High-Pressure Softening of the Out-of-Plane A₂ᵤ(Transverse-Optic) Mode of Hexagonal Boron Nitride Induced by Dynamical Buckling

We investigate the highly anisotropic behavior of the in-plane and out-of-plane infrared-active phonons of hexagonal boron nitride by means of infrared reflectivity and absorption measurements under high pressure. Infrared reflectivity spectra at normal incidence on high-quality single crystals show strict fulfillment of selection rules and an unusually long E₁ᵤ[transverse-optic (TO)] phonon lifetime. Accurate values of the dielectric constants at ambient pressure e₀⊥ = 6.96, e∞⊥ = 4.95, e₀∥ = 3.37, and e∞∥ = 2.84 have been determined from fits to the reflectivity spectra. The out-of-plane A₂ᵤ phonon reflectivity band is revealed in measurements on an inclined facet, and absorption measurements at an incidence angle of 30° allow us to observe both the transverse- and longitudinal-optic A₂ᵤ modes. Pressure coefficients and Gruneisen parameters for all infrared-active modes are determined and compared with ab initio calculations. While Gruneisen parameters are generally small in this layered crystal, the A₂ᵤ(TO) displays an exceptionally large and negative Gruneisen parameter that results in widening of the type I hyperbolic region with increasing pressure. Softening of the A₂ᵤ(TO) mode is induced by dynamical buckling of the flat honeycomb layers.