Pressure Effects on the Charge Carrier Transportation of BaF_2 Nanocrystals

The charge transport behavior of barium fluoride nanocrystals is investigated by in situ impedance measurement up to 35 GPa. It is found that the parameters change discontinuously at about 6.9 GPa, corresponding to the phase transition of BaF2 nanocrystals under high pressure. The charge carriers in BaF_2 nanocrystals include both F~-ions and electrons. Pressure makes the electronic transport more difficult. The defects at grains dominate the electronic transport process. Pressure could make the charge-discharge processes in the Fm3m phase more difficult.     

Pressure-induced quenching of the Jahn-Teller distortion and insulator-to-metal transition in LaMnO(3)

LaMnO(3) was studied by synchrotron x-ray diffraction, optical spectroscopies, and transport measurements under pressures up to 40 GPa. The cooperative Jahn-Teller (JT) distortion is continuously reduced with increasing pressure. There is strong indication that the JT effect and the concomitant orbital order are completely suppressed above 18 GPa. The system, however, retains its insulating state to approximately 32 GPa, where it undergoes a bandwidth-driven insulator-metal transition. Delocalization of electron states, which suppresses the JT effect but is insufficient to make the system metallic, appears to be a key feature of LaMnO(3) at 20-30 GPa.     

The pressure induced insulator to metal transition in FeSb2

The evolution of the ground state properties of FeSb(2) has been investigated via temperature (4.2-300 K), magnetic field (0-12 T) and pressure (0-8.8 GPa) dependent electrical resistivity studies. The temperature dependence of the resistivity follows activated behavior in the high temperature (HT) regime (T > 60 K), while variable range hopping (VRH) dictates the transport in the intermediate temperature (IT) regime (10 K > T > 45 K) and power law behavior is observed in the low temperature (LT) regime (T < 10 K). The pressure profoundly affects the resistivity in all the temperature regimes. The energy gap (Δ) extracted in the HT regime initially increases with pressure and then decreases, while the VRH parameter T(0) deduced in the IT regime is seen to decrease monotonically and vanish beyond 5 GPa leading to an insulator to metal transition (MIT) on account of delocalization of the electronic states in the gap. The analysis of the logarithmic derivative of the conductivity indicates the MIT to occur at ~6 GPa. The magnetoresistivity is found to be positive. The analysis of the resistivity behavior under pressure and magnetic field indicates that the former induces delocalization, while the latter tends to assist localization of the defect states inside the gap of FeSb(2).     

高压下ZnSe直流和交流电学性质的研究

采用高压原位测量技术在0–35 GPa压力范围内对ZnSe直流和交流电学性质进行了研究. 通过分析直流电学测量结果可知,在实验压力区间内ZnSe经历了由纤锌矿转变为朱砂相再转变为岩盐相的两次相结构转变. 分析温度与材料电阻率的变化关系表明ZnSe在高压下的相变为金属化相变,并通过交流阻抗谱的测量实验证实了这个结论. 进一步比较低压条件下晶粒和晶界电阻的变化,表明朱砂相结构的ZnSe更接近各向同性材料. 关键词： 高压 ZnSe 电学     

Superconductivity in SnO: a nonmagnetic analog to Fe-based superconductors?

We discovered that under pressure SnO with α-PbO structure, the same structure as in many Fe-based superconductors, e.g., β-FeSe, undergoes a transition to a superconducting state for p?6 GPa with a maximum Tc of 1.4 K at p=9.3 GPa. The pressure dependence of Tc reveals a domelike shape and superconductivity disappears for p?16 GPa. It is further shown from band structure calculations that SnO under pressure exhibits a Fermi surface topology similar to that reported for some Fe-based superconductors and that the nesting between the hole and electron pockets correlates with the change of Tc as a function of pressure.     

Phase transformations in calcite from electrical impedance measurements

The phase transformation in calcite I-IV-V and calcite ? aragonite have been characterized by electrical impedance measurements at temperatures 600–1200°C and pressures 0.5–2.5?GPa in a piston cylinder apparatus. The bulk conductivity σ has been measured from Argand plots in the frequency range 10⁵–10^?2?Hz in an electric cell representing a coaxial cylindrical capacitor. The synthetic polycrystalline powder of CaCO₃ and natural crystals of calcite were used as starting materials. The transformation temperature T_c was identified from resistivity-temperature curves as a kink point of the activation energy. At pressure above 2?GPa in ordered phase calcite I, the activation energy E _σ is c. 1.05?eV, and in disordered phase calcite V E _σ is c. 0.75?eV. The pressure dependence of T_c for the rotational order–disorder transformation in calcite is positive for pressures <1?GPa and negative for pressures >1?GPa. The transformation boundary of calcite 1–IV is observed only during first heating in samples after a long annealing at low temperatures. The activation energy of calcite I???IV decreases gradually from 1.8 to 1.05?eV with the pressure increase from 0.5 to 2?GPa. The kinetics of calcite ? aragonite transformation has been monitored by measuring a time-variation of the electrical resistance of a calcite sample at 10³?Hz in the stability P-T field of aragonite. The variation of the impedance correlates with the degree of phase transformation, estimated from X-ray powder diffraction studies on quenched products of experiments. The kinetics of calcite ? aragonite transformation may be fitted to the Avrami kinetics with the exponent m???1–1.5.     

Electro-and magnetotransport properties of disordered carbon phases synthesized from C60 fullerite at moderate pressures P syn < 1.5 GPa

Charge transport has been investigated in new disordered carbon phases synthesized from C₆₀ at “moderate” pressures P _syn < 1.5 GPa and high temperatures T _syn > 1000 K. Under these synthesis conditions, a structure transition from nanographite phases to new amorphous modifications is induced when pressure decreases. The transition from hopping transport to diffusive electron transport typical of “dirty” metals is observed in a number of samples synthesized at various pressures and temperatures in the indicated ranges. The magnetotransport properties of these phases under intermediate synthesis conditions can be described by diffusive electron transport.     

百吉帕压力下固态氩的状态方程

 利用最新的X光衍射实验数据和Ar原子的弹性散射实验结果，得到了Ar原子间的相互作用和350 GPa下的固态氩的状态方程。这个状态方程可以作为超高压力下的压标。在150 GPa以上的压标，是目前高压研究中急需的。固态氩既可作为准静水压传压介质，又可作为高压X光衍射中的压标使用。本文计算了固态氩的Grüneisen参数、Debye特征参数、定容比热和等温体模量随压力的变化，压力范围为0~150 GPa。     

Freezing of glycerol-water mixtures under pressure

We investigated freezing of pure glycerol as well as glycerol-water (GW) mixtures with 3:1 and 3:2 volume fractions as a function of pressure in the 0-10?GPa range by ruby fluorescence spectroscopy and neutron scattering. We find that the glass transition pressure increases from 5.5?GPa for pure glycerol to 6.5?GPa for the 3:1?GW mixture, with unusually small pressure gradients above. For higher water concentrations close to 3:2, phase separation occurs above 2?GPa where most of the water is expelled in the form of ice VII. The results suggest that glycerol is able to effectively hydrogen bond not more than ≈2.5 H(2)O molecules per glycerol, which seems to support conclusions from molecular dynamics simulations. The data indicate that these fluids could become important as pressure transmitting media for neutron scattering in the 0-7?GPa range, including at low temperatures.     

Pressure-induced isostructural phase transition in Bi2Sr2CaCu2O8+δ

The high-pressure structures of an underdoped cuprate superconductor Bi₂Sr₂CaCu₂O_8+δ have been studied by synchrotron X-ray diffraction at pressures up to 36.5 GPa. We find that this superconductor retains its orthogonal structure with the space group Amaa in the pressure range studied. Upon compression, both the a and b axes first shrink monotonically up to 17.4 GPa from their ambient pressure values and keep these behaviors with positive compressibilities up to 36.5 GPa after experiencing expansion with negative compressibilities in the pressure regime between 17.4 and 23.7 GPa. However, the c axis decreases continuously with increasing pressure with a slow change at about 23.7 GPa. The results indicate an isostructural phase transition starting at 17.4 GPa and a structural collapse at around 23.7 GPa.     

Variations of lattice constants and thermal expansion coefficients of indium at high pressure and high temperature

ABSTRACT

The effects of pressure and temperature on the lattice constants and thermal expansion coefficients of Indium were studied up to 18.6?GPa and 506?K based on in situ X-ray diffraction method with an externally heated diamond anvil cell. The results show that the measured axial ratio (c/a) decreases with increasing temperature and its temperature dependence decreases with increasing pressure. The thermal expansion coefficient of the a-axis decreases with increasing pressure up to 7?GPa and remains almost constant above 7?GPa, whereas that of the c-axis increases monotonously with pressure and changes from negative to positive at around 7?GPa. The observed behavior suggests that temperature reduces the tetragonal distortion on the lattice, and its effect is dominant below 7?GPa; in contrast, pressure enhances lattice distortion, and tends to have a stronger effect above 7?GPa.     

High pressure studies on α-cristobalite form of Al0.5Ga0.5PO4: hydrostatic versus non-hydrostatic conditions

High pressure angle-dispersive X-ray diffraction investigations have been carried out on α-cristobalite form of Al_0.5Ga_0.5PO₄. Our investigations show that the structural stability of this phase under high pressure depends on the nature of pressure conditions in the diamond anvil cell. Under hydrostatic pressure conditions using neon as a pressure transmitting medium, ambient orthorhombic C222₁ phase transforms to orthorhombic Cmcm phase at 4.9?GPa. The high pressure Cmcm phase remains stable up to the highest pressure in the experiment, i.e. 19?GPa. The values of bulk modulus for C222₁ and Cmcm phases are 19(2) and 126(4)?GPa, respectively. In contrast to this, under non-hydrostatic pressure conditions, transformation of ambient C222₁ phase to Cmcm phase has not observed up to 17.4?GPa. Instead, a new monoclinic phase P2₁ is observed which contains layers of six coordinated Al/Ga ions separated by less dense five coordinated Al/Ga ions.     

Mechanical,electronic and thermodynamic properties of TE-C36 under high pressure

The structural parameters, mechanical, electronic and thermodynamic properties of TE-C36 under high pressure were calculated via the density functional theory in combination with the quasi-harmonic Debye model. The results show that the pressure has significant effects on the equilibrium structure parameters, mechanical, electronic and thermodynamic properties of TE-C36. The obtained ground state structural parameters are in good agreement with previous theoretical results. The mechanically and dynamically stable under pressure were confirmed by the calculated elastic constants and phonon dispersion spectra. The elastic constants, elastic modulus, B/G ratio, Poisson’s ratio and Vicker’s hardness were determined in the pressure range of 0–100?GPa. The elastic anisotropy of TE-C36 under pressure are also determined in detail. The electronic structure calculations reveal that TE-C36 remains a direct band gap semiconductor when the pressure changes from 0 to 100?GPa, and the band gap decreases with increasing pressure. Furthermore, the pressure and temperature dependence of thermal expansion coefficient, heat capacity and Debye temperature are predicted in a wide pressure (0–90?GPa) and temperature (0–2500?K) ranges. The obtained results are expected to provide helpful guidance for the future synthesis and application of TE-C36.     

窗口声阻抗对锆相变动力学的影响

基于磁驱动加载装置CQ-4开展了锆的斜波压缩相变实验,研究了锆样品后表面窗口声阻抗对相变波形的影响.实验结果显示,锆后表面为较低声阻抗窗口(自由面和LiF窗口)时,相变起始对应的特征粒子速度约331.0 m/s,而高阻抗蓝宝石窗口时,特征粒子速度约301.9 m/s,特征速度对应的压力从约9.14 GPa下降到8.27 GPa.相变对应的速度特征拐点是与多种因素相关的实验信息,因此它对应的压力并不是材料属性参数相变压力.结合基于热力学Helmholtz自由能的多相状态方程和非平衡相变动力学方程开展了锆的相变动力学数值模拟研究,相变弛豫时间为30 ns,计算结果与三种情况的实验结果符合良好,可以较好地模拟斜波压缩下锆的弹塑性转变、相变等物理过程.在压力-比容和温度-压力热力学平面,相变前锆的准等熵线与冲击绝热线差异很小,相变后准等熵线都位于冲击绝热线下方,随着压力的增加准等熵线和冲击绝线偏差越来越大,温度-压力平面中在20 GPa时相差约100 K.相变开始后,由于相变引起比容的间断,导致锆的拉氏声速迅速下降约7%,相变完成后拉氏声速恢复到体波声速.     

Pressure-induced polyamorphism in Nd60Fe30Al10 and Ce70Al10Cu20 metallic glasses by high-energy X-ray diffraction and electrical resistance measurements

In situ high-energy X-ray diffraction of Nd₆₀Fe₃₀Al₁₀ and Ce₇₀Al₁₀Cu₂₀ metallic glasses is carried out under high pressure. During compression, Ce₇₀Al₁₀Cu₂₀ and Nd₆₀Fe₃₀Al₁₀ exhibit a polyamorphic transition from low-density state below 2.0 and 10.0?GPa, to high-density state above 8.4 and 21.1?GPa, respectively. The intermediate hysteresis regions are the mixture of both phases. Electrical resistance measurements under high pressure show that Ce₇₀Al₁₀Cu₂₀ display a discontinuous change in pressure dependence curve of resistivity at around 1.7?GPa. The addition of Fe atom gives a significant standoff of phase transition pressure in Nd₆₀Fe₃₀Al₁₀. The results in this work suggest that the solute element and microstructure of lanthanide solvent aggregates have implications on the polyamorphic transition in metallic glasses.     

Phase transitions of YbX (X = P,As and Sb) with a NaCl-type structure at high pressures

The powder X-ray diffraction of YbX (X?=?P, As and Sb) with a NaCl-type structure has been studied with synchrotron radiation up to 63?GPa at room temperature. YbSb undergoes the first-order structural phase transition from the NaCl-type (B1) to the CsCl-type (B2) structure at around 13?GPa. The structural change to the B2 structure occurs with the volume collapse of about 1% at 13?GPa. The transition pressure of YbSb is surprisingly lower than that of any other heavier LnSb (Ln?=?Dy, Ho, Er, Tm and Lu). The pressure-induced phase transitions in YbP and YbAs are observed at around 51?GPa and 52?GPa respectively. The transition pressure of both compounds is much higher than that of YbSb. The high-pressure structural behaviour of YbX (X?=?P, As and Sb) is discussed. The volume versus pressure curve for YbX with the NaCl-type structure is fitted by a Birch equation of state. The bulk moduli of these compounds with the NaCl-type structure are 104?GPa for YbP, 85?GPa for YbAs and 52?GPa for YbSb.     

Synchrotron X-ray diffraction of SiO2 to multimegabar pressures

Abstract

α-Quartz was compressed at room temperature in a diamond-anvil cell without a medium to maximum pressures of 31 to 213 GPa and was studied by energy-dispersive synchrotron X-ray diffraction. Broad peaks observed in a previous high-pressure diffraction study of silica glass are evident in the present study of quartz compression, providing in situ confirmation of pressure-induced amorphization above 21 GPa. The 21-GPa crystalline-crystalline (quartz 1–11) transformation previously observed on quasihydrostatic compression of quartz is found to also occur under the current nonhydrostatic conditions, at the identical pressure. With nonhydrostatic compression, however, new sharp diffraction lines are observed at this pressure. The measurements show the coexistence of at least one amorphous and two crystalline phases above 21 GPa and below 43 GPa. The two crystalline phases are identified as quartz II and a new, high-pressure silica phase. The high-pressure phases, both crystalline and amorphous, can be quenched to ambient conditions from a maximum pressure of 43 GPa. With compression above 43 GPa, the diffraction pattern from quartz II is lost and the second crystalline phase persists to above 200 GPa.     

Ab initio predicted elastic and thermodynamic properties of Imm2-BN under high pressure

The structural parameters, elastic constants, thermodynamic properties of Imm2-BN under high pressure were calculated via the density functional theory in combination with quasi-harmonic Debye approach. The results showed that the pressure has the significant effect on the equilibrium lattice parameters, elastic and thermodynamic properties of Imm2-BN. The obtained ground state structural parameters are in good agreement with previous theoretical results. The elastic constants, elastic modulus, and elastic anisotropy were determined in the pressure range of 0–90?GPa. Furthermore, by analyzing the B/G ratio, the brittle/ductile behavior under high pressure is evaluated and the elastic anisotropy of the Imm2-BN up to 90?GPa is studied in detail. Moreover, the pressure and temperature dependence of thermal expansion coefficient, heat capacity, Debye temperature, and Grüneisen parameter are predicted in a wide pressure (0–90?GPa) and temperature (0–1600?K) ranges. The obtained results are expected to provide helpful guidance for the future synthesis and application of Imm2-BN.     

高压下AlN纳米线的介电性质

利用高压X射线衍射和高压原位电化学阻抗谱法研究了AlN纳米线的高压结构和电学性质.研究结果表明:AlN纳米线的电学参数均在22.6 GPa发生了突变,该突变与样品发生了从纤锌矿到岩盐矿的结构相变相关.岩盐相中晶格对电子的散射在电传导中起主导作用;不同于纤锌相中的长程传导,岩盐相中存在短程传导,且其作用随压力增加而增大;纤锌相的介电损耗峰在1Hz且不受压力影响;岩盐相的介电损耗峰随压力减小且向低频方向移动.本研究将为AlN及其相关材料的进一步研究和应用提供实验依据.     

Electrical resistivity of fluid methane multiply shock compressed to 147 GPa

Shock wave experiments were carried out to measure the electrical resistivity of fluid methane. The pressure range of 89–147?GPa and the temperature range from 1800 to 2600?K were achieved with a two-stage light-gas gun. We obtained a minimum electrical resistivity value of 4.5?×?10^?2?Ω?cm at pressure and temperature of 147?GPa and 2600?K, which is two orders of magnitude higher than that of hydrogen under similar conditions. The data are interpreted in terms of a continuous transition from insulator to semiconductor state. One possibility reason is chemical decomposition of methane in the shock compression process. Along density and temperature increase with Hugoniot pressure, dissociation of fluid methane increases continuously to form a H₂-rich fluid.