Raman spectroscopy study of the ferrielectric-paraelectric transition in layered ${\mathrm{CuInP}}_2{\mathrm{S}}_6$

The dynamics of the ferrielectric-paraelectric transition in lamellar ${\mathrm{CuInP}}_2{\mathrm{S}}_6$ is studied using Raman scattering. Band assignments for this system are established by comparison with published vibrational spectroscopy information on related chalcogenophosphates as well as with new data on ${\mathrm{SnP}}_2{\mathrm{S}}_6$ herein described. Relaxational rather than resonant behavior is indicated by the temperature dependence of the spectral characteristics, in agreement with the assumed order-disorder character of the transition. It is suggested that a coupling between ${\mathrm{P}}_2{\mathrm{S}}_6$ deformation modes and ${\mathrm{Cu}}^{\mathrm{I}}$ vibrations enables the copper hopping motions which lead to the loss of polarity and the onset of ionic conductivity in this material.