The First Ferroelectric Flouride with a Tungsten Bronze-Type Structure (NOT KNOWN IF TALK GIVEN BECAUSE AUTHOR HAS LEFT AT&T)

Ferroelectric oxides with structures related to that of tetragonal potassium tungsten bronze, such as Ba sub 2 NaNb sub 5 O sub 15, K sub 3 Li sub 2 Nb sub 5 O sub 15 and PbNb sub 2 O sub 6, 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. in Ba sub 2-x Na sub 1+x Nb sub 5 O sub 15-x F sub x, T sub c =835K for x=0 and 100K for x = 1. Ferroelectricity in K sub 3 Fe sub 5 F sub 15, an orthorhombic material with distorted tetragonal tungsten bronze-type structure, had been predicted on the basis of its previously reported atomic arrangement. The predicted T sub c was 535K whereas the experimental T sub c = 490+- 10K as given by the dielectric permittivity maximum and the lambda-type heat capacity anomaly. K sub 3 Fe sub 5 F sub 15 is both ferroelectric and ferroelastic, with full coupling between these properties below T sub c. The ferroelastic domains disappear sharply on heating above T sub c as the symmetry changes from mm2 to 4/mm. The phase transition was also predicted to be accompanied by a change from order to disorder among the Fe sup 2+, Fe sup 3+ ions on heating above T sub c, and the prediction is confirmed by the thermal dependence of the Mossbauer effect in which lines due to Fe sup 2+ broaden as as anomaly appears in the hyperfine structure at T sub c.