Shock-wave compression of x-cut quartz as determined by electrical response measurements

The mechanical response of x-cut quartz in the vicinity of the Hugoniot elastic limit is determined from measurements of the piezoelectric current from samples impact loaded from 26 to 130 kbar. The Hugoniot elastic limit is determined to be 60_?1·5⁺³ kbar at a compression of 0·066_?0·002^+0·004 This Hugoniot elastic limit corresponds to a shear strength of 5·5 per cent of the C₄₄ shear modulus. For stresses well above the Hugoniot elastic limit the electrical current measurements show that the material exhibits a substantial reduction of shear strength. The pressure derivative of the bulk modulus is determined to be 4·5, substantially less than the ultrasonic value. The experimental records show evidence for a time delay for reduction of shear strength which varies from about 10^?7 sec immediately above the 60 kbar Hugoniot elastic limit to about 10^?8 sec for stresses well above the Hugoniot elastic limit. The measurements also show stress relaxation below the Hugoniot elastic limit between 40 and 60 kbar.     

Hydrogen vibrations in γ-TiH and γ-ZrH under high pressure

Abstract

The effect of high hydrostatic pressure on the vibrational spectra of monohydrides γ-TiH and γ-ZrH was studied by inelastic neutron scattering, the maximum experimental pressure being, respectively, P=15 and 17·5 kbar. There was observed a uniform linear pressure-induced energy shift of each spectrum compared to that at ambient pressure with proportionality factors of 1·02 for y-TiH and 1·03 for γ-ZrH. The observed linear shift of the one-phonon hydrogen modes and the bound multiphonons (biphonons and triphonons) is indicative of a pressure-induced increase of the anharmonicity parameters in the model Hamiltonian representing the strength of two-phonon and three-phonon interactions.     

Elastic properties of Se and As2Se3 glasses under pressure and temperature

The sound velocities and their pressure and temperature variations of Se and As₂Se₃ glasses have been determined by means of a pulse superpositions method, and other elastic constants and their pressure and temperature derivatives were calculated from these data. The bulk modulus was found to be 94·6 kbar for Se glass and 143·7 kbar As₂Se₃ glass, both of which are higher than the values calculated from the previous compression data. No anomaly was observed in any of the pressure and temperature dependence of elastic behavior of these glassses. Furthermore, the comparison of the pressure and temperature derivatives of the bulk modulus indicates that the thermodynamic self-consistency is satisfied on these materials. The bulk moduli of these glasses and crystalline As and Se were used to obtain an empirical bulk modulus-volume relationship for compounds in the As?Se system. The acoustic Grüneisen parameter was calculated and compared with the thermal Grüneisen parameter.     

Pressure effect on the anomalous electrical conductivity of magnetite

The temperature dependence of the electrical conductivity of magnetite was measured under hydrostatic pressure up to 18.4 kbar. It is found that the temperature of the conductivity maximum in the high temperature phase is more rapidly reduced by pressure (dT_m/dP = -4.1 K/kbar) than the Verwey temperature (dT_v/dP = -0.27 K/kbar). The discontinuous change of the conductivity at T_v appears to increase with applied pressure as a result of a lowering of T_m.     

A possible tricritical line in Ni(NO3)2 · 2H2O

We obtain the behaviour of the critical (possibly tricritical) point for metamagnetic Ni(NO₃)₂·2H₂O as a function of several applied hydrostatic pressures up to 11 kbar. The obtained line of possible tricritical points greatly suggests a pressure induced metamagnetic transition in a 0.8 kbar range.     

High pressure equation of state of rubidium

The pressure-volume relation of rubidium metal is studied by high-pressure x-ray diffraction up to 110 kbar at room temperature. In addition, pressure scans of the near-infrared reflectivity are recorded up to 250 kbar. Rubidium undergoes a bcc to fcc structural transition (Rb I → Rb II) at 70 ± 2 kbar. Other phase transitions occur at 128 ± 3, 160 ± 5 and 190 ± 5 kbar on the ruby pressure scale. The pressure-volume relation and the near-infrared reflectivity provide evidence for a pressure-induced 5s → 4d electronic transition similar to the well-known 6s → 5d transition in cesium metal.     

Commensurability and fluctuating conductivity in the organic conductor TSF-TCNQ

We report the pressure dependence of the phase transition in TSF-TCNQ as determined by resistivity measurements. We find a narrow pressure domain centered on 6.25 kbar where the transition temperature peaks above a monotonous pressure dependence. We suggest that there is a commensurate × 3 superlattice in this pressure regime resulting from an increase in charge transfer from 0.63 under ambient pressure to $\text{2}\text{3}$.A drop of longitudinal conductivity related to the × 3 commensurability is visible in the same temperature region where X-ray diffuse scattering data show 1-D features for the precursor fluctuations. We suggest that like TTF-TCNQ, the metallic conductivity of TSF-TCNQ is also influenced by the fluctuating Fröhlich mode.     

Lutetium: High pressure polymorph and compression

The effect of pressure on the lattice parameters of the h.c.p. phase of Lu has been studied up to 230 kbar at 23 ± 3°C by means of X-ray diffraction employing a diamond-anvil cell. The data fitted to the Birch equation yield an isothermal bulk modulus of 460 ± 20 kbar with a pressure derivative of 2.8 ± 0.5. The axial ratio $\text{(}\text{c}\text{a}\text{)}$ decreases nonlinearly with increasing pressure, the decrease being 2.4 per cent at 230 kbar. When the pressure exceeds 230 kbar, Lu transforms reversibly from the h.c.p. phase to the Sm-type structure. The transition occurs with increasing pressure in the range of 230 ± 5 kbar. The lattice parameters of the Sm-type structure at about 231 kbar are a = 3.176 ± 0.006 Å and c = 21.77 ± 0.04 Å, and the volume change is $\text{?0.21}\text{cm}{}^3\text{mole}$ or ?1.6 per cent of the volume of the h.c.p. phase.     

EPR of 3d 5 and 3d 8 Ions in Crystals With Perchlorate Structure at High Pressure

EPR spectra for ions Ni 2+ (3d 5 ) and Mn 2+ (3d 5 ) substituting Zn 2+ in crystals Zn(BF 4 ) 2 ·6H 2 O are studied within a wide range under hydrostatic pressure in X and Q bands. It is shown that the pressure changes to a considerable extent the spectrum parameters of ion Mn 2+ , reducing axial parameter b_{2}^{0} and increasing cubic one b_{4}^{0} at 9 kbar pressure, there is no temperature dependency of parameter b_{2}^{0} . Hydrostatic pressure changes linearly the initial splitting of ion Ni 2+ and results to a change in D(b_{2}^{0}) sign, which means the spin levels inversion at 3.5 kbar.     

On the behaviour of TSeF-TCNQ under pressure

We report b-axis electrical conductivity data for TSeF-TCNQ single crystals from 12 to 300 K under hydrostatic pressures up to 9 kbar. The single anomaly visible in the conductivity at 29 K and the low temperature conductivity gap rise under pressure at the same rate of ～ 6% kbar^-1. It has been found that the pressure dependence of the metal-insulator phase transition is qualitatively consistent with a mean field formulation of the Peierls transition. A Gruneisen constant of 0.64 for TSeF-TCNQ has been derived from this pressure study together with recent optical and compressibility investigations. The pressure dependence of the conductivity anisotropies at room temperature in TSeF-TCNQ and TTF-TCNQ are reported. The magnitude of the anisotropies in the two compounds are found to be essentially the same. The striking result, however, is that the anisotropies in both compounds are found to be independent of pressure up to 9 kbar.     

Raman scattering and Hall effect in layer InSe under pressure

Abstract

Experimental results of Raman scattering, Hall effect and conductivity measurements of ε-InSe crystals at 300K and pressures up to 10.2 kbar are presented. Changes of the shear force constants under pressure were analyzed using a linear-chain model. The decrease of the shear force constants between metallic planes with increasing pressure are explained qualitatively by charge transfer from intralayer to interlayer space. Changes in the slopes of the Hall constant versus pressure and conductivity versus pressure dependence at about 3.5 kbar are explained by the existing representation of InSe as a direct band gap semiconductor.     

GaP at ultrahigh pressure

An x-ray diffraction experiment has been carried out on GaP compressed in a quasi-hydrostatic pressurizing medium to 296 ± 5 kbar. The transformation pressure was found to be increased from the value of 220 kbar (22 GPa) reported in a less hydrostatic environment, since two phases of GaP (I and II) were found in coexistence at 250 ± 5 kbar. No evidence of disproportionation was found at this pressure, or 296 kbar, where sharp lines of phase II were found (body centered tetragonal). No evidence of amorphization was found on release of pressure.     

Superconductivity in polymeric sulfur nitride (SN)x at high pressure

Investigations of the pressure dependence of the superconducting transition temperature T_c up to 17 kbar, and of the normal conductivity up to 50 kbar are reported. It is observed that below 8 kbar, the value of T_c increases linearly with the pressure. In addition, there is a significant drop of T_c at about 9 kbar which may be due to a phase transition.     

Phases of silicon at high pressure

X-ray diffraction studies are reported on silicon at pressures up to 250 kbar (25 GPa). A transition to the β-Sn structure (II) initiates at 112 ± 2 kbar and two phases (I + II) coexist to 125 ± 2 kbar. At 132 ± 2 kbar a new phase (V) initiates, and the transition is complete at 164 ± 5 kbar. This phase persists to 250 kbar. Its structure is tentatively assigned as primitive hexagonal with c/a = 0.941 ± 0.002 at 250 kbar. On release of pressure, the sequence is V → (V + II) (145 - 110 kbar) → II → (II + III) (108 - 85 kbar) → III, the last phase persisting to room pressure.     

Pressure dependence of transport and optical properties of Ag2HgI4

Silver mercury iodide, Ag₂HgI₄, has been studied to pressures of 75–100 kbar at 25°C. There are now four high pressure phases known. Transport studies indicate predominantly electronic conduction, with the highest pressure phase showing rectification in an asymmetric conductivity cell. Optical studies now corroborate the four high pressure phases. For two of the phases the energy gap shifts to lower energies with increasing pressure.     

Compression and polymorphism of potassium to 400 kbar

Both the compression and polymorphism of K were investigated to about 400 kbar at room temperature in a diamond-anvil pressure cell by optical and X-ray diffraction techniques. The ambient b.c.c.-K(I) is stable to about 120 kbar. The compression data for K(I), fitted to the Birch equation of state, yielded the zero-pressure bulk modulus B₀ = 29.9 ± 0.2 kbar and its first pressure derivative B'₀ = 4.15 ± 0.10. These values agree very well with those of recent data derived from the direct measurement of length changes using piston-displacement (to 20 kbar) and laser interferometer (to 7 kbar) techniques. On the basis of the compression data for K(I) alone, Bridgman's pressure scale may be overestimated by about 10 kbar at 100 kbar and that of Kennedy and LaMori may be underestimated by about 5 kbar at 50 kbar. Two high-pressure polymorphs of K were revealed in the pressure range 100–200 kbar. The b.c.c. → f.c.c. transition in K was observed to occur near 120 kbar, accompanied by a −2.5% discontinuous change in volume. The sample changes gradually from silver to gold in the range 130–150 kbar. The f.c.c.-K.(II) transforms further into an as yet undetermined structure between 170 and 190 kbar. No change in colour was observed in the latter transition. K(III) is stable up to at least about 400 kbar. The equation of state for the f.c.c. phase of K cannot be established. The volume of K was compressed more than 60% in the vicinity of 200 kbar, however.     

Conductivity of C60 fullerene crystals under dynamic compression up to 200 kbar

The conductivity σ of C₆₀ fullerene crystals is measured under quasi-isentropic loading by a spread shock wave to a pressure of 200 kbar at the initial temperatures 293 and 77 K. A sharp increase in σ by seven to eight orders of magnitude is detected: from 10^?6?10^?7 Ω^?1 cm^?1 at normal conditions to 5 Ω^?1 cm^?1 under pressure from 100 to 200 kbar. The conductivity of samples under pressure decreases with decreasing temperature, which is characteristic of semiconductors. On pressure release, σ regains its initial value.     

Pressure-induced structural transitions in PbI2: A high-pressure Raman and optical absorption study

The effect of pressure on the 2H and 4H polytype of PbI₂ has been investigated by Raman and optical absorption spectroscopy, using the diamond anvil cell. The 2H-polytype undergoes pressure-induced phase transitions at 5 kbar and near 30 kbar. The 4H-polytype exhibits phase transitions near 8 kbar and above 30 kbar. The Raman modes abruptly change at these pressures. The optical absorption edge shifts red at the rate of 15±1 MeV/kbar in the 2H-PbI₂ and at the rate of 7 MeV/kbar in phase II. The latter phase is most likely to possess a 3d-structure and not a layer type. The possible structures for the high pressure phases are discussed.     

Phase transformations in equiatomic alloy TiZr at pressure up to 70 kbar

The effect of pressure on the α ? β and ω ? β transformations in the equiatomic alloy TiZr is studied by the differential thermal analysis (DTA) and calorimetric technique. The α-β equilibrium at atmospheric pressure occurs at a temperature of 579°C, and the heat of transition ΔH is 40.9±2.0 J/g. As the pressure increases up to 28 kbar, the temperature of the α-β equilibrium linearly decreases, dT/dP=?2.2±0.3 K/kbar. In the pressure range 28–48 kbar, the β-phase undergoes a transition to the two-phase (α + ω) state upon cooling to room temperature. At pressures above the triple point with the coordinates P=49±3 kbar and T=460±30°C, the cooling of the β-phase gives rise to only the hexagonal ω-phase with the unit cell parameters a=4.843 Å, c=2.988 Å, and c/a=0.617 under normal conditions. The slope of the ω-β equilibrium boundary is positive at pressures up to 70 kbar, dT/dP≈0.46 K/kbar. The ω → α transformation at atmospheric pressure proceeds in the temperature range T=425–470°C with the enthalpy of transition ΔH=2.8 J/g.     

半磁半导体Cd1-xMnxTe光吸收边的压力效应

本文研究了室温下1—40kbar流体静压力范围内三元化合物半磁半导体Cd_1-xMn_xTe光吸收边的压力效应。实验结果给出:x<0.5的样品,吸收边随压力增加向高能方向以α=6—8×10^-3eV/kbar的速率漂移,并具有10^-5/kbar²量级的二级非线性系数;x≥0.5的样品,表观吸收边随压力增加向低能方向漂移,压力系数为α-5×10^-3eV/kbar。高压下所研究的样品均有一从闪锌矿结构到NaCl结构的相变发生。这一相变可以是不可逆的,相变压力与样品组分有关,大致在25—40kbar范围内。根据半导体能带畸变势效应和晶体场理论模型估计了压力系数的理论值,讨论了不同压力系数的物理原因。 关键词：