Melting curve of NaCl0.5Br0.5 up to 4.5 GPa

Abstract

The melting curve of NaCl_0.5Br_0.5 has been measured under pressure up to 4.5 GPa. The melting temperatures of Ag and NaCl have been used to determine the pressure in the sample at its melting temperature.     

Radial X-Ray Diffraction Study of Static Strength of Tantalum to 80 GPa

We study the strength and texture of tantalum(Ta) under uniaxial compression up to 80 GPa using an angledispersive radial x-ray diffraction technique together with the lattice strain theory in a diamond anvil cell at ambient temperature. The ratio of differential stress to shear modulus(t/G) is found to remain constant above～60 GPa, indicating that the Ta starts to experience macro yield with plastic deformation at this pressure.Combined with independent constraints on the high-pressure shear modulus, we find that the Ta sample could support a differential stress of～4.67 GPa when it starts to yield with plastic deformation at～60 GPa under uniaxial compression. The differential stress in Ta ranges from 0.216 GPa to 4.67 GPa with pressure increasing from 1 GPa to 60 GPa and can be expressed as t-0.199(33)十 0.075(1)P, where P is the pressure in GPa. A maximum differential stress as high as～5.37 GPa can be supported by Ta at the high pressure of～80 GPa. In addition, we investigate the texture of Ta under nonhydrostatic compression to 80 GPa using the software package material analysis using diffraction. It is proven that the plastic deformation due to stress under high pressures is responsible for the development of texture.     

Lindemann criterion and the anomalous melting curve of sodium

Recent reports of the melting curve of sodium at high pressure have shown that it has a very steep descent after a maximum of around 1000 K at 31 GPa. This maximum does not occur due to a solid-solid phase transition. According to the Lindemann criterion, this behaviour should be apparent in the evolution of the Debye temperature with pressure. In this work, we have performed an “ab initio” analysis of the behaviour of both the Debye temperature and the elastic constants up to 102 GPa, and find a clear trend at high pressure that should cause a noticeable effect on the melting curve.     

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.     

铅的高压状态方程与熔化曲线

基于Gray三相状态方程模型,采用GRIZZLY的Grüneisen系数模型,对铅的冲击雨贡纽线、等温压缩线、等熵压缩线和熔化曲线进行了系统计算,优化参数,获得与实验数据一致的冲击雨贡纽线、等温压缩线与等熵压缩线,计算所得熔化温度线与X射线衍射实验结果吻合得很好。     

Lindemann''s criterion and the necessary condition for a melting curve maximum

Dielectric and differential microcalorimetry measurements have been realized on the $\text{Bi}{}_{\mathrm{1?x}}\text{Te}{}_{\mathrm{x}}\text{O}{}_{\mathrm{<mtext>3+x</mtext><mtext>2</mtext>}}$ (0,33 ? x ? 0,50) solid solution. A phase transition has been detected. The variation of the transition temperature with the composition has been determined. The low temperature phase, stable at 20°C, has piezoelectric and non-linear optical properties.     

First-principles simulation of Raman spectra and structural properties of quartz up to 5 GPa

The crystal structure and Raman spectra of quartz are calculated by using first-principles method in a pressure range from 0 to 5 GPa. The results show that the lattice constants(a, c, and V) decrease with increasing pressure and the a-axis is more compressible than the c axis. The Si–O bond distance decreases with increasing pressure, which is in contrast to experimental results reported by Hazen et al. [Hazen R M, Finger L W, Hemley R J and Mao H K 1989 Solid State Communications 725 507–511], and Glinnemann et al. [Glinnemann J, King H E Jr, Schulz H, Hahn T, La Placa S J and Dacol F 1992 Z. Kristallogr. 198 177–212]. The most striking changes are of inter-tetrahedral O–O distances and Si–O–Si angles. The volume of the Si O4-4tetrahedron decreased by 0.9%(from 0 to 5 GPa), which suggests that it is relatively rigid.Vibrational models of the quartz modes are identified by visualizing the associated atomic motions. Raman vibrations are mainly controlled by the deformation of the Si O4-4tetrahedron and the changes in the Si–O–Si bonds. Vibrational directions and intensities of atoms in all Raman modes just show little deviations when pressure increases from 0 to 5 GPa.The pressure derivatives(dνi/d P) of the 12 Raman frequencies are obtained at 0 GPa–5 GPa. The calculated results show that first-principles methods can well describe the high-pressure structural properties and Raman spectra of quartz. The combination of first-principles simulations of the Raman frequencies of minerals and Raman spectroscopy experiments is a useful tool for exploring the stress conditions within the Earth.     

Shifts in the 3He melting curve due to pore condensation

We have measured the ³He melting curve, first using a cell with an open geometry, then using the same cell filled with MgO powder. In the latter case, we found that below 200 mK the apparent melting curve is shifted.     

Pressure effects on EXAFS Debye-Waller factor and melting curve of solid krypton

The pressure effects on atomic mean-square displacement, extended X-ray absorption fine structure (EXAFS) Debye-Waller factor and melting temperature of solid krypton have been investigated in within the statistical moment method scheme in quantum statistical mechanics. By assuming the interaction between atoms can be described by Buckingham potential, we performed the numerical calculations for krypton up to pressure 120?GPa. Our calculations show that the atomic mean-square displacement and EXAFS Debye-Waller factor of krypton crystal depend strongly on pressure. They make the robust reduction of the EXAFS peak height. Our results are in good and reasonable agreements with available experimental data. This approach gives us a relatively simple method for qualitatively calculating high-pressure thermo-physical properties of materials. Moreover, it can be used to verify future high-pressure experimental and theoretical works.     

Anomalous influence of an external magnetic field on spin fluctuations in magnetic semiconductors with strong p-d hybridization and the colossal magnetoresistance effect

The colossal magnetoresistance effect in magnetic semiconductors based on lanthanum manganites has been investigated in terms of the model allowing for the effects of p-d hybridization and electronelectron Coulomb correlations. The influence of an external magnetic field on spin fluctuations has been considered under the conditions where the chemical potential is in a narrow heavy-fermion band formed in the hybridization gap. It has been shown that, in the vicinity of the Curie point T _C, the strong spin anharmonicity leads to an anomalously strong suppression of spin fluctuations by the external magnetic field, a phenomenon contributing significantly to the formation of colossal negative magnetoresistance.     

Triple point on the melting curve and polymorphism of nitrogen at high pressure

Raman spectra of solid and fluid nitrogen to pressures up to 120 GPa and temperatures up to 2500 K reveal that the melting line exhibits a maximum near 70 GPa, followed by a triple point near 87 GPa, after which the melting temperature rises again. Fluid nitrogen remains molecular over the entire pressure range studied, and there is no sign of a fluid-fluid transition. Solid phases obtained on quenching from the melt above 48 GPa are identical to the recently discovered iota and zeta' phases. We find that kinetics plays a major role in the experimentally observed phase changes and account for the metastability of various crystalline molecular phases and the existence of an amorphous single bonded eta-N.     

Lindemanns' criterion and the necessary condition for a melting curve maximum

Assuming that the generalized Lindemann's criterion adequately describes the melting process, it is shown that the softening of a branch of the frequency spectrum with pressure is associated with the occurrence of fusion curve maximum as well as a negative melting slope. By characterizing the number of “soft” modes through 〈γ〉 and σ^{$\text{1}\text{2}$}, the average and approximate width of the mode gamma distribution of the solid, a qualitative as well as a quantitative condition for a fusion curve maximum can be established for a model potential.     

Strength and equation of state of molybdenum triboride under 80 GPa pressure,using radial X-ray diffraction

The strength and equation of state of molybdenum triboride have been determined under nonhydrostatic compression up to 80?GPa, using an angle-dispersive radial X-ray diffraction technique in a diamond anvil cell (DAC). The RXD data yield a bulk modulus and its pressure derivative as K_0?=?342(6)?GPa with K₀′?=?2.11(17) at ψ?=?54.7°. Analysis of diffraction data using the strain theory indicates that the ratio of differential stress to shear modulus (t/G) ranges from 0.002 to 0.050 at pressures of 4–80?GPa. Together with theoretical results on the high pressure shear modulus, our results here show that molybdenum triboride sample under uniaxial compression can support a differential stress of ～10?GPa when it started to yield with plastic deformation at ～30?GPa. In addition, we draw a conclusion that MoB₃ is not a superhard material but a hard material.     

CO气体在5 000 K和25 GPa条件下的动力学研究

 利用第一性原理分子动力学方法,对炸药降解产物CO在温度5 000 K、压力25 GPa条件下的化学过程进行了研究。计算得到的最终产物主要由C₃O₂、CO₂和C₂O₂组成。通过对CO、C₃O₂、CO₂和C₂O₂组成的混合物体系的振动性质进行分析,发现计算得到的红外光谱与一些理论和实验结果一致;体系的径向分布函数表明,最终产物具有高密度流体的特征;同时还分析了CO、C₃O₂和CO₂的态密度。结果表明：CO在温度5 000 K、压力25 GPa 这一特定条件下的反应主要归因于高温高压下CO分子间的剧烈碰撞;在形成这些产物的过程中没有发现有明显的过渡态存在。