Brillouin Scattering and Elastic Constants of Potassium Bromide

The Brillouin scattering of KBr crystals was measured by using a helium–neon laser and a pressure-scanned Fabry-Perot interferometer. The Brillouin shifts were measured for various crystal orientations with the scattering vector in the [110] plane. In addition to the strong Brillouin peaks due to the longitudinal acoustic mode, relatively weak Brillouin peaks due to the transverse mode with admixture of the longitudinal mode were clearly resolved for a certain range of crystal orientations. From these Brillouin shifts, the frequencies and velocities of thermally excited sound waves of the wavelength 286.9 nm were obtained, and the three independent elastic constants of the KBr crystal for sound waves of microwave frequencies 6–12 GHz were determined as C11=3.37×1011, C12=0.61×1011 and C44=0.51×1011 dyn/cm2. The elastic constants determined in the present study were close to the ultrasonic values, indicating little dispersion of the sound velocities for the regions between 10 MHz and 10 GHz.