Pressure behavior of polar phonons in hexagonal and wurtzite boron nitride

58th European High Pressure Research Group Meeting, 9-11, september 2020, Puerto de la Cruz,TenerifeThis abstract report an investigation of the pressure behavior, up to 20 GPa, of the polar phonons in hexagonal (h) and wurtzite (w) boron nitride, by means of FTIR reflection and transmission measurements in diamond anvil cell. The in-plane E1u and out-of-plane A2u infrared-active phonons of layered hexagonal boron nitride exhibit a highly anisotropic behavior. 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 ε0⊥ = 6.96, ε∞⊥ =4.95, ε0∥ = 3.37, and ε∞∥ = 2.84 have been determined from fits to the reflectivity spectra, including interference fringe pattern. 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°, allowing us to observe both the transverse- and longitudinal-optic A2u modes.This work has been supported by the Spanish MINECO/FEDER under contracts nos. MAT2015-71035-R and MAT2016-75586-C4-1-P. T.T. acknowledges support from the Elemental Strategy Initiative conducted by the MEXT, Japan and JSPS KAKENHI grant number JP15K21722. B.G.acknowledges support from the Government of the Russian Federation, contract #14.W03.31.0011 at Ioffe Institute