Tu, Chi-ShunKatiyar, R.S.Schmidt, V. HugoGuo, R.Bahalla, A.S.2016-04-182016-04-181999-01-01C.‑S. Tu, R.S. Katiyar, V.H. Schmidt, R. Guo, and A.S. Bhalla, “Hypersonic anomalies and optical properties of RbTiOAsO4 and KTiOPO4 single crystals,” Phys. Rev. B 59, 251-256 (1999).1098-0121https://scholarworks.montana.edu/handle/1/9703The 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.Article