Hypersonic anomalies and optical properties of RbTiOAsO4 and KTiOPO4 single crystals

The longitudinal ~LA! Brillouin back-scattering spectra along the @001# phonon direction have been measured as a function of temperature for RbTiOAsO4 ~RTA! and KTiOPO4 ~KTP! single crystals. As temperature increases, the acoustic-phonon frequencies of RTA and KTP show a clear softening ~which reaches a turning point at Tc;800 °C for RTA!. We conclude that the transitions in RTA and KTP are either weakly first order or of second order. It seems likely that the alkali ions (K or Rb) are strongly involved in the soft mode. A broad damping evolution ~which attains a maximum at Tc;800 °C in RTA! was observed for both crystals and can be attributed to the dynamic order-parameter fluctuations. The optical transmissions, refractive indices (nx ,ny ,nz) and the Cauchy equations ~for RTA! were obtained as a function of wavelength. The LA@001# sound velocities VLA and elastic constants C331(e33 2 /«33 s ) were also calculated at room temperature for both crystals. @S0163-1829~99!10501-0#