高压电导研究

高压下物质结构和性质会发生很大的改变，电导对于认识压力的作用效应和寻找新的导电材料具有重要意义．高压电导研究所采用的设备主要有金刚石压腔、超高压大腔体装置(大压机)以及高压电导池三种．金刚石压腔可达550GPa压力；超高压大腔体装置压力一般在30GPa以下，可以提供较大体积的样品；高压电导池主要进行流体的电导研究，压力在400MPa以下，有静止和流动两种类型，流动式高压电导池是近年来才发展起来的，其准确方便．文章对目前高压电导的研究进行简单的介绍和分析．     

高压电导研究

高压下物质结构和性质会发生很大的改变,电导对于认识压力的作用效应和寻找新的导电材料具有重要意义.高压电导研究所采用的设备主要有金刚石压腔、超高压大腔体装置（大压机）以及高压电导池三种.金刚石压腔可达550GPa压力;超高压大腔体装置压力一般在30GPa以下,可以提供较大体积的样品;高压电导池主要进行流体的电导研究,压力在400MPa以下,有静止和流动两种类型,流动式高压电导池是近年来才发展起来的,其准确方便.文章对目前高压电导的研究进行简单的介绍和分析.     

原位高压微米氧化锌电学性质的研究

利用集成有金属薄膜电极的金刚石对顶砧,对微米氧化锌样品进行了原位高压电导率测量.结果表明,在919 GPa时样品电导率达到最小值,在919—1122 GPa时样品电导率急剧增大,说明此时样品从纤锌矿结构向岩盐矿结构转变直至完全相变,1122　GPa为相变点.通过测量不同条件下高温退火处理的样品电导率,明显看到氧空位对电导率的影响. 关键词： 高压 微米氧化锌 电导率 金刚石对顶砧     

Diamond cleavage: importance to high pressure research

The extraordinary properties of diamond make it the number-one choice for anvils in high pressure experiments involving anvil cells. In much of the literature on the properties of diamond the only cleavage mentioned is {111}. However, experience has shown that diamond anvils made with their [001] axis oriented in line with the principal stress axis of the anvil or at a small angle to it often failed with flat faces having {110} orientations; a cleavage plane is reported in some publications. Analysis of the anisotropy of strength and Poisson ratio in diamond has shown that such orientations do, indeed, favor initial failure on {110} cleavage planes. This analysis, in conjunction with stereographic projections of the {111} and {110} cleavage planes, suggests that a 27° tilt of the [001] axis with respect to the linear stress axis by rotation around the [100] or [010] axis should provide significantly greater resistance to failure by cleaving.     

The dehydration kinetics of gypsum at high pressure and high temperature

An in situ dehydration kinetics study of gypsum under water-saturated condition was performed in the temperature and pressure ranges of 383–423?K and 343–1085?MPa by using a hydrothermal diamond anvil cell and Raman spectroscopy. Kinetic analysis shows that the dehydration rate k increases with pressure, suggesting a negative pressure dependence on dehydration rate. The elevation of temperature can contribute to the dehydration. The n values increase with pressure, indicating that the nucleation process becomes slower relative to the growth process. According to the n values of ～1.0, the dehydration of gypsum is dominated by an instantaneous nucleation and diffusion-controlled growth mechanism. The obtained average activation volume ?V is equal to 5.69?cm³/mol and the calculated activation energy E_a and the pre-exponential factor A are 66.9?kJ/mol and 4.66?×?10⁵?s^?1. The activation energy may be dependent upon grain size, shape, temperature and pressure, and surrounding water.     

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).     

Raman study of datolite CaBSiO4(OH) at simultaneously high pressure and high temperature

Using an in situ method of Raman spectroscopy and resistance‐heated diamond anvil cell, the system datolite CaBSiO₄(OH) – water has been investigated at simultaneously high pressure and temperature (up to Р ~5 GPa and Т ~250 °С). Two polymorphic transitions have been observed: (1) pressure‐induced phase transition or the feature in pressure dependence of Raman band wavenumbers at P = 2 GPа and constant T = 22 °С and (2) heating‐induced phase transition at T ~90 °С and P ~5 GPа. The number of Raman bands is retained at the first transition but changed at the second transition. The first transition is mainly distinguished by the changes in the slopes of pressure dependence of Raman peaks at 2 GPa. The second transition is characterized by several strong changes: the wavenumber jumps of major bands, the merging of strong doublets at 378 and 391 cm⁻¹ (values for ambient conditions), the splitting of the intermediate‐intensity band at 292 cm⁻¹, and the transformation of some low‐wavenumber bands at 160–190 cm⁻¹. No spectral and visual signs of overhydration and amorphization have been observed. No noticeable dissolution of datolite in the water medium occurred at 5 GPa and 250 °С after 3 h, which corresponds to typical conditions of the ‘cold’ zones of slab subduction. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis of Ge-Sn at high pressure and high temperature in a laser-heated diamond anvil cell

Ge–Sn compound is predicted to be a direct band gap semiconductor with a tunable band gap. However, the bulk synthesis of this material by conventional methods at ambient pressure is unsuccessful due to the poor solubility of Sn in Ge. We report the successful synthesis of Ge–Sn in a laser-heated diamond anvil cell (LHDAC) at ~7.6 GPa &; ~2000 K. In situ Raman spectroscopy of the sample showed, apart from the characteristic Raman modes of Ge TO (Г) and β-Sn TO (Г), two additional Raman modes at ~225 cm^?1 (named Ge–Sn₁) and ~133 cm^?1 (named Ge–Sn₂). When the sample was quenched, the Ge–Sn₁ mode remained stable at ~215 cm^?1, whereas the Ge–Sn₂ mode had diminished in intensity. Comparing the Ge–Sn Raman mode at ~225 cm^?1 with the one observed in thin film studies, we interpret that the observed phonon mode may be formed due to Sn-rich Ge–Sn system. The additional Raman mode seen at ~133 cm^?1 suggested the formation of low symmetry phase under high P–T conditions. The results are compared with Ge–Si binary system.     

Anvil design for slip-free high pressure deformation experiments in a rotational diamond anvil cell

A rotational diamond anvil cell is the most suitable deformation apparatus with which to investigate the rheological properties of deep-Earth materials at pressures similar to those found in the lower mantle and core. However, slip between the sample and piston is still a problem, since the slip prevents the attainment of a constant strain rate and interferes with the uniform deformation of a sample. In this paper, we report that using a diamond anvil with deep grooves results in a marked improvement in the coupling between the sample and the diamond anvils.     

A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.     

Raman G‐mode of single‐wall carbon nanotube bundles under pressure

Raman studies of nanotubes under pressure have been a lively area of research. However, the results are not always as expected and at times have not been adequately explained. One example of the diversity of the results is the higher energy Raman mode (the graphitic mode, GM) shift to higher wavenumber under pressure. Here we report a new high‐pressure Raman study showing that the effects of the variation in the tube diameters and the pressure transmitting medium are both crucial for understanding the outcomes of such high‐pressure experiments. Copyright © 2011 John Wiley & Sons, Ltd.     

Momentum‐resolved resonant inelastic X‐ray scattering on a single crystal under high pressure

A single‐crystal momentum‐resolved resonant inelastic X‐ray scattering (RIXS) experiment under high pressure using an originally designed diamond anvil cell (DAC) is reported. The diamond‐in/diamond‐out geometry was adopted with both the incident and scattered beams passing through a 1 mm‐thick diamond. This enabled us to cover wide momentum space keeping the scattering angle condition near 90°. Elastic and inelastic scattering from the diamond was drastically reduced using a pinhole placed after the DAC. Measurement of the momentum‐resolved RIXS spectra of Sr_2.5Ca_11.5Cu₂₄O₄₁ at the Cu K‐edge was thus successful. Though the inelastic intensity becomes weaker by two orders than the ambient pressure, RIXS spectra both at the center and the edge of the Brillouin zone were obtained at 3 GPa and low‐energy electronic excitations of the cuprate were found to change with pressure.     

Ionic liquids based on the bis(trifluoromethylsulfonyl)imide anion for high‐pressure Raman spectroscopy measurements

Assembling a diamond anvil cell for high‐pressure measurements involves placing in a gasket hole the sample of interest, a pressure transmitting fluid, and a material for pressure calibration. In this communication, we propose the use of ionic liquids containing the bis(trifluoromethylsulfonyl)imide anion ([Tf₂N]^‐), [(CF₃SO₂)₂ N]^‐, as a simultaneous pressure transmitting and calibrant material for high‐pressure Raman spectroscopy measurements of solid samples that are not soluble in ionic liquids. The position of the characteristic Raman band of the [Tf₂N]^‐ anion at 740 cm⁻¹ exhibits linear frequency shift for pressures up to 2.5 GPa. High‐pressure Raman spectra of different ionic liquids containing the same anion indicate that the actual magnitude of the pressure‐induced frequency shift of the [Tf₂N]^‐ normal mode depends on the counterion, the typical shift being 4.2 cm⁻¹/GPa. Ionic liquids based on the [Tf₂N]^‐ anion are also good pressure transmitting mediums because hydrostatic condition is kept at high pressure, and no crystallization is observed up to 4.0 GPa. Copyright © 2012 John Wiley & Sons, Ltd.     

Equal compressibility structural phase transition of molybdenum at high pressure

We have studied the high-pressure compression behavior of molybdenum up to 60 GPa by synchrotron radial x-ray diffraction (RXRD) in a diamond anvil cell (DAC). It is found that all diffraction peaks of molybdenum undergo a split at around 27 GPa, and we believe that a phase transition from a body-centered cubic structure to a rhombohedral structure at room pressure has occurred. The slope of pressure-volume curve shows continuity before and after this phase transition, when fitting the pressure-volume curves of the body-centered cubic structure at low pressure and the rhombohedral structure at high pressure. A bulk modulus of 261.3 (2.7) GPa and a first-order derivative of the bulk modulus of 4.15 (0.14) are obtained by using the nonhydrostatic compression data at the angle ψ = 54.7° between the diffracting plane normal and stress axis.     

X-ray absorption spectroscopy under high pressures in diamond anvil cells

Abstract

Although potentially extremely important for understanding the high-pressure microscopic behaviour of materials, over the years the area of high-pressure EXAFS in particular using diamond anvil cells has proved to be technically difficult. This has significantly hampered its development. The interference of X-ray dimaction from the diamonds in the diamond anvil cell with the absorption signal has proved to be a challenging problem to tackle, restricting the use of high-pressure EXAFS to energies below about 11 key Below 11 keV however the technique is also limited due to absorption of incident X-rays by the diamonds making it virtually impossible to conduct X-ray absorption experiments below about 9keV In this paper we describe in detail the methodology for scanriirig high-pressure EXAFS in diamond anvil cells and examine the origins of the associated problems and ways of dealing with them. We also demonstrate that it is possible to extend the useful range of studied absorption edges from 7keV up to at least 30keV This brings about new opportunities for high pressure EXAFS using diamond anvil cells.     

High-energy X-ray diffraction of a hydrous silicate liquid under conditions of high pressure and temperature in a modified hydrothermal diamond anvil cell

In situ high-energy X-ray diffraction measurements were made for the first time on a water-saturated silicate melt at high pressure and temperature. A modified hydrothermal diamond anvil cell (HDAC), designed to minimize the path length of the X-ray beam within a diamond anvil and to increase the solid angle of the diffracted beam, was used to reduce high background contributions and extend X-ray diffraction data collection in Q space. Quantitative differential pair distribution function (PDF) analysis of X-ray diffraction data show that the first measurable (Si–O) peak is 0.095 Å greater in length in the hydrous melt than in the starting glass. Contributions from the H₂O O–O correlations, as well as from the second nearest neighbor O–O correlations within the silicate melt, are evident within the second peak of the differential PDF. The procedure described opens new opportunities to directly investigate volatile-rich melts at high pressure and temperature.     

Preparation of a macrocrystalline pressure calibrant SrB4O7:Sm 2+ suitable for the HP-HT powder diffraction

A new simplified synthesis of monocrystalline chips of SrB₄O₇: Sm²⁺ pressure calibrant, well-suited for the diamond anvil cell (DAC) powder diffraction experiments, is proposed. It consists of ordinary solid-state synthesis of fine-grained SrB₄O₇: Sm²⁺ and subsequent annealing near melting temperature. The obtained material was characterized and tested in HP-HT DAC experiment.     

基于拉曼频移的白宝石压腔无压标系统高温高压实验标定

在高压实验科学中, 各类宝石压腔是最为常见的高压设备之一, 其样品腔中压力的精确标定是实验的关键. 目前, 人们主要通过加入红宝石等压标物质来进行定压, 但压标物质的加入会增加实验的装样难度, 改变样品腔中的物理化学环境, 甚至直接与实验样品发生反应, 从而对实验结果产生影响. 在0–6.3 GPa和300–573 K下, 利用共聚焦拉曼显微镜, 根据白宝石压砧砧面的ν₁₂ 拉曼频移与温度和压力的变化关系, 建立了一套适用于高温高压水热体系的无压标白宝石压腔系统. 实验结果表明: 白宝石砧面的ν₁₂ 峰随着压力的升高发生线性蓝移, 而随着温度升高则发生线性红移, 且温度和压力对拉曼频移的影响存在耦合效应. 利用本实验结果, 可在高温高压下根据白宝石砧面的拉曼频移计算出样品腔的压力P=(Δλ-0.01913×ΔT)/(1.9158-0.00105×ΔT), 在物理学、材料学和地球科学等领域具有重要应用.     

ZnO纳米线在压力下的电学性质

 在金刚石压砧上测量了ZnO纳米线R-p和C-p关系,并和原先测量的纳米晶及体相ZnO的结果进行了比较。研究结果表明,ZnO纳米线在5.0 GPa左右发生相转变。其间的电阻-压力曲线呈指数下降趋势,电容呈指数上升趋势;在相转变之后电阻迅速上升,电容急速下降（指数下降）,这与一般的块状和纳米晶的相转变都有所不同,表现出了纳米线与非线状纳米晶的差别。