Aminoguanidinium hexafluorozirconate: a new ferroelectric

Abstract

Analysis of the atomic arrangement in anhydrous aminoguanidinium hexafluorozirconate, CN4H8ZrF6, reported by Bukvetskii, Gerasimenko & Davidovich [Koord. Khim.(1990),16, 1479–1484], led to the prediction that it is a new ferroelectric [Abrahams, Mirsky & Nielson (1996).Acta Cryst.B52, 806–809]. Initial attempts to verify the prediction were inconclusive because of the variety of closely related materials produced under the original preparation conditions. Clarification of these conditions led to the formation of pure CN4H8ZrF6and the growth of single crystals with dimensions as large as 7 × 7 × 2 mm. Highly reproducible calorimetric and dielectric permittivity anomalies reveal the Curie temperatureTc= 383 (1) K. At this temperature, the heat capacityCpexhibits an entropy change of 0.7 (1) J mol−1 K−1, while the relative permittivity ∊rexhibits an inflection and the dielectric loss a distinct peak; the dielectric anomaly atTcis observed only at the lowest (0.1–1 kHz) frequencies used. Dielectric hysteresis is demonstrable at 295 K under the application of ∼1 MV m−1alternating fields and remains observable at allT<Tcbut not atT≥Tc; the prediction of ferroelectricity is hence confirmed. The value of the spontaneous polarizationPsis 0.45 (9) × 10−2 C m−2at 298 K, with piezoelectric coefficientd33= 1.9 (5) pC N−1and pyroelectric coefficientp3= 4 (1) µC m−2 K−1. Tilts of less than ∼11° by the two symmetry-independent CN4H{}_{8}^{2+} ions, combined with rotations of ∼20° or less by the N—NH3and C—(NH2)2groups about the central C—N bond in each cation, as all H atoms rotate into or become symmetrically distributed about the planes atz= 0 or ½, allow them to conform to mirror symmetryviapolar atomic displacements of ∼0.4 Å or less by N or C, and of 0.7 Å or less by H. Corresponding displacements of less than 0.08 Å within the two symmetry-independent ZrF{}_{6}^{2-} anions also result in mirror symmetry, satisfying the structural criteria required for the development of ferroelectricity.