a wide pressure range. The predicted high pressure melting curve is in good agreement with the experimental ones, and the melting curve up to the core–mantle boundary pressure, being very steep at lower pressures, rapidly flattens on increasing pressure. The present results also suggest the validity of the experimental data of Zerr and Boehler.     

High pressure X-ray diffraction studies on UAI2, UAI2, and UAI4 systems

Abstract

The UAI₂, UAI₂, AND UAI₄ compounds have been studied by high pressure X-ray diffraction up to a maximum pressure of ～ 35 GPa. The compressibility behaviour of UAI₂ has been found to be consistent with the itinerant 5f states, whereas that of UAI₂ and UAI₄, indicate a more localized nature. Further, UAI₂ has been found to undergo a structural transition at ～ 11 GPa and the structure of the high pressure phase has been identified to be of MgNi₂ type with space group P6₃/mmc. The structure of UAI₂ at ambient pressure is of MgCu₂ type with space group Fd3m. From the electronic considerations, for instance, free electrons per atom ratio e/a, it is anticipated that it may transform back to MgCu₂ type structure at still higher pressures. On similar considerations, it is expected that most of the AB₂ type Laves phase compounds of the ‘f’ electron systems may undergo the structural sequence: MgCu₂ – MgZn₂ (or MgNi₂) – MgCu₂ due to increased delocalization of their ‘f’ electron states.     

High-pressure Raman study of SnGeS3

Abstract

We report Raman-scattering studies of SnGeS₃ under hydrostatic pressures up to 19.5 GPa. An assignment to internal-external modes is proposed, based on the pressure slopes obtained. Our data show evidence for two critical pressures, one around 7 GPa and a second one around 12 GPa. The material renders itself Raman inactive at 19.5 GPa. The observed changes are reversible upon pressure release.     

Phase transitions in poly(ethylene oxide) and polystyrene at high pressure

Abstract

The temperature and enthalpy of melting for poly(ethy1ene oxide) have, for the first time, been studied as a fuction of pressure up to 1 GPa by means of differential scanning calorimetry. The initial increase of the temperature of melting with increasing pressure is 64 K/GPa, whereas the enthalpy decreases by 40% in the 1 GPa pressure range. Using Clausius-Clapeyrons equation the volume change on melting is estimated to be 1.5 cm³/mol. The glass transition temperature T_g for polystyrene has also been studied by the same technique for pressures up to 0.1 GPa. The measurements show that T_g increases with increasing pressure by 250 K/GPa.