Aminoguanidinium hexafluorozirconate: a new ferroelectric

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 CN4H8ZrF6 and the growth of single crystals with dimensions as large as 7 x 7 x 2 mm. Highly reproducible calorimetric and dielectric permittivity anomalies reveal the Curie temperature T-c = 383 (1) K. At this temperature, the heat capacity C-p exhibits an entropy change of 0.7 (1) J mol(-1) K-1, while the relative permittivity epsilon (r) exhibits an inflection and the dielectric loss a distinct peak; the dielectric anomaly at T-c is observed only at the lowest (0.1-1 kHz) frequencies used. Dielectric hysteresis is demonstrable at 295 K under the application of similar to1 MV m(-1) alternating fields and remains observable at all T < T-c but not at T <greater than or equal to> T-c; the prediction of ferroelectricity is hence confirmed. The value of the spontaneous polarization P-s is 0.45 (9) x 10(-2) C m(-2) at 298 K, with piezoelectric coefficient d(33) = 1.9 (5) pC N-1 and pyroelectric coefficient p(3) = 4 (1) muC m(-2) K-1. Tilts of less than similar to 11 degrees by the two symmetry-independent CN4H82+ ions, combined with rotations of similar to 20 degrees or less by the N-NH3 and CD (NH2)(2) groups about the central C-N bond in each cation, as all H atoms rotate into or become symmetrically distributed about the planes at z = 0 or 1/2, allow them to conform to mirror symmetry via polar atomic displacements of similar to0.4 Angstrom or less by N or C, and of 0.7 Angstrom or less by H. Corresponding displacements of less than 0.08 Angstrom within the two symmetry-independent ZrF62- anions also result in mirror symmetry, satisfying the structural criteria required for the development of ferroelectricity.