Abstract
The high-pressure behavior of optical phonons in wurtzite zinc oxide is studied using room-temperature Raman spectroscopy and ab initio calculations based on a plane-wave pseudopotential method within the density-functional theory. The pressure dependence of the zone-center phonons and was measured for the wurtzite structure up to the transition near 9 GPa. Above this pressure no active mode was observed. The only negative Grüneisen parameter is that of the mode. and (TO) frequencies increase with increasing pressure. The corresponding perpendicular tensor component of the Born’s transverse dynamic charge is experimentally found to increase under compression like whereas calculations give (in units of the elementary charge e, P in GPa). In both cases, the pressure variation is small, indicating a weak dependence of the bond ionicity with pressure. The pressure dependence of the optical mode energies is also compared with the prediction of a model that treats the wurtzite-to-rocksalt transition as an homogeneous shear strain. There is no evidence of an anomaly in the and mode behaviors before the phase transition.
- Received 10 July 2001
DOI:https://doi.org/10.1103/PhysRevB.65.092101
©2002 American Physical Society