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

Abstract

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/TaO6group. The soft optical phonon mode concerns the extraordinary transverse optical phonons propagating along the [110] direction. The thermal expansion experiments show a displacive phase transition and a big thermal contraction in thecdirection of the crystal, with an average linear expansion coefficient αc= −4.52 × 10−5K−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.