LAUSR

ABSTRACT The lattice instability and elastic anomaly of a ferroelectric and incommensurate (IC) phase transitions of Sr2(Nb1-xTax)2O7 single crystals with x = 0.20 and 0.40 were studied by Raman and Brillouin scattering over the broad temperature range from room temperature to 1400°C. For a ferroelectric phase transition, Brillouin scattering of the x = 0.20 and 0.40 crystals show the discontinuous change in Brillouin frequency shift of a LA mode related to elastic constant c22 in the vicinity of TC = 1160°C and 910°C, respectively. The Raman scattering of the x = 0.20 and 0.40 crystals shows the remarkable softening of the lowest frequency A1(z) mode at about 50 cm−1 towards TC similar to the Sr2Nb2O7 (x = 0.00) crystal reported previously. It indicates that the displacive nature of a soft optic mode still remains for the composition up to x = 0.40. For the normal to incommensurate (IC) phase transitions, Brillouin frequency shifts of LA modes show the noticeable changes at TIC = 220°C and 265°C, respectively.