Phase transitions and ferroelectric behavior in the Pb3(MF6)2 family (M=Ti, V, Cr, Fe, Ga)

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Abstract
The atomic arrangement in Sr3(FeF6)2 determined by von der Mühll [C. R. Acad. Sci. Paris 2 7 8, 713 (1974)] differs from a structure of opposite polarity by atomic displacements of 1.3 Å or less for F and 0.4 Å or less for Sr and Fe. The structure is hence potentially ferroelectric, with an estimated transition temperature TC to the paraelectric state of about 1160 K: experimentally, TC=922 K. The Pb3(MF6)2 family is isomorphous with Sr3(FeF6)2 and the corresponding TC(M) has been measured as 695 (Ti), 635 (V), 550 (Cr), 740 (Fe), and 645 (Ga) K. The respective nonlinear dielectric susceptibilities are 0.010, 0.010, 0.005, 0.045, and 0.010×10−12 V−1 m. A dielectric anomaly is observed close to TC in each material at 1 kHz with a maximum permittivity ε′r of 2740 (Ti), 300 (V), 2015 (Cr), 4000 (Fe), and 7800 (Ga). Dielectric losses in each material reach a minimum at the phase transition temperature.