The first ferroelectric fluoride with a tungsten bronze-type structure

K 3 Fe 5 F 15 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 K 3 Fe 5 F 15 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 Fe 2+ , Fe 3+ ions on heating above T c , and the prediction is confirmed by the thermal dependence of the Mössbauer effect in which lines due to Fe 2+ broaden as an anomaly appears in the hyperfine structure at T c .