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

The paraelectric–ferroelectric structural transition of potassium lithium tantalite niobate has been studied by both Raman scattering and thermal measurements. 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-plane bending of the Nb/TaO 6 group. The soft optical phonon mode concerns the extraordinary transverse optical phonons propagating along the [1 1 0] direction. The thermal expansion experiments show a displacive phase transition and a big thermal contraction in the c direction of the crystal, with an average linear expansion coefficient α c = −4.52 × 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.