Abstract
The longitudinal acoustic phonon propagating along the [100] direction in a potassium alum KAl(SO4)2·12H2O single crystal from 297 to 77 K has been studied on the basis of Brillouin light scattering. On cooling, inverse sigmoid-shaped hardening of the Brillouin shift (and the associated elastic constant C11) was observed. An anomalous hump in the full-width at half-maximum (FWHM) appeared in the vicinity of 233 K. These anomalous changes in the acoustic properties were wholly unexpected, and may be attributed to coupling between the longitudinal acoustic phonon waves and the orientational degree of freedom of sulfate anions in the crystal. The temperature dependence of the relaxation time of the relevant relaxation process was derived on the basis of a single-relaxation-time approximation, which showed an increase in the relaxation time upon cooling, indicating dynamic slowing down.
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