The first ferroelectric fluoride with a tungsten bronze-type structure

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Abstract
K3Fe5F15 has been predicted to be both ferroelectric and ferroelastic, with a phase transition at 535 K, on the basis of the atomic coordinates. 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 Δ S = 5.5 (2) J mol−1 K−1. Ferroelastic domains present at room temperature disappear sharply on heating above 490 (10) K, as K3 Fe5 F15 transforms from orthorhombic to tetragonal symmetry, and reappear on cooling below 480 (10) K. The phase transition was also predicted to be accompanied by a change from order to disorder among the Fe2+, Fe3+ ions on heating above Tc, and the prediction is confirmed by the thermal dependence of the Mössbauer effect in which lines due to Fe2+ broaden as an anomaly appears in the hyperfine structure at Tc.