Nonreversible Transition from the Hexagonal to Wurtzite Phase of Boron Nitride under High Pressure: Optical Properties of the Wurtzite Phase

Abstract

We present an infrared reflectance and transmission study of a pressure-induced phase transition of boron nitride from the hexagonal layered structure to the wurtzitic phase. The transition is completed at about 13 GPa. The phase transition is nonreversible and the optical features of the metastable wurtzitic phase are retained after a pressure cycle from 20.5 GPa down to ambient pressure. This allows the infrared-active optical phonons and the dielectric properties of the cold-pressed wurtzitic boron nitride sample to be studied over the whole range of pressures. Experimental permittivity values of ϵ0 = 6.65 ± 0.03 and ϵ∞ = 4.50 ± 0.05 are determined from fits to the reflectance spectra at ambient pressure. Accurate values of the refractive index in the mid-infrared and visible-ultraviolet regions are evaluated from the interference patterns. Contrary to the h-BN case, the refractive index of w-BN decreases slightly with pressure, on account of the much lower compressibility of the close-packed structure. The pressure coefficients for the longitudinal optical and transverse optical modes are determined, and an overall good agreement with ab initio calculations is found.