KCaNb2O6F from a combined synchrotron X-ray and neutron powder diffraction study

Abstract

Potassium calcium diniobium hexa­oxide fluoride, KCaNb2O6F, crystallizes in the cubic pyrochlore-type structure. In the crystal structure, NbO6 octa­hedra are linked together by common corners to form a three-dimensional [Nb2O6]2− network. (K,Ca) and F atoms, which are located in the voids of the [Nb2O6]2− network, form zigzag [(K,Ca)F]2+ chains. The K and Ca atoms are distributed statistically over the same crystallographic position (site-occupancy factor = 0.5), having an eightfold coordination by six O atoms from the NbO6 octa­hedra and by two F atoms. All atoms lie on special positions, viz. Nb has \overline{3}m, the (K,Ca) site has \overline{3}m, O has 2mm and F has \overline{4}3m site symmetry.