³¹P MAS NMR Study of the Ferrielectric−Paraelectric Transition in Layered CuInP₂S₆

Abstract

The order-disorder ferrielectric-paraelectric transition in lamellar CuInP 2 S 6 is studied using 31 P 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 P 2 S 6 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 (≈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.

Broadband dielectric spectroscopy of CuInP2Se6 crystals

“…More accurate broadband dielectric investigations showed only nearly second order phase transition at T c =226 K [9]. From a single-crystal X-ray diffraction study follows that the high-and low-temperature structures of CuInP 2 Se 6 (trigonal space group P-31c and P31c, respectively) are very similar to those of CuInP 2 S 6 in the paraelectric and ferrielectric phases, with the Cu I off-centering shift being smaller in the former than in the latter [3,8]. There the thermal evolution of the cell parameters of CuInP 2 Se 6 was obtained by full profile fits to the X-ray diffractograms.…”

“…CuInP 2 S 6 crystals represent an unusual example of a anticollinear two-sublattice ferrielectric system [1,2,3,4]. Here a first-order phase transition of the orderdisorder type from the paraelectric to the ferrielectric phase is realized (T c = 315 K).…”

“…The spontaneous polarization arising at the phase transition to the ferrielectric phase is perpendicular to the layer planes. These thiophosphates consist of lamellae defined by a sulphur framework in which the metal cations and P -P pairs fill the octahedral voids; within a layer, the Cu, In, and P-P form triangular patterns [1,2,3]. The cation off-centering, 1.6Å for Cu I and 0.2 A for In III , may be attributed to a second-order JahnTeller instability associated with the d 10 electronic configuration.…”

Investigation of longitudinal 31P relaxation in metal selenophosphate compounds

“…Interesting properties and uses exhibited by some [P 2 Q 6 ] 4À containing compounds are: ferroelectricity for use in memory devices (Sn 2 P 2 S 6 , CuInP 2 S 6 ) [11,12]; non-linear optical properties (Mn 2 P 2 S 6 ) [5]; and photoconductivity (In 1.33 P 2 Se 6 ) [13,14].…”

“…Studies of this type are relatively rare [12,[15][16][17][18] and a wide compilation of NMR data on [P x Q y ] nÀ anions should yield new insights into chalcophosphate compounds. In a previous NMR study, we focused on correlations between 31 P chemical shifts (CSs) or chemical shift anisotropies (CSAs) and local structure and bonding in metal selenophosphates [19].…”