High-pressure melting of molybdenum

The melting curve of the body-centered cubic (bcc) phase of Mo has been determined for a wide pressure range using both direct ab initio molecular dynamics simulations of melting as well as a phenomenological theory of melting. These two methods show very good agreement. The simulations are based on density functional theory within the generalized gradient approximation. Our calculated equation of state of bcc Mo is in excellent agreement with experimental data. However, our melting curve is substantially higher than the one determined in diamond anvil cell experiments up to a pressure of 100 GPa. An explanation is suggested for this discrepancy.     

Lattice dynamics of MgSiO3 perovskite

A lattice dynamical study of the geophysically important mineral MgSiO₃ in its orthorhombic perovskite phase, with space group Pnma (D _2h ¹⁶ ) has been carried out using a rigid ion model, with the potential consisting of Coulombic and short-ranged interactions. With the help of program DISPR, the ionic charges and radii were optimized using the equilibrium conditions. The resulting potential model is employed to predict the elastic constants and the phonon dispersion relations. The computed long wavelength optic modes are in good agreement with the corresponding experimental Raman and infrared active bands. The phonon density of states has been obtained and is used to evaluate the specific heat, the mean square displacements and thermal parameters of atoms.     

MgSiO3状态方程的分子动力学模拟

利用分子动力学方法,研究了高温高压下钙钛矿结构MgSiO_3的状态方程。研究表明,分子动力学模拟结果精确地再现了广泛温度和压强范围内MgSiO_3的摩尔体积。在300 K压强上升到140 GPa模拟的MgSiO_3状态方程和有效的实验值、他人的拟合值以及基于局域密度近似的第一原理计算结果基本一致。并且更高温度和更高压强下模拟的MgSiO_3状态方程和他人的计算值吻合的很好。另外,还分别计算了300、900、2000和3000 K压强上升到120 GPa时MgSiO_3的体积压缩率。     

High-pressure melting curve of nitrogen and the liquid-liquid phase transition

The melting curve of nitrogen was measured up to 71 GPa, a fourfold increase in pressure over previous measurements. The measurements were made using the laser-heated diamond anvil cell and melting was detected in situ by the laser speckle method. The melting temperature rises linearly up to a maximum at 50 GPa and 1920 K, and with increasing pressure suddenly decreases linearly to 1400 K at 71 GPa. This sharp drop in the melting slope (dT/dP) above 50 GPa indicates the appearance of a liquid denser than the solid and of a liquid-liquid phase transition. The sharpness of the changes suggests that the transition is first order and is a liquid-liquid polymer transition. This conclusion is consistent with earlier theoretical studies and experimental evidence that pressure transforms molecular nitrogen into a chainlike polymeric form.     

Thermoluminescence and defect study of MgSiO3 ceramics

Enstatite (MgSiO₃) ceramic powders were synthesised by a low-temperature initiated self-propagating, gas-producing solution combustion process. The prepared powders were characterised by powder X-ray diffraction, scanning electron microscopy and Brunauer–Emmer–Teller specific surface area measurements. Defect centres induced by radiation were studied using the techniques of thermoluminescence (TL) and electron spin resonance (ESR). A well-resolved glow with peak at 178°C and a shouldered peak at 120°C were observed. Two defect centres were identified by ESR measurements, which were carried out at room temperature, and these were assigned to an O^? ion and F⁺ centre. The O^? ion (hole centre) appears to correlate with the main TL peak at 178°C.     

High-pressure differential thermal analysis measurements of the melting curve of lithium

The pressure effect on the melting behavior of lithium has been measured by observing the latent heat signatures of melting and freezing using differential thermal analysis (DTA). Samples were repeatedly melted and recrystallized at selected pressures up to 15 GPa in a multianvil press. Despite the weak DTA signals due to small sample sizes at high pressure, the melting and freezing temperatures were clearly determined from the derivatives of the DTA traces. We measured a drop in the melting temperature between 9 and 12 GPa, yielding a maximum at 10(2) GPa and 245(2) ^°C. This work highlights both the successes and failures of recent theoretical models for the melting behavior of this and related systems.     

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.     

Shear response of Fe-bearing MgSiO3 post-perovskite at lower mantle pressures

We investigate the shear response of possible slip systems activated in pure and Fe-bearing MgSiO₃ post-perovskite (PPv) through ab initio generalized stacking fault (GSF) energy calculations. Here we show that the [100](001) slip system has the easiest response to plastic shear among ten possible slip systems investigated. Incorporation of Fe²⁺ decreases the strength of all slip systems but does not change the plastic anisotropy style. Therefore, pure and Fe-bearing MgSiO₃ PPv should demonstrate similar LPO patterns with a strong signature of the [100](001) slip system. An aggregate with this deformation texture is expected to produce a V_SH > V_SV type polarization anisotropy, being consistent with seismological observations.     

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.     

High-pressure single-crystal studies of MnTiO3

Abstract

Recent high-pressure single-crystal x-ray diffraction experiments have provided new information on the crystal chemistry of MnTiO₃ and have provided insight into polymorphic transitions among phases of this composition. The stable polymorph of MnTiO₃ at room P and T has the ilmenite structure. At high P and T, MnTiO₃ ilmenite (MnTiO₃ II) transforms to a LiNbO₃ structure through a cation reordering process, and the quenched LiNbO₃-structure phase transforms to a perovskite structure as the pressure is again increased. This transition is unique in that twinned MnTiO₃ II crystals transform under pressure to untwinned crystals having the perovskite structure. The back-transformation of perovskite to the LiNbO₃ structure as pressure is released is similar to that observed previously for the rutile-type dioxides TiO₂ and SnO₂.     

下地幔条件下钙钛矿结构MgSiO3熔化行为的研究

利用分子动力学方法结合有效的对势,模拟了下地幔条件下钙钛矿结构MgSiO3的熔化曲线．研究表明,分子动力学模拟结果精确地再现了广泛压强范围内钙钛矿结构MgSiO3的状态方程,并且熔化曲线与最新的实验结果也符合的很好．在压强上升到下地幔压强范围内,压强低于60 GPa时的钙钛矿结构MgSiO3熔化曲线比较陡,接着变得平缓．在核幔边界压强135 GPa时,钙钛矿结构MgSiO3的熔化温度是6500 K,明显低于Zerr和Boehler实验结果的外推结果．     

新的相互作用势对MD模拟钙钛矿结构MgSiO3热力学性质的影响

通过分析势能曲线解释了钙钛矿结构MgSiO3熔化模拟过程中模拟熔化温度存在较大差异的原因，并进一步研究了对势参数在分子动力学模拟中的影响. 通过调整已有的经验势得到了一组新的势参数，以此来进行分子动力学研究，得到的常温常压下摩尔体积与Belonoshko和Dubrovinsky的结果符合较好，并且其状态方程、常压下热容和常压下热膨胀系数与他人的实验值都较好地吻合. 另外，所得到的熔化温度也与以前的研究进行了比较.     

Spin-state crossover and hyperfine interactions of ferric iron in MgSiO(3) perovskite

Using density functional theory plus Hubbard U calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O(3) perovskite, the major mineral phase in Earth's lower mantle, has high-spin ferric iron (S=5/2) at both dodecahedral (A) and octahedral (B) sites. With increasing pressure, the B-site iron undergoes a spin-state crossover to the low-spin state (S=1/2) between 40 and 70 GPa, while the A-site iron remains in the high-spin state. This B-site spin-state crossover is accompanied by a noticeable volume reduction and an increase in quadrupole splitting, consistent with recent x-ray diffraction and M?ssbauer spectroscopy measurements. The anomalous volume reduction leads to a significant softening in bulk modulus during the crossover, suggesting a possible source of seismic-velocity anomalies in the lower mantle.     

三氧化二镓的高压拉曼光谱研究

β-Ga2O3是一种宽禁带半导体材料(Eg=4.8 eV).研究β-Ga2O3在高压(高应力)条件下的相稳定性和晶格动力学特性对其材料应用具有重要的参考价值.目前关于Ga2O3在高压下的晶格动力学特性研究较少,且Ga2O3的β→α的高压相变压力仍然具有争议.本工作采用基于金刚石压砧(DAC)的高压拉曼光谱技术研究了Ga...     

High-pressure melting curves of argon, krypton, and xenon: deviation from corresponding states theory.

The melting curves of argon, krypton, and xenon were measured in a laser heated diamond-anvil cell to pressures of nearly 80 GPa reaching melting temperatures around 3300 K. For the three gases, we observed a considerable decrease in the melting slopes (dT/dP) from the predictions based on corresponding states scaling starting near 40, 30, and 20 GPa, respectively. The melting anomaly can be understood in terms of a model in which hcp stacking faults act as solutes in a binary system.     

MgSiO3钙钛矿高温高压特性的第一性原理研究

本文采用第一性原理方法,计算了MgSiO3钙钛矿在零温和0 ~ 150 GPa静水压范围内的晶体结构和弹性模量,并利用准简谐近似Debye模型,拟合三阶Birch-Murnaghan物态方程得到了其高温高压下的热力学性质。通过与现有的理论和实验的结果数据比较,确认在0 ~ 2000 K的温度区间内,第一性原理计算结合Debye模型能够较可靠地模拟在下地幔压力范围内MgSiO3钙钛矿的热力学性质。     

含铁后钙钛矿(Fe_xMg_(1-x))SiO_3的自旋、结构以及地震波特性的第一性原理研究

运用基于密度泛函的第一性原理方法,对含Fe2+后钙钛矿相MgSiO3在高压下的性质进行了计算。计算结果表明,在0~160GPa的压强范围内Fe2+始终保持高自旋状态,但在高压下有转向中间自旋或低自旋的趋势。对于含有多个Fe2+的结构,通过比较可能结构的能量以及结合力学稳定性判据,发现Fe2+倾向于以最紧密方式替位占据Mg2+的位置。还计算了120GPa(D〃层压强)下的弹性波速,表明无论压缩波(P波)还是剪切波(S波),随着Fe2+浓度的增加,地震波速明显减小,并且S波的横向各向异性明显增强。