A. Jayaraman

Elastic Moduli of GaAs at Moderate Pressures and the Evaluation of Compression to 250 kbar

Ultrasonic wave velocities in GaAs have been measured at 25°C as a function of hydrostatic pressure. The adiabatic ``zero‐electric‐field stiffness moduli calculated from these velocities are as follows: c11=1.1877± 0.0006, c12=0.5372±0.0009, c44=0.5944±0.0003. All moduli are in units of 1012 dyn/cm2 and are based on a density of 5.3169 g/cm3 taken as exact with respect to estimates of errors. Hydrostatic pressure derivatives for c11, c12, and c44, are, respectively, 4.63±0.03, 4.42±0.05, and 1.10±0.02. From the isothermal bulk modulus (BT) of 746.6 kbar and its pressure derivative (∂BT/∂P) of 4.67 calculated using the above data, the static compression of GaAs has been computed to a pressure of 250 kbar.

Resonance Raman scattering and optical absorption studies of CdSe microclusters at high pressure

The pressure dependence of the HOMO–LUMO transition energy and the frequency of the longest wavelength longitudinal optical vibration of 45 A diameter CdSe clusters in methanol–ethanol solution have been measured up to 50 Kbar. The LO mode shifts to higher frequency at a rate of 0.43 cm−1/Kbar, which corresponds to a Gruneisen parameter of 1.1. The HOMO–LUMO transition shifts to higher energy at 4.5 meV/Kbar, yielding a deformation potential of 2.3 eV. The pressure dependence of these properties closely resemble those of the corresponding bulk solid, confirming the point of view that the lattice properties of these clusters resemble those of the bulk, even though the optical properties are quite distinct.

Magnetism, Metal‐Insulator Transition, and Optical Properties in Sm‐ and Some Other Divalent Rare‐Earth Monochalcogenides

In Sm2+X compounds (X = S, Se, Te) an unusualy large dependence of the low‐temperature Van Vleck susceptibility on the ligand has been observed. SmSe, SmTe, and also TmTe undergo a continuous insulator‐metal transition under pressure. In contrast, SmS shows a discontinuous transition to the metal at 6.5 kbar. In all cases the transition is accompanied by a large volume change of around 20%, which is due to the promotion of a 4f electron into the 5d‐6s conduction band. Optical spectroscopy yields three distinct absorption peaks. They may be interpreted as transitions from 4f6↔4f55dt2g and 4f6↔4f55deg and probably an excitonic peak. The activation energy of a 4f electron is found to be 0.63, 0.46, 0.20, and 0.22 eV for SmTe, SmSe, SmS, and TmTe, respectively, from both the pressure‐dependent electrical resistivity at room temperature and the observed absorption edge.

A high-pressure Raman study of yttrium vanadate (YVO4) and the pressure-induced transition from the zircon-type to the scheelite-type structure

The pressure dependences of the Raman active modes in yttrium vanadate (YVO4) crystallizing in the zircon-type structure D4h19 (I41amd), have been studied using a diamond anvil cell up to 15 GPa. At room temperature the zircon-type structure transforms to the scheelite-type structure C4h6(I41a) with a = 5.04 A and c = 11.24 A near 7.5 GPa. The density changes from 4.24 to 4.74 gcm3 in the transition, and the corresponding volume decrease is ΔV = 5.07 cm3mole. The high-pressure phase is retained on release of pressure. The optical absorption edge shifts from 3.87 to 2.77 eV at the transition, presumably due to charge transfer, or due to V-O bond length change of the VO4 tetrahedra. The internal mode frequencies of the VO4 tetrahedra also decrease abruptly at the transition. The V-O bond stretching frequency decreases from 891 to 828 cm−1 in the scheelite-type phase, which suggests a fundamental change in the VO4 unit. The general question of the stability of the zircon-type structure under high pressure is discussed in the light of the high-pressure behavior of a large number of rare earth vanadates and arsenates.

Raman and optical absorption studies of the pressure-induced zircon to scheelite structure transformation in TbVO4 and DyV04

Abstract The pressure-induced phase transition in terbium vanadate (TbVO 4 ) and dysprosium vanadate (DyVO 4 ) from the zircon-type structure D 19 4 h to the scheelite-type structure C 6 4 h has been studied at room temperature by high-pressure Raman spectroscopy in a diamond anvil cell. The pressure dependencies of the Raman active modes are presented up to 18 GPa in TbVO 4 and 10 GPa in DyVO 4 . From these data the transition pressures are found to be 6.6 ± 0.6 GPa for TbVO 4 and 6.5 ± 0.3 GPa for DyVO 4 . X-ray powder diffraction studies confirm that the high-pressure phase, quenchable in polycrystalline form to atmospheric pressure in both cases, has the scheelite-type structure. The effect of pressure on the optical absorption edge was also investigated. It is found that d E g /d P is linear to 30 GPa, with slopes of −37 meV GPa -1 for TbVO 4 and −33meV GPa -1 for DyVO 4 . These results are discussed in terms of the phase transition kinetics, and changes in the electronic structure both at the transition and in the scheelite-type phase.

Band Structure of InGaP from Pressure Experiments

Positive identification of the Γ and X conduction‐band minima in InGaP has been made by performing hydrostatic‐pressure experiments on forward‐biased p‐n junction diodes. The Γ and X valleys are coincident in energy at a composition of In0.37Ga0.63P, and the corresponding bandgap is (2.17±0.02) eV at 300°K. The indirect bandgap EX in InP is inferred from the measurements to be 2.0 eV at 300°K. In addition, the pressure coefficients of the direct and indirect bandgaps, ∂EΓ/∂P and ∂EX/∂P, respectively, have been measured at various In(1−x)GaxP compositions. For InP, ∂EΓ/∂P is 8.7×10−3 eV/kbar and this coefficient increases to 13×10−3 eV/kbar for compositions close to In0.5Ga0.5P. On the other hand, ∂EX/∂P = −1.25×10−3 eV/kbar for GaP and shows little change for compositions in the range 0.4<x<1. The deformation potential is 5.7 eV for the Γ valley in InP, and increases to about 9 eV for direct bandgap ternary compositions.

Effect of pressure on the Raman modes in LiNbO3 and LiTaO3

The pressure dependence of the optical phonons in LiNbO3 to 21 GPa and in LiTaO3 to 10 GPa has been investigated by Raman spectroscopy, using the diamond anvil cell. All the observed modes increase in frequency with pressure, and no mode softening occurs. This is in contrast to the behavior observed in BaTiO3 and PbTiO3, which are well‐known displacive‐type ferroelectric. It is suggested that this difference in pressure behavior may stem from a different type of phase transition in LiNbO3 and LiTaO3, namely the order‐disorder type.

High‐Pressure Decomposition of Synthetic Garnets

The stability of synthetic garnets {Y3}[Fe2](Fe3)O12, {Y3}[Al2](Al3)O12, and {Y3}[Ga2](Ga3)O12 has been investigated at high pressures and high temperatures. Decomposition to the YXO3 compound (X=Fe or Al) with the perovskitelike structure and to the sesquioxide with the corundum structure has been found to occur in the former two garnets: {Y3}[X2](X3)O12→3 YXO3+X2O3, while in the case of yttrium—gallium garnet no breakdown was detected at pressures up to 44 kbar and 1000°C. The three garnets seem to behave differently at high pressure and high temperature. This appears to be related to the differences in the preference of the three cations for the tetrahedral site.Results of preliminary studies of the effect of high pressure and high temperature on Li0.5Fe2.5O4 are presented.The geophysical implications of the decomposition under high pressure and high temperature of the garnet structure are discussed.

Elastic Moduli of Gallium Antimonide under Pressure and the Evaluation of Compression to 80 kbar

Ultrasonic wave velocities in GaSb have been measured at 25°C as a function of hydrostatic pressure to 2 kbar. The adiabatic ``zero‐field stiffness moduli calculated from these velocities (for 1 atm) are as follows: c11 = 8.839±0.008, c12 = 4.033±0.010, c44 = 4.316±0.004, and Bs (bulk modulus) = 5.635±.012. All moduli are in units of 1011 dyn/cm2 and are based on a density of 5.6137 g/cm3 taken as exact with respect to estimates of errors. Hydrostatic pressure derivatives for c11, c12, c44, and Bs are, respectively, 4.96±0.10, 4.64±0.15, 1.01±0.02, and 4.75±0.15. The static compression of GaSb to 80 kbar has been evaluated using the isothermal bulk modulus (BT) of 5.614×1011 dyn/cm2 and (∂BT/∂P) of 4.78.

Optical microscopic, X-ray diffraction, and electrical resistance studies of CuCl at high pressure

Electrical resistance and X-ray diffraction measurements and also optical observations under a polarizing microscope were made on CuCl to pressures in excess of 12.5 GPa at room temperature using a diamond anvil cell. Resistance measurements were also performed in a piston-cylinder apparatus to pressures of approximately 5.5 GPa at room temperature. Three samples of CuCl prepared by different methods were examined. No anomalous pressure dependence in electrical resistance was found in the pressure range studied, and no dramatic changes in optical transmission were observed up to pressures of approximately 10.0 GPa. Optical observations and X-ray diffraction measurements indicate the existence of four phases in the pressure range studied, including a nonconducting black opaque phase which grows with time when CuCl is left for several days at the highest pressures.