Aminoguanidinium hexafluorozirconate: a new ferroelectric

摘要: Analysis of the atomic arrangement in anhydrous aminoguanidinium hexafluorozirconate, CN4H8ZrF6, reported by Bukvetskii, Gerasimenko & David­ovich [Koord. Khim. (1990), 16, 1479–1484], led to prediction that it is a new ferroelectric [Abrahams, Mirsky Nielson (1996). Acta Cryst. B52, 806–809]. Initial attempts verify were inconclusive because variety closely related materials produced under original preparation conditions. Clarification these conditions formation pure CN4H8ZrF6 and growth single crystals with dimensions as large 7 × 2 mm. Highly reproducible calorimetric dielectric permittivity anomalies reveal Curie temperature Tc = 383 (1) K. At this temperature, heat capacity Cp exhibits an entropy change 0.7 (1) J mol−1 K−1, while relative ∊r inflection loss distinct peak; anomaly at observed only lowest (0.1–1 kHz) frequencies used. Dielectric hysteresis demonstrable 295 K application ∼1 MV m−1 alternating fields remains observable all T < but not ≥ Tc; ferroelectricity hence confirmed. The value spontaneous polarization Ps 0.45 (9) 10−2 C m−2 298 K, piezoelectric coefficient d33 1.9 (5) pC N−1 pyroelectric p3 4 (1) µC m−2 K−1. Tilts less than ∼11° two symmetry-independent CN4H{}_{8}^{2+} ions, combined rotations ∼20° or N—NH3 C—(NH2)2 groups about central C—N bond each cation, H atoms rotate into become symmetrically distributed planes z 0 ½, allow them conform mirror symmetry via polar displacements ∼0.4 A N C, 0.7 A H. Corresponding 0.08 A within ZrF{}_{6}^{2-} anions also result symmetry, satisfying structural criteria required for development ferroelectricity.