K3Fe5F15: Ferroelectric and ferroelastic behavior

Abstract

Ferroelectric oxides with structures related to that of tetragonal potassium tungsten bronze, such as Ba2NaNb5O15, K3Li2Nb5O15 and PbNb2O6, have been well known for over twenty years. Partial substitution of oxygen by fluorine has recently led to the preparation of new oxyfluorides for which the Curie temperature is strongly depressed even by minor substitution, e.g. inBa2-xNa1+xNb5O15-xFx,Tc = 835K for x = 0 and 100K for x = 1. Ferroelectricity in K3Fe5F15, an orthorhombic material with distorted tetragonal tungsten bronze-type structure, had been predicted on the basis of its previously reported atomic arrangement. The predicted Tc was 535K whereas the experimental Tc = 490 ± 10K as given by the dielectric permittivity maximum and the λ -type heat capacity anomaly. K3Fe5F15 is both ferroelectric and ferroelastic, with full coupling between these properties below Tc. The ferroelastic domains disappear sharply on heating above Tc as the symmetry changes from mm2 to 4/mmm. 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.