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

For CuInP$_2$S$_6$ layered ferrielectric crystal, that is most investigated among wide family of van der Waals ferroics MM$'$P$_2$X$_6$ (M - Cu, Ag; M$'$ In, Bi, Cr, $\dots$; X S, Se), by Brillouin spectroscopy the temperature dependence longitudinal hypersound velocity was for acoustic phonons propagated in plane crystal layers. Pronounced softening phonon branch observed paraelectric phase below $T^{*} \approx 330~$K at cooling to $T_C 312~$K first order transition into phase. Below $T_C$ growth correlation with spontaneous polarization rise and its anomaly similar earlier behavior ultrasound wave normally structural Determined anomalous part thermal conductivity vicinity also demonstrate lowering heat transferring group from $T^{*}$ $T_C$. Observed obviously induced flexoelectric coupling relaxational soft polar optic branches related inhomogeneously polarized state appearing between phases.