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 $^{67}\mathrm{ssbauer}$ spectroscopy at high external pressures. In ZnSe, the recoilfree fraction f increases from f=0.50% at ambient pressure to 1.19% at 6.1 GPa. It then decreases to f=0.92% as the pressure is further raised to 8.2 GPa. This decrease of f is caused by softening of phonon modes which occurs far below the crystallographic phase transition (13.5 GPa). In the high-pressure phase of ZnO (NaCl structure), low-frequency acoustic-phonon modes become harder and high-frequency optic modes become softer as compared to ZnO (wurtzite structure). Modern theoretical Hartree-Fock cluster and full potential scalar-relativistic linearized-augmented plane-wave calculations have been performed. These calculations reveal that in both systems covalent contributions to the chemical bond determine the change of the s electron density \ensuremath{\rho}(0) at the Zn nucleus between the different crystallographic phases as well as the electric-field-gradient tensor in ZnO (wurtzite). In particular, \ensuremath{\rho}(0) in ZnO (NaCl phase) is reduced compared to \ensuremath{\rho}(0) in ZnO (wurtzite phase) by -1.15e/${\mathit{a}}_{0}^{3}$. Thus, contrary to observation for ZnSe, the electrical conductivity in ZnO (NaCl phase) is not expected to increase in comparison with the low-pressure wurtzite structure. \textcopyright{} 1996 The American Physical Society.