The longitudinal ͑LA͒ Brillouin back-scattering spectra along the ͓001͔ phonon direction have been measured as a function of temperature for RbTiOAsO 4 ͑RTA͒ and KTiOPO 4 ͑KTP͒ single crystals. As temperature increases, the acoustic-phonon frequencies of RTA and KTP show a clear softening ͑which reaches a turning point at T c ϳ800°C for RTA͒. 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 ϩ or Rb ϩ ) are

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... trongly involved in the soft mode. A broad damping evolution ͑which attains a maximum at T c ϳ800°C in RTA͒ was observed for both crystals and can be attributed to the dynamic order-parameter fluctuations. The optical transmissions, refractive indices (n x ,n y ,n z ) and the Cauchy equations ͑for RTA͒ were obtained as a function of wavelength. The LA͓001͔ sound velocities V LA and elastic constants C 33 ϩ(e 33 2 / 33 s ) were also calculated at room temperature for both crystals.