Phonon spectra and phase transitions in van der Waals ferroics MM’P2X6

Abstract

For CuInP2S6 ferrielectrics by Brillouin spectroscopy, the temperature dependence of the longitudinal hypersound velocity was investigated for the acoustic phonons propagated in plane of the crystal layers. Pronounced softening of acoustic phonon branch is observed in the paraelectric phase below T* ≈ 330 K at cooling to temperature Tc ≈ 312 K of the first order phase transition into ferrielectric phase. Below Tc the hypersound velocity growth in correlation with spontaneous polarization rise and its temperature anomaly is similar to earlier observed temperature behavior of longitudinal ultrasound velocity for acoustic wave propagated normally to the structural layers. Determined anomalous part of the CuInP2S6 crystal thermal conductivity in the vicinity of the ferrielectric transition also demonstrates lowering of the heat transferring phonons group velocity with cooling from T* to Tc. Observed acoustic softening obviously is induced by flexoelectric coupling of relaxational soft polar optical and acoustic branches that is related to an in ingomogeneously polarized state appearing between the paraelectric and ferrielectric phases.