Brillouin and rayleigh scattering in the plastic crystalline phase of pivalic acid

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Abstract
Brillouin spectroscopy has been used to determine the hypersonic velocities, phonon lifetimes, elastic constants and pockels (strain-optic) coefficients of pivalic acid in its plastic crystal phase at a frequency of order 109 Hz. The elastic constants at 25°C are C 11 = (3·43 ± 0·04), C 12 = (1·38 ± 0·06) and C 44 = (0·642 ± 0·003) in units of 1010 dynes cm-2 §, and vary little with temperature. The pockels coefficients are |P 12| = (1·330 ± 0·010), |P 44| = (0·210 ± 0·003) and |P 11 - P 12| = (0·348 ± 0·012). This is thought to be the first determination of the elastic constants and the pockels coefficients for a plastic crystal by the Brillouin scattering method. High resolution measurements of the linewidth of the spectrum of the depolarized Rayleigh scattering show that the molecules are reorienting with a correlation time 0·62 × 10-8 s at 33·2°C and that the reorientational activation energy is (60 ± 4) kJ mole-1.

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