Crystallographic, magnetic and dielectric studies of the potential multiferroic cryolite (NH₄)₃FeF₆

The cryolite phase (NH4)(3)FeF6 has been studied using variable temperature X-ray powder diffraction, together with dielectric and magnetic measurements, in order to resolve its potential multiferroic properties. The X-ray data reveal a direct transition from cubic to triclinic symmetry between 270 and 250 K, consistent with earlier reports of a ferroelectric T-c of 264 K. The crystal structure of the low temperature phase has been modelled approximately in a pseudo-tetragonal, triclinic space group F1, a = 8.98258(9) b = 8.99631(9) angstrom, c = 9.27961(9) angstrom. alpha = 90.039(1)degrees, beta = 90.451(1)degrees and gamma = 90.339(1)degrees (at 100 K) but weak superlattice reflections remain unindexed, suggesting the true unit cell to be larger. Electrical measurements indicted an increase in capacitance above 260 K, however this was accompanied by an increase in dielectric loss and so is inconclusive evidence of a ferroelectric-paraelectric phase transition. Polarisation-field loops were not possible due to overriding leakage. Magnetic susceptibility data show paramagnetic behaviour with no indication of long-range order above 2 K, and only weak antiferromagnetic interactions (theta similar to -7 K). This therefore clearly demonstrates the material is not multiferroic. (C) 2011 Elsevier Masson SAS. All rights reserved.