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 T_c ≈ 312 K of the first order phase transition into ferrielectric phase. Below T_c 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 T_c. 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.

Keywords:

• Acoustic phonons
• Brillouin scattering
• CuInP₂S₆
• flexoelectric coupling
• frustrations
• thermal conductivity
• van der Waals ferroics