Ferroelectric-ferroelastic properties of K3Fe5F15 and the phase transition at 490 K

Abstract

K3Fe5F15 has previously been predicted to be both ferroelectric and ferroelastic, with a phase transition at 535 K, on the basis of the atomic coordinates given in Acta Crystallogr. Sect. B 29, 1654 (1973). Subsequently, the dielectric permittivity has been found to reach a maximum at 495(10) K as the dielectric loss undergoes a change in slope, characteristic of ferroelectric behavior. Furthermore, the heat capacity exhibits a λ-type anomaly at 490(10) K, with a corresponding entropy change of ΔS=5.5(2) J mol−1 K−1. The entropy change at the phase transition calculated from the predicted change in structure is 5.42 J mol−1 K−1. Ferroelastic domains present at room temperature disappear sharply on heating above 490(10) K, as K3Fe5F15 transforms from orthorhombic to tetragonal symmetry, and reappear on cooling below 480(10) K. The ferroelectric-ferroelastic properties in the orthorhombic phase are shown to be fully coupled.