LATTICE-DYNAMICS AND HYPERFINE INTERACTIONS IN ZNO AND ZNSE AT HIGH EXTERNAL PRESSURES

The II-VI semiconductors ZnO and ZnSe have been investigated by x-ray and Zn-67-Mossbauer spectroscopy at high external pressures. In ZnSe, the recoilfree fraction f increases from f=0.50% at ambient pressure t o 1.19% at 6.1 GPa. It then decreases to f=0.92% as the pressure is fu rther raised to 8.2 GPa. This decrease of f is caused by softening of phonon modes which occurs far below the crystallographic phase transit ion (13.5 GPa). In the high-pressure phase of ZnO (NaCl structure), lo w-frequency acoustic-phonon modes become harder and high-frequency opt ic modes become softer as compared to ZnO (wurtzite structure). Modern theoretical Hartree-Fock cluster and full potential scalar-relativist ic linearized-augmented plane-wave calculations have been performed. T hese calculations reveal that in both systems covalent contributions t o the chemical bond determine the change of the s electron density rho (0) at the Zn nucleus between the different crystallographic phases as well as the electric-field-gradient tensor in ZnO (wurtzite). In part icular, rho(0) in ZnO (NaCl phase) is reduced compared to rho(0) in Zn O (wurtzite phase) by -1.15e/a(0)(3). Thus, contrary to obser vation f or ZnSe, the electrical conductivity in ZnO (NaCl phase) is not expect ed to increase in comparison with the low-pressure wurtzite structure.