  In order to explore the properties of a two-sublattice ferroelectric, we
measured the infrared and Raman scattering response of CuInP2S6 across the
ferroelectric and glassy transitions and compared our findings to a symmetry
analysis, calculations of phase stability, and lattice dynamics. In addition to
uncovering displacive character and a large hysteresis region surrounding the
ferroelectric transition temperature T_C, we identify the vibrational modes
that stabilize the polar phase and confirm the presence of two ferroelectric
variants with opposite polarizations. Below TC, a poorly understood
relaxational or glassy transition at Tg is characterized by local structure
changes in the form of subtle peak shifting and activation of low frequency
out-of-plane Cu- and In-containing modes. The latter are due to changes in the
Cu/In coordination environments and associated order-disorder processes.
Moreover, Tg takes place in two steps with another large hysteresis region and
significant underlying scattering. Combined with imaging of the room
temperature phase separation, this effort lays the groundwork for studying
CuInP2S6 under external stimuli and in the ultra-thin limit.
