Phase transition and the soft lattice mode of a perovskite crystal studiedby Raman scattering and thermal measurements

The paraelectric-ferroelectric structural transition of potassium lithium t antalite niobate has been studied by both Raman scattering and thermal meas urements, A condensed soft lattice vibrational mode at the phase transition has been analyzed. It originates from the symmetric O2/O3-Nb/Ta-O3/O2 in-p lane bending of the Nb/TaO6 group. The soft optical phonon mode concerns th e extraordinary transverse optical phonons propagating along the [1 (1) ove r bar 0] direction. The thermal expansion experiments show a displacive pha se transition and a big thermal contraction in the c direction of the cryst al, with an average linear expansion coefficient alpha(c), = -4.52 x 10(-5) K-1. The phase transition temperature and enthalpy are 358 K and 0.50 J/g, respectively. Curie temperature measured by four methods is within 353 and 360 K.