Temperature Dependence of Single-Crystal Spinel (MgAl_2O_4) Elastic Constants from 293 to 423°K Measured by Light-Sound Scattering in the Raman-Nath Region

The temperature dependence of single-crystal elastic constants of synthetic stoichiometric MgAl_2O_4 spinel has been measured by the light-sound scattering technique in the Raman-Nath region. The crystal is set into forced vibration by a single crystal LiNbO_3 transducer coupled to one crystal face. A He-Ne Laser beam is diffracted by the stress-induced birefringence inside the crystal. The diffraction angle is determined from the distance between two spots exposed on a photographic plate by the first order diffracted beams as measured by a microdensitometer. The sound wavelength inside the crystal is then inferred from the laser diffraction angle. Combining the sound wavelength with the measured transducer frequency, the velocity inside the crystal is determined typically to a precision of 0·05 per cent. In this method, the measurement of velocity is not dependent on either the determination of sample length or on phase shifts at sample-transducer interface. Velocities of four pure modes, L//[001], T//[001], L//[110], and T//[110](P//[110] are measured in the temperature range between 293 and 423°K. A linear temperature dependence is fit to the data by a least square method