Pressure dependence of the interlayer and intralayer E2g Raman-active modes of hexagonal BN up to the wurtzite phase transition

We present a Raman-scattering study of the interlayer and intralayer E2g Raman-active modes of hexagonal boron nitride (h−BN) under hydrostatic pressure for pressures up to the transition to the wurtzite phase (10.5 GPa). Pressure coefficients and Grüneisen parameters are determined for both modes, and are compared to ab initio calculations based on density functional perturbation theory. The pressure coefficient of the low-energy interlayer mode is higher than that of the high-energy intralayer mode owing to the large compressibility of the h−BN crystal along the c direction. Both modes exhibit a sublinear phonon frequency increase with pressure, which is more marked in the case of the low-energy mode. The intensity of the low-energy mode increases with pressure, suggesting an enhancement of the mode polarizability as the honeycomb layers become closer. The Raman spectrum of the metastable wurtzite phase is observed at ambient pressure in the transited sample after decompression.