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 E-1u[transverse-optic (TO)] phonon lifetime. Accurate values of the dielectric constants at ambient pressure epsilon(perpendicular to)(0) = 6.96, epsilon(perpendicular to)(infinity) = 4.95, epsilon(parallel to)(0) = 3.37, and epsilon(parallel to)(infinity) = 2.84 have been determined from fits to the reflectivity spectra. The out-of-plane A(2u) phonon reflectivity band is revealed in measurements on an inclined facet, and absorption measurements at an incidence angle of 30 degrees allow us to observe both the transverse- and longitudinal-optic A(2u) 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 A(2u)(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 A(2u) (TO) mode is induced by dynamical buckling of the flat honeycomb layers.