Aminoguanidinium hexafluorozirconate: a new ferroelectric

Analysis of the atomic arrangement in anhydrous aminoguanidinium hexafluoro zirconate, CN4H8ZrF6, reported by Bukvetskii, Gerasimenko & Davidovich [Koo rd. Khim. (1990), 16, 1479-1484], led to the prediction that it is a new fe rroelectric [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 preparatio n conditions. Clarification of these conditions led to the formation of pur e CN4H8ZrF6 and the growth of single crystals with dimensions as large as 7 x 7 x 2 mm. Highly reproducible calorimetric and dielectric permittivity a nomalies 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) alterna ting fields and remains observable at all T < T-c but not at T <greater tha n 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 pyroele ctric 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 rot ations 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 b ecome symmetrically distributed about the planes at z = 0 or 1/2, allow the m to conform to mirror symmetry via polar atomic displacements of similar t o0.4 Angstrom or less by N or C, and of 0.7 Angstrom or less by H. Correspo nding displacements of less than 0.08 Angstrom within the two symmetry-inde pendent ZrF62- anions also result in mirror symmetry, satisfying the struct ural criteria required for the development of ferroelectricity.