High-pressure phase transition in CaTiOSiO4 titanite

The P2₁/a to A2/a transition in titanite has been studied at high pressure (and room temperature) by X-ray powder diffraction. On the basis of the disappearance of k + l= odd reflections from the diffraction pattern, the transition was located between 3·351(3) and 3·587(3) GPa. The variation with pressure of the spontaneous strain in the P2₁/a phase indicates that the transition has an effective critical exponent significantly less than 1/2. Within the uncertainties of the data, the transition could be weakly first-order in character or be continuous with an effective critical exponent in the range of ～0·13-0·25. The A2/a to P2₁/a transition is accompanied by a significant expansion of the a-axis as a result of the bond-valence sum requirements of the Ti atoms: in the high-pressure phase they occupy the centres of the TiO₆ octahedra, but they are displaced along the a-axis in the P2₁/a phase to form alternating short and long Ti-O bonds.     

High-pressure phase transition of carbon disulfide

The high-pressure phase transition of CS₂ was studied by combing ab initio molecular dynamics with total energy calculations. At 300 K the pieces of polymer structure were found to appear at 10 GPa in the molecular dynamics run, and further the CS₄ tetrahedral structure to appear at about 20 GPa. The phase transition was then studied in the structure of Cmca, α-quartz and β-quartz by using the first-principle total energy calculation method. A phase transition from Cmca to β-quartz was found at 10.6 GPa. The calculated lattice constants of β-quartz at atmospheric pressure are a=5.44 and c/a=1.138 with B₀=95 GPa. The calculation has also indicated that CS₂ decomposes at 20 GPa and below 1000 K.     

Low temperature phase transition in ZnSe doped with nickel

Neutron diffraction and ultrasonic experiments as well as measurements of heat conductivity in ZnSe and Zn_1−xNi_xSe (x=0.0025) semiconductors have been carried out. As a result, a structural transition induced by Ni impurity has been found at      

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.     

ZnSe弹性常数和相变的从头计算

利用平面波密度泛函理论研究了ZnSe从闪锌矿结构到盐石结构的相变.结果发现通过H相等得到的相变压力为16.8 GPa,与通过高压弹性常数值判断所得到的结果相符.     

High-pressure phase transition of Bi2Fe4O9

The high-pressure behaviour of Bi2Fe4O9 was analysed by in situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8) GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6) GPa and the high-pressure structure is stable up to 26.3(1) GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1) GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16 GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch–Murnaghan equation of state to the p–V data results in K0 = 74(3) GPa for the low-pressure phase and K0 = 79(2) GPa for the high-pressure phase. The mode Grüneisen parameters were obtained from Raman-spectroscopic measurements.     

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.     

Identification of the pressure-induced phase transition of ZnSe with the positron annihilation method

This paper studies the pressure-induced phase transition between zincblende (B3) and NaCl (B1) structure ZnSe by using the hydrostatic pressure first-principles pseudopotential plane wave method. The energy-volume and enthalpy-pressure curves are employed to estimate the transition pressure. It is found that ZnSe undergoes a first-order phase transition from the B3 structure to the B1 structure at approximately 15 GPa derived from the energy-volume relation and 14 GPa based on deduction from enthalpy-pressure data. The pressure-related positron bulk lifetimes of the two ZnSe structures are calculated with the atomic superposition approximation method. In comparison with the 13.4% reduction in volume of ZnSe at the transition pressure, the positron bulk lifetime decreases more significantly and the relative value declines up to 22.3%. The results show that positron annihilation is an effective technique to identify and characterize the first-order phase transition and can give valuable information about changes in micro-scale, such as volume shrinkage and compressibility.     

High-pressure phases and pressure-induced phase transition of MoN6 and ReN6

Transition metal nitrides have been widely used in many scientific and technical areas because of their unique physical and mechanical properties. We report two new nitrogen-rich transition metal nitrides, MoN₆ and ReN₆, by crystal structure searching technique. Under high pressure, MoN₆ will undergo phase transition (from R-3m to Pm-3 phase) at 54 GPa, and ReN₆ always keep the R-3m phase in the pressure range from 50 to 100 GPa. There are benzene-like six-membered “N6” rings with nitrogen single bonds in the R-3m phase structures, indicating that MoN₆ and ReN₆ are expected to be the high-energy-density materials.     

High-pressure phase transitions in UP1-xSx

Abstract

High-pressure X-ray diffraction using synchrotron radiation has been performed on UP_1-x -S_x (X=0.1; 0.25; 0.4) up to 53 GPa UP_1-x S_x is a solid solution with a B1 (NaCl) structure. For all compositions a second order phase transition is observed around 10 GPa to a distorted B1 structure of rhombohedral symmetry. For UP_1-x S_x with x 0.25 a second phase transition is observed, which takes place in the region of 35 GPa This phase transition occurs when the nearest U-U distance reaches the Hill limit of 330–340 pm. The high-pressure phase seems to have orthorhombic or even monoclinic symmetry. It has some similarities to the high pressure phase of UP. UP^1-x S_x 4 shows only weak indications for an additional phase at 53 GPa. In conclusion, we observe that the second phase transition and the bulk modulus B, in UP shift to higher pressure, when phosphorus is replaced by sulfur.     

Effects of grinding-induced grain boundary and interfaces on electrical transportation and structure phase transition in ZnSe under high pressure

Interface and scale effects are the two most important factors which strongly affect the structure and the properties of nano-/micro-crystals under pressure.We conduct an experiment under high pressure in situ alternating current impedance to elucidate the effects of interface on the structure and electrical transport behavior of two Zn Se samples with different sizes obtained by physical grinding.The results show that(i) two different-sized Zn Se samples undergo the same phase transitions from zinc blend to cinnabar-type phase and then to rock salt phase;(ii) the structural transition pressure of the859-nm Zn Se sample is higher than that of the sample of 478 nm,which indicates the strong scale effect.The pressure induced boundary resistance change is obtained by fitting the impedance spectrum,which shows that the boundary conduction dominates the electrical transport behavior of Zn Se in the whole experimental pressure range.By comparing the impedance spectra of two different-sized Zn Se samples at high pressure,we find that the resistance of the 478-nm Zn Se sample is lower than that of the 859-nm sample,which illustrates that the sample with smaller particle size has more defects which are due to physical grinding.     

Thermodynamics of vapor phase Al doped ZnSe

In order to understand the processes involved in the doping of zinc selenide from an aluminum vapor phase, we present a theoretical study of such a heat treatment. The quenching effects due to the cooling of the crystal are considered, giving results which can be compared with experimental values. Mechanisms of compensation are described in terms of their relative importance. The influence of the composition of the vapor phase and of the temperature of the doping treatment is considered. The calculated values are compared with experimental ones, and show good agreement between our model and the behaviour of doped crystals.     

Pressure-induced phase transition in transition metal trifluorides

As a fundamental thermodynamic variable, pressure can alter the bonding patterns and drive phase transitions leading to the creation of new high-pressure phases with exotic properties that are inaccessible at ambient pressure. Using the swarm intelligence structural prediction method, the phase transition of TiF₃, from R—3c to the Pnma phase, was predicted at high pressure, accompanied by the destruction of TiF₆ octahedra and formation of TiF₈ square antiprismatic units. The Pnma phase of TiF₃, formed using the laser-heated diamond-anvil-cell technique was confirmed via high-pressure x-ray diffraction experiments. Furthermore, the in situ electrical measurements indicate that the newly found Pnma phase has a semiconducting character, which is also consistent with the electronic band structure calculations. Finally, it was shown that this pressure-induced phase transition is a general phenomenon in ScF₃, VF₃, CrF₃, and MnF₃, offering valuable insights into the high-pressure phases of transition metal trifluorides.