Brillouin scattering study of transverse elastic waves in the α, incommensurate and β phases of AlPO4

Transverse and pseudo-transverse elastic waves have been studied in several scattering geometries in order to investigate the temperature dependences of CE 66 and CE 14 over the range 300-1100 K, including the transitions near 860 K. These results complete those on CE 44 we have obtained in a previous work. All these constants display discontinuities at the lock-in transition. In the α phase, the results are analysed in term of lowest order couplings between strains (e) and the order parameter (Q). The main features are described by the lowest order biquadratic e2Q2 coupling, in particular for CE 44 in a large temperature range. However, it appears that a contribution of the next coupling term arises for CE 66 below ~ 710 K and that the first two lowest order terms have to be taken into account even just below the lock-in transition in the case of CE 14 . The temperature dependence of Q has been deduced and it can be well described in the framework of Landau's theory.