Small angle and large-angle Brillouin scattering have been used to measure the sound velocity and damping of longitudinal waves propagating along (C, and perpendicular to (C'J the incommensurate axis in Ba,NaNb,O,, . For each mode strong dispersion is observed, with relaxation time 2 m of order 1.3 x lo-" s near T. For C , , T (~ satisfies with 2nr, = (1.08 & 0.13) x lo-'' s;2zr1 = (8.5 + 0.7) x lo-" s; and thediffusivity D = 1.3 cm'/s. This approximately 65 times larger than the thermal diffusivity of entropy fluctuations and implies a dynamic "central mode" for right-angle geometry light scattering of -60GHz. This mode is ten times the width observed for phason (soliton) diffusion by Lyons et al. in BaMnF,; it is not directly observed in the present work. For LA sound waves perpendicular to the incommensurate axis r;i(T) = T ; ' + Dq';that is, T, and D are the same as for C , , but ro' is neligible. T,, is interpreted as due to amplitude mode-acoustic mode interaction; T, and D are interpreted as interaction of acoustic modes with mobile defects, presumably soliton-like kinks in the incommensurate structure. +This paper summarizes the

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Brillouin spectra of lithium potassium sulfate have been obtained in its hexagonal phase from room temperature to the orthorhombic phase transition temperature at 435 OC. The V,, longitudinal sound velocity corresponding to the C,, elastic coefficient undergoes a smooth decrease with increasing temperature; the maximum decrease is 6.0% at To = 435 O C and the temperature dependence of V,, satisfies an expression of form with B=(1.13f0.04)x10-4K-', C =(0.9*0.1)K and Tc=(467f6)OC=(740+6)K.

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