High-pressure Raman spectroscopy study of wurtzite ZnO

Abstract

The high-pressure behavior of optical phonons in wurtzite zinc oxide $(w\ensuremath{-}\mathrm{ZnO})$ 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 ${(E}_{2},{A}_{1},$ and ${E}_{1})$ was measured for the wurtzite structure up to the $\mathrm{hexagonal}\ensuremath{\rightarrow}\mathrm{cubic}$ transition near 9 GPa. Above this pressure no active mode was observed. The only negative Gr\"uneisen parameter is that of the ${E}_{2}^{\mathrm{low}}$ mode. ${E}_{1}(\mathrm{LO})$ and (TO) frequencies increase with increasing pressure. The corresponding perpendicular tensor component of the Born's transverse dynamic charge ${e}_{T}^{*}$ is experimentally found to increase under compression like ${e}_{T}^{*}(P)=2.02+6.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}P,$ whereas calculations give ${e}_{T}^{*}(P)=2.09--2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}P$ (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 ${E}_{2}$ and ${A}_{1}$ mode behaviors before the phase transition.