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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 ) aredoi:10.1103/physrevb.59.251 fatcat:dv6yawxmzzghjkl7toowflzvvm