Exploring lattice symmetry evolution with discontinuous phase transition by Raman scattering criteria: The single-crystalline (K, Na)NbO3 model system

We have developed the multiperspective methods for discussing phase transition and evolution of molecular vibration using unpolarized and polarized Raman scatterings. There are three scattering characteristics serving as the criteria to judge the vibrational modes, molecular symmetry, and first-order phase transition. Except for the frequency collection of one or more phonon modes, remarkably, we first demonstrate in theory that polarized Raman scattering exhibits high sensitivity on dissecting first-order phase transition, such as the symmetry breaking and discontinuity during phase transition. Experimental results confirm that temperature dependencies of peak intensity and the depolarization ratio extracted from polarized spectra present the abrupt changes associated with the discontinuity of first-order phase transition. Thermal discontinuous transformation of the (K, Na)NbO3 lattice brings into physical correspondence with the molecule polarizability and order parameter of the system, i.e., spontaneous polarization, which is the first derivative of Gibbs free energy. Hence, these methods based on the scattering criteria could be broadly adapted for more extensive studies on the phase transition in condensed matter.