Raman spectroscopic study of the phase transitions induced by hydrostatic pressure in a Rb2KScF6 crystal

The Raman spectra of Rb₂KScF₆ elpasolite crystals are studied in the pressure range up to 7 GPa. A phase transition is revealed at a pressure of approximately 1 GPa. Analysis of the changes in the spectral parameters shows that the phase transition is accompanied by a doubling of the volume of the primitive cell in the initial cubic phase. Judging from the character of the variations in the pressure dependences of the frequency of the observed vibrations, there can exist another transition to the phase with a lower symmetry at a pressure of approximately 2.1 GPa.     

Kinetics of phase transitions under high pressure

Abstract

Phase transitions under high pressure have attracted increasing attention i n connection with high pressure synthesis of new materials (e.g. superhard materials, ceramics, semiconductors, high temperature superconductors) and the exploration of geological processes like the formation of rocks and minerals. For t h e investigation of thermodynamic and electronic equilibrium properties a broad spectrum of methods for pressure generation and physical measurements have been developed ^1,2,3,4. Tha measurement of equilibrium properties, however, gives only poor insight into the detailed mechanisnts of phase changes. The accurate determination of phase equilibria and phase diagrams becomes inore and more difficult a t low temperatures due to increasing hysteresis effects end t h e extrapolation of experimental data to zerc, teiiipwature a nd the compar ison with quantumiiiechanical ah in.itio calculations are questionable.     

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.     

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.     

A raman study of hydrostatic pressure induced phase transitions in Rb2KInF6 crystals

The Raman spectra of the elpasolite (Rb₂KInF₆) crystal have been studied in the pressure range from 0 to 5.3 GPa at a temperature of 295 K. A phase transition at a pressure of approximately 0.9 GPa has been found. An analysis of the variations in the spectral parameters has led to the conclusion that the phase transition to a distorted phase is accompanied by the doubling of the volume of the primitive cell of the initial cubic phase. Numerical calculations of the lattice dynamics in the Rb₂KInF₆ crystal have been performed. The numerical simulation has established that the phase transition at a pressure of 0.9 GPa is associated with condensation of the F _lg mode. A probable high-pressure phase is the phase with space group C2/m.     

The high pressure phase transitions in KF and RbF

The high pressure B1 ? B2 transitions in KF at ~ 40 kbar and RbF at ~ 30 kbar have been studied using hydrostatic X-ray diffraction. No other transitions have been observed. The addition of the $\text{ΔV}\text{V}$ for these transitions to the already existing body of literature on B1–B2 transitions in alkali halides permits an extension of Pauling's theory to larger values of radius ratios. It also permits the modified Born criterion for predicting phase transitions to be further verified. Values of ionic radii for 8 coordination we suggest are 1.33 Å for F^?, 1.84 Å for Cr^?, 2.00 Å for Br^? and 2.27 Å for I^?.     

Raman scattering study of temperature and hydrostatic pressure phase transitions in Rb2KTiOF5 crystal

Raman spectra of Rb₂KTiOF₅ crystal were obtained and analyzed in the temperature range from 77 to 297 K and under hydrostatic pressure up to 4.2 GPa (at T = 295 K). The experimental results were compared with quantum‐chemical simulation of TiOF₅ pseudo‐octahedron. To interpret effects of lattice ordering, phonon spectra of several ordered phases of Rb₂KTiOF₅ were calculated within ab initio generalized Gordon–Kim model, and ordering of TiOF₅ molecular groups were simulated within Monte Carlo approach. The spectra exhibited orientation disordering in the cubic phase under ambient conditions. Cooling below the phase transition temperature (215 K) leads to partial ordering of the structure. The isotropic perovskite‐like phase was found to undergo first‐order transition into a low‐symmetry anisotropic phase at about 1 GPa. Further compression up to 4.1 GPa did not show any effects associated with phase transitions. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman study of phase transitions in rare earth vanadates

The phase transitions in DyVO₄ at about 14K and in TbVO₄ at about 33K have been investigated by Raman scattering. The E_g phonons are split below the transition temperature but the singlet phonon modes are not appreciably affected. No soft phonon mode is observed but large changes are seen in the low lying electronic levels. These results appear to be a consequence of a cooperative Jahn-Teller effect.     

A CCD based detector for quick detection of pressure induced phase transitions

Abstract

A high sensitivity CCD based two dimensional angle dispersive X-ray are a detector has been developed for quick detection of pressure induced phase transitions for a laboratory X-ray source such as a rotating anode generator. The performance of this detector was tested by successfully carrying out powder X-ray diffraction measurements on element Pd, intermetallics AuIn², AuGa² and low Z scatterer adamantane (C₁₀H_l6) at ambient conditions. Its utility for quick detection of phase transitions at high pressures with diamond anvil cell (DAC) is demonstrated by reproducing the known pressure induced structural phase transitions in RbI and KI. The importance of this detector system in search of unknown phase transitions has been established by observing new structural phase transitions in In_0.25Sn_0.75 and AuGa₂. Various softwares have also been developed such as interactive location of centre of diffraction rings, radial integration and image enhancement to analyze data from this detector.     

Structures and phase transitions of ScH3 under high pressure

The structures and the phase transitions of ScH3 under high pressure are investigated using first-principles calculations. The calculated structural parameters at zero pressure agree well with the available experimental data. With increasing pressure, the transition sequence hcp (GdH3 -type)→ C2/m →fcc→hcp (YH3-type)→Cmcm of ScH3 is predicted first; the corresponding transition pressures at 0 K are 23 GPa, 25 GPa, 348 GPa, and 477 GPa, respectively. The C2/m symmetry structure is a possible candidate but not a good one as the intermediate state from hexagonal to cubic in ScH3 . On the other hand, via the analysis of the structures of hexagonal ScH2.9 , cubic ScH3 , and cubic ScH2 , we find that the repulsive interactions of H-H atoms must play an important role in the transition from hexagonal to cubic.     

A bibliographical review of the high pressure studies of ferroelectric phase transitions

A brief review of the various high pressure studies on the phase transitions in ferroelectric crystals carried out over the past two decades is presented. The theoretical framework in which high pressure studies have to be viewed is given. The pressure data of ferroelectric crystals are organized in the form of tables. High pressure spectroscopic and structural studies are included. The review contains a bibliography of over two hundred references.     

Anomalous superconductivity and pressure induced phase transitions in black phosphorus

Pressure induced superconductivity in black phosphorus was measured along three different paths in P-T diagram. High-Tc was observed when pressure was continuously increased at liquid helium temperature. Pressure induced phase transitions has been also studied. A mechanism of the phase transition is closely related to the anomalous superconductivity. The high-Tc superconductivity of black-P may be attributed to the existence of superconducting phase in metallic phase.     

A new differential thermal analysis setup for measuring high pressure phase transitions

A new setup has been recently developed in the toroidal opposed-anvil device ‘Conac 40’ to perform differential thermal analysis in a high pressure range (0–6 GPa). To evaluate the precision and the reliability of the setup, the high pressure melting curve of germanium and the transition points of α-iron have been investigated up to 5.3 GPa and compared with previous results.     

一种高密度玻璃的多形性高压相变和物态方程研究

在二级轻气炮上用无氧铜飞片直接撞击重玻璃平板样品(密度为4.817 g/cm³,材料牌号:ZF6)开展了冲击压缩实验研究,压力范围为52.1—167.8 GPa,并采用多通道瞬态辐射高温计和光分析技术测量了其雨贡纽线、高压声速和冲击波温度等动态特性.实验结果显示,上述性质在三个不同压力区间出现不连续性变化,表明冲击压缩下该样品材料存在多形性高压相变,相变起始压力分别为23,78和120 GPa.实测声速先是随冲击压力的增高而增加,并在78 GPa附近出现急剧下降,之后又随压力增长,并在120 GPa之后下降到体波声速,表明材料进入高压熔化相.温度数据同样在78和120 GPa处出现明显的不连续变化,并在120 GPa之后变化趋于平缓与计算的Lindeman熔化线相符,进一步印证了上述相变行为.实测雨贡纽数据与LASL数据库中的重玻璃数据相符,结果显示除23 GPa附近有一明显的突变外,高压区数据几近线性变化,表明重玻璃的两个高压相变均为二级相变.本文报道的重玻璃材料高压物性数据和序列相变认识对于发展反向加载技术、提高材料声速测量精度和适用压力范围具有实用价值. 关键词： 重玻璃 冲击温度 卸载声速 冲击相变     

高温高压下方解石相转变的拉曼光谱原位实验研究

用热液金刚石压腔装置结合拉曼光谱技术研究了高温高压下方解石的相变过程及拉曼光谱特征。结果表明:常温条件下,体系压力增至1 666和2 127 MPa时,方解石的拉曼特征峰155cm-1消失,1 087cm-1峰分裂为1 083和1 090cm-1两个谱峰、282cm-1峰突然降至231cm-1,证明其转变为方解石-Ⅱ和方解石-Ⅲ。在起始压力为2 761MPa和低于171℃的升温过程中,方解石-Ⅲ的拉曼散射的各个特征振动峰没有变化。当温度达到171℃,方解石晶体完全变成不透明状,其对称伸缩振动峰1 087cm-1、面内弯曲振动峰713cm-1和晶格振动峰155和282cm-1均发生突变,说明方解石-Ⅲ相变生成一种碳酸钙新相。体系降至常温,该新相一直保持稳定不变,表明高温高压下方解石向碳酸钙新相的转变过程是不可逆的。方解石-Ⅲ与碳酸钙新相之间的相变线方程为P(MPa)=9.09.T(℃)+1 880。碳酸钙新相的对称伸缩振动峰(ν1 087)随压力、温度的变化率分别为dν/dP=5.1(cm-1.GPa-1),dν/dT=-0.055 3(cm-1.℃-1)。     

Raman study of phase transitions in double orthophosphates of lutetium and potassium

This study by Raman spectrometry of double orthophosphates of lutetium and potassium consists of a systematic analysis of double orthophosphates of the potassium and lanthanide family. In the 10-300 K temperature range three solid-solid phase transitions in this compound are obtained which demonstrate the application of Raman spectrometry to the study of the nature and mechanism of such transitions.     

Volume collapse phase transitions in cerium-praseodymium alloys under high pressure

Cerium-12at%Praseodymium(Ce_0.88Pr_0.12) and Ce-50at%Praseodymium(Ce_0.50Pr_0.50) alloy samples that contain a random solid-solution of Ce (4f¹ (J?=?5/2)) and Pr (4f² (J?=?4)) localized f-states have been studied by angle dispersive x-ray diffraction in a diamond anvil cell to a pressure of 65?GPa and 150?GPa respectively using a synchrotron source. Ce_0.88Pr_0.12 alloy crystallizes in a face-centered cubic (γ-phase) structure at ambient conditions, while Ce_0.50Pr_0.50 alloy crystallizes in the double hexagonal close packed (dhcp) structure at ambient conditions. Two distinct volume collapse transitions are observed in Ce_0.88Pr_0.12 alloy at 1.5?GPa and 18?GPa with volume change of 8.5% and 3% respectively. In contrast, Ce_0.50Pr_0.50 alloy shows only a single volume collapse of 5.6% at 20?GPa on phase transformation to α-Uranium structure under high pressure. Electrical transport measurements under high pressure show anomalies in electrical resistance at phase transitions for both compositions of this alloy.