Lattice Infrared Spectra of Boron Nitride and Boron Monophosphide

Abstract

The fabrication of highly dense, highly polished compacts consolidated from monocrystalline cubic boron nitride at very high pressures and temperatures has enabled us to record the infrared reflection spectrum of cubic boron nitride from 80 to 550\ifmmode^\circ\else\textdegree\fi{}K. For annealed compacts, the reststrahlen peak reflectivity of about 85% could be obtained. Analysis of data by a damped-oscillator model yields $\mathrm{TO} (\mathbf{k}\ensuremath{\simeq}0):1065$ ${\mathrm{cm}}^{\ensuremath{-}1}$, ${\ensuremath{\epsilon}}_{0}=7.1$, ${\ensuremath{\epsilon}}_{\ensuremath{\infty}}=4.5$; and $\mathrm{LO} (\mathbf{k}\ensuremath{\simeq}0):1340$ ${\mathrm{cm}}^{\ensuremath{-}1}$. Infrared transmission measurements on small single crystals were obtained with a beam condenser. Eleven absorption peaks were observed between 600 and 3000 ${\mathrm{cm}}^{\ensuremath{-}1}$. These are assigned as combinations of four Brillouin-zone boundary modes: LO 1232 ${\mathrm{cm}}^{\ensuremath{-}1}$, TO 1000 ${\mathrm{cm}}^{\ensuremath{-}1}$, LA 685 ${\mathrm{cm}}^{\ensuremath{-}1}$, and TA 348 ${\mathrm{cm}}^{\ensuremath{-}1}$. The reflection spectrum of hot-pressed compacts of boron monophosphide single crystals measured between 600 and 1000 ${\mathrm{cm}}^{\ensuremath{-}1}$ showed a monotonic decrease of reflectivity from about 15 to 9% with an extremely weak reststrahlen band around 825 ${\mathrm{cm}}^{\ensuremath{-}1}$. The estimated values of long-wavelength optical modes are $\mathrm{TO} (\mathbf{k}\ensuremath{\simeq}0):820$ ${\mathrm{cm}}^{\ensuremath{-}1}$, and $\mathrm{LO} (\mathbf{k}\ensuremath{\simeq}0):835$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The results thus suggest an extremely low effective ionic charge (\ensuremath{\sim}0.25) for the crystal. Additional deduced physical properties of cubic boron nitride and boron phosphide are discussed.