Hypersonic anomalies and optical properties of RbTiOAsO4 and KTiOPO4 single crystals

The longitudinal (LA) Brillouin back-scattering spectra along the [001] pho non direction have been measured as a function of temperature for RbTiOAsO4 (RTA) and KTiOPO4 (KTP) single crystals. As temperature increases, the aco ustic-phonon frequencies of RTA and KTP show a clear softening (which reach es a turning point at T-c similar to 800 degrees C for RTA). We conclude th at the transitions in RTA and KTP are either weakly first order or of secon d order. It seems likely that the alkali ions (K+ or Rb+) are strongly invo lved in the soft mode. A broad damping evolution (which attains a maximum a t T-c similar to 800 degrees C in RTA) was observed for both crystals and c an be attributed to the dynamic order-parameter fluctuations. The optical t ransmissions, refractive indices (n(x),n(y),n(z)) and the Cauchy equations (for RTA) were obtained as a function of wavelength. The LA[001] sound velo cities V-LA and elastic constants C-33 + (e(33)(2)/epsilon(33)(s)) were als o calculated at room temperature for both crystals. [S0163-1829(99)10501-0] .