Raman spectroscopy study of the ferrielectric-paraelectric transition in layered<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">CuInP</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">S</mml:mi></mml:mrow><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

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.