No abstract

HgTe and HgS have been investigated with the use of the high-pressure x-ray diffraction technique to 20 GPa. HgTe undergoes three pressure-induced phase transitions in the (0 -20)-GPa range, from zinc blende to cinnabar to rocksalt to P-Sn, at 1.4, g, and 12 GPa, respectively. HgS does not show any evidence for a transition from the cinnabar structure up to 20 GPa, either in high-pressure x-ray diffraction or in high-pressure Raman studies. The above transition sequence agrees with the pressure-induced sequence for CdTe except for the intrusion of the cinnabar structure. Evaluation of the bulk modulus Bo and Bo from high-pressure x-ray data reveals that the cinnabar phases of HgTe and HgS are very compressible; Bo and Bo are 19.4+0.5 GPa and 11.1 for HgS, and 16.0+0.5 GPa and 7.3 for HgTe. Bulk moduli, volume changes for transitions, and lattice parameters of the high-pressure phases have all been determined from the x-ray data. The observed transition sequence for Hg Te appears to be in agreement with the predictions of recent pseudopotential total-energy calculations for phase stability in III-V and II-VI compounds under pressure.

The dependence on hydrostatic pressure of the direct and indirect optical-absorption edges of GaAs has been measured for pressures up to the first structural transition near 17 GPa (T =300 K).The energy of the lowest direct gap Eo (I &'5~I &) increases sublinearly with pressure whereas its dependence on density p is linear (dEo/d lnp=8. 5). The energy E; of the indirect edge (I I5~X&) decreases with pressure at a rate of -1.35&(10 eV/GPa in the range 4.2 to 17.5 GPa. The strength of the direct absorption grows linearly with increasing gap Eo due to excitonic coupling. A strong enhancement of the indirect absorption with pressure is attributed, in part, to an increase of the transition-matrix element for virtual direct transitions involved in the indirect absorption process.