High-pressure Raman spectroscopy study of wurtzite ZnO

The high-pressure behavior of optical phonons in wurtzite zinc oxide $(w-\mathrm{ZnO})$ 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 $\mathrm{hexagonal}\to \mathrm{cubic}$ transition near 9 GPa. Above this pressure no active mode was observed. The only negative Grüneisen parameter is that of the $E_2^{\mathrm{low}}$ mode. $E_1\left(\mathrm{LO}\right)$ 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 $e_T^{*}\mathrm{}\left(P\right)=2.02+6.4\mathrm{}\times {10}^{-3}P,$ whereas calculations give $e_T^{*}\mathrm{}\left(P\right)=2.09\mathrm{}–2.5\times {10}^{-3}P$ (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.