Collapsing cristobalitelike structures in silica analogues at high pressure

The cristobalitelike forms of the ternary silica analogues BPO4 and BAsO4 were investigated at high pressure by x-ray diffraction and theoretical methods. The behavior of these compounds represents an extreme case in which the tilt angle of the constituent tetrahedra increases in a spectacular way at high pressure resulting in a major change in topology from a cristobalitelike framework towards a "collapsed cristobalite" structure. These compounds provide the first examples of the collapse of a framework structure to a close-packed form in a continuous manner without an intervening phase transition.     

Crystalline and amorphous states in Zn-Sb and Cd-Sb alloys at high pressure

The structures of Zn-Sb and Cd-Sb alloys at high pressures up to 15 GPa are investigated by means of a diamond anvil. It is shown that the equiatomic compounds ZnSb and CdSb decompose at elevated pressures with the formation of a new high-pressure phase containing ∼60 at. % Sb with segregation of any Zn or Cd in excess with respect to antimony. The high-pressure phase has a simple hexagonal structure. Plots of the pressure dependence of the volume, the lattice parameters, and the axis ratio c/a are constructed for the phase in the Cd-Sb system. It is shown that the alloys amorphize when the load is removed. Fiz. Tverd. Tela (St. Petersburg) 39, 1509–1512 (September 1997)     

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.     

高温高压下过渡金属Ru的结构相变

采用基于密度泛函理论的第一性原理和准简谐晶格动力学方法对Ru的六角密排 (hcp)、面心立方 (fcc)、体心四方 (bct) 和体心立方 (bcc) 结构的磁性、晶格结构稳定性和高温高压下的相变进行了系统的研究. 计算获得了各相结构的磁性基态及其稳定性范围, 结果表明: 零温下在计算的压力范围内, NM-hcp 结构是Ru最稳定的结构, 压力的单独作用下并没有相变的发生; NM-fcc结构是Ru的亚稳定结构, 而NM-bcc和FM-bct结构在动力学上并不稳定. 高温高压下Ru将发生从NM-hcp到NM-fcc结构的相变, 并给出了Ru的温度压力相图. 关键词： 相变 晶格稳定性 磁性 第一性原理     

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.     

First principles calculations of structural phase transformation in CaTe at high pressure

First principles calculations of the total energy of CaTe as a function of unit cell volume have been carried out for the NaCl, MnP and CsCl structures on the basis of density functional theory (DFT). All these calculations are performed with the CRYSTAL06 program package. The sequence of high-pressure phases for CaTe transforms from NaCl phase to an intermediate state with a mixture of NaCl and MnP phases and then to the CsCl phase is obtained, which is in good agreement with the previous experimental results. Several structural properties (equilibrium lattice constant, bulk modulus, etc.) of NaCl structure have been calculated, which are also in agreement with the previous experimental results.     

Electroresistance of some semiconductors in the phase transition region at high pressure

Abstract

In apparata of liquid type piston-cylinder up to 2GPa and toroid with solid anvil up to 25 GPa pressure (P) electrial resistance R(P) in sphere of phase transition (PT) CdSnAs₂,SnTeInP,GaAs is investigated, and GaP at T = 300K, and also R(T,P) VTSP YB₂Cu₃0_6+x in sphere of superconductive transition. In order to describe the substance behaviour in the vicinity of PT, that is, the region of gapped change of R the approximation geterophased structure is used - the effective environment (model GSEE) which checks different configuration of phases inserts and their dependence from P. In order to picture the solid substance behaviour in the vicinity of point PT at high pressure (HP) different methods are used [1–8].     

Neutron scattering study of the phase transition in s-triazine at high pressure

The elastic transition in s-triazine (C₃N₃H₃) from a trigonal ($\text{R}\text{3}\text{c}$) high temperature (low pressure) structure to a monoclinic (C2/c) low temperature (high pressure) phase has been investigated at pressures up to 5 kbar using neutron scattering techniques. Neutron diffraction was used to measure the pressure dependence of the order parameter and inelastic scattering to study the softening of the transverse acoustic phonon modes on three isotherms. In both cases the effect of pressure on the transition is found to be described primarily by that on the temperature of the transition.     

Structural phase transition and elastic properties of ZnSe at high pressure

We have evolved an effective interionic interaction potential to investigate the pressure-induced phase transitions from zinc blende (B3) to rock salt (B1) structure in II-VI [ZnSe] semiconductors. The elastic constants, including the long-range Coulomb and van der Waals (vdW) interactions and the short-range repulsive interaction of up to second-neighbor ions within the Hafemeister and Flygare approach, are deduced. Keeping in mind that both of the ions are polarisable, we employed the Slater-Kirkwood variational method to estimate the vdW coefficients. The estimated value of the phase transition pressure (P _t ) is higher than in the reported data, and the magnitude of the discontinuity in volume at the transition pressure is consistent with that data. The major volume discontinuity in the pressure-volume phase diagram identifies the structural phase transition from zinc blende to rock salt structure.

The variation of second-order elastic constants with pressure resembles that observed in some binary semiconductors. It is inferred that the vdW interaction is effective in obtaining the thermodynamic parameters such as the Debye temperature, the Gruneisen parameter, the thermal expansion coefficient and the compressibility. However, the inconsistency between the thermodynamic parameters as obtained from present model calculations and their experimental values is attributed to the fact that we have derived our expressions by assuming the overlap repulsion to be significant only up to the nearest second-neighbor ions, as well as neglecting thermal effects. It is thus argued that full analysis of the many physical interactions that are essential to binary semiconductors will lead to a consistent explanation of the structural and elastic properties of II–VI semiconductors.     

方解石高压相变的拉曼光谱研究

应用金刚石压腔结合拉曼光谱技术研究了方解石-Ⅰ在静水高压作用下相转变为方解石-Ⅲ的过程.结果表明,压力增大的过程中,方解石-Ⅰ晶体的三个拉曼特征峰均向高频移动;在1 103 MPa条件下,体系中的水介质结冰,冰点处方解石-Ⅰ晶体性质没有变化;继续加压至1 752 MPa时矿物的拉曼特征峰发生了突变,表明晶体由方解石-Ⅰ相转变为方解石-Ⅲ相中的的A型方解石;相变后矿物的拉曼特征峰显示了从矿物内部向边缘的过渡中,相变程度逐渐增大的趋势;该研究也体现了金刚石压腔结合拉曼光谱技术在定性分析矿物结构相变过程中原位测试的优势.     

Thermodynamic properties and plasma phase transition of xenon at high pressure and high temperature

By using recent Padé approximations for the Coulombic contribution and hard-core, Van der Waals and polarisation approximations for the atomic contribution, the thermodynamic functions for the nonideal xenon plasma are constructed. These formulae cover a large range of pressure and temperature. Single ionisation is studied by minimising the free energy with respect to the particle numbers. A second critical point is predicted. The coexistence line for the corresponding first order phase transition (weakly ionised plasma - strongly ionised plasma), the lines indicating the soft transition to a higher degree of ionisation at overcritical temperatures, as well as the border of the coexistence area are presented. The influence of the different interactions on the critical data is discussed.     

Raman scattering studies on phase transition in barium sodium niobate at high pressure

Phase transition in barium sodium niobate at high pressure is investigated by Raman scattering experiments. Polarized Raman spectra from polished single domain crystal plates of definite orientation are obtained under pressure produced by a diamond anvil device. Anomalous hardening and intensity decrease of 32 cm^-1B₂ mode are found with increasing pressure. A remarkable pressure dependence of the frequencies of the bending modes of NbO₆ octahedrons is observed.     

Magnetic collapse and electronic phase transitions at high pressure in transition metal oxides

A review of electronic and magnetic phase transition in metal oxides with strong electron correlations (SEC) is given. The bandwidth control of the insulator gap is expected in the Hubbard model when the decreasing of the interatomic distance results in the bandwidth W(P) increase and at some critical value P_c, W(P_c)∼U and the Mott–Hubbard gap disappears. The other situation takes place in transition metal boroxides FeBO₃ and GdFe₃(BO₃)₄, where the increase of crystal field parameter Δ(P) results in the high spin–low spin crossover.     

Equation of state and structural phase transition in FeBO3 at high pressure

The evolution of X-ray diffraction patterns in FeBO₃ under high pressures up to 63 GPa has been investigated at room temperature in a diamond anvil cell. A structural phase transition at a pressure of 53±2 GPa was found for the first time. The transition is of the first-order type with a hysteresisless drop of the reduced unit cell volume of about 8.6%. Apparently, the transition is isostructural. At pressures below the transition, the equation of state for FeBO₃ was fitted. In the third-order approximation of the Birch-Murnagan equation of state, the bulk modulus K and its first pressure derivative K′ were found to be 255±25 GPa and 5.0±1.2, respectively.     

Energy spectrum and phase transitions in C70 fullerite crystals at high pressure

Measurements have been made of the Raman, optical absorption, and luminescence spectra of single crystals and pellets of the fullerite C₇₀ at T=300 K and at pressures up to 12 GPa. The baric shift dω/dP and the Grüneisen parameters of the Raman-active intramolecular phonon modes have been determined. It has been established that the d ω/dP value for certain phonon modes abruptly changes at pressures of P ₁≈2 GPa and P ₂≈5.5 GPa, as do the half-widths of the Raman lines. These features in the Raman spectrum are associated with phase transitions at high pressure. The baric shifts of the absorption and luminescence edges of C₇₀ crystals have been determined and are −0.12 eV/GPa and −0.11 eV/GPa, respectively, for absorption and luminescence. The baric shift of the absorption edge decreases significantly with increasing pressure and is −0.03 eV/GPa at 10 GPa. These data have been used to determine the deformation potential of the fullerite C₇₀, which is about 2.1±0.1 eV. Zh. éksp. Teor. Fiz. 111, 262–273 (January 1997)     

Brillouin scattering of vitreous silica under high pressure

The pressure dependence of Brillouin spectra of vitreous silica exhibits a maximum in the hypersonic absorption and a minimum in the sound velocity at about 2 GPa almost independent of temperature. The results are discussed on the basis of relaxing defects residing in double-well potentials with a Gaussian distribution of barrier heights and a Lorentzian distribution in the asymmetry. Our analysis suggests that both the distribution functions are strongly changed at higher pressures. It is striking that a phase boundary between α-quartz and coesite exists just at the pressure where absorption and sound velocity of vitreous silica pass their extremum. This might be a hint that a corresponding structural change also occurs in the glass.     

Structural phase transition and elastic properties of cerium chalcogenides at high pressure

The structural and elastic properties of cerium chalcogenides (CeZ, Z = S, Se, Te) under high pressure have been investigated by using the potential model considered up to third nearest neighbor interaction. The computed values of B1-B2 phase transition pressure, equation of state (compression curve), bulk modulus, its first order pressure derivative and elastic constants in the case of cerium chalcogenides agree well with the experimental results. The present study shows the anomalous behavior of cerium chalcogenides in comparison to the alkaline earth chalcogenides, due to the presence of Kondo effect and reentrant valence behavior of Ce in cerium chalcogenides.