High-pressure Raman spectroscopy study of wurtzite ZnO

The high-pressure behavior of optical phonons in wurtzite zinc oxide (w-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 (E₂,A₁, and E₁) was measured for the wurtzite structure up to the hexagonal→cubic transition near 9 GPa. Above this pressure no active mode was observed. The only negative Gr\"uneisen parameter is that of the E₂ˡᵒʷ mode. E₁(LO) and (TO) frequencies increase with increasing pressure. The corresponding perpendicular tensor component of the Born's transverse dynamic charge eT* is experimentally found to increase under compression like eT*(P)=2.02+6.4×10^-3P, whereas calculations give eT*(P)=2.09--2.5×10^-3P (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 E₂ and A₁ mode behaviors before the phase transition.