31P MAS NMR study of the ferrielectric-paraelectric transition in layered CuInP2S6

The order-disorder ferrielectric-paraelectric transition in lamellar CuInP2S6 is studied using P-31 solid-state MAS NMR spectroscopy. Spectra from a powder sample were recorded at various temperatures between 255 and 355 K with a probe precalibrated for heating or cooling due to magic angle spinning. Two center bands are observed at the lowest measured temperature while only one is detected at the highest temperature. The former two represent the inequivalent positions for the P atoms of the P2S6 group which reflect the antiparallel displacements of the polar Cu-I and In-III sublattices in the ferrielectric phase. The latter corresponds to the appearance of a 2-fold axis through the P-P bond as the Cu-I ions undergo double-well hopping motions, and the In-III ions occupy on-center sites in the paraelectric phase. At intermediate temperatures, both ferrielectric and paraelectric type resonances contribute to the spectrum at ratios which are T-dependent, indicating a transition temperature T-c = 312 +/- 1 K (310 +/- 1 K) for a warming (cooling) cycle. The chemical shifts of the center bands characteristic of the ferrielectric phase are asymmetrically disposed with respect to that of the paraelectric type signal and exhibit distinct thermal variations; the line widths likewise evolve differently with temperature. Line-narrowing attributable to thermally enhanced motions is observed for the paraelectric type resonance upon warming across the transition. The temperature range for the coexistence of the center bands representative of the two phases is unusually wide (approximate to 70 K), lying mostly below T-c. The presence of the ferrielectric type resonance in the paraelectric regime may be ascribed to the nucleation of polar order, while the persistence of the paraelectric signal well below T-c implies residual hopping motions occurring in the ferrielectric regime.