Hypersonic anomalies and optical properties of RbTiOAsO4 and KTiOPO4 single crystals

Abstract

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 C33+(e233/ɛs33) were also calculated at room temperature for both crystals.