A structural study of a nonstoichiometric niobium-zirconium oxyfluoride with the ReO3 type structure

Within the ZrNbOF quaternary system a nonstoichiometric compound of the ReO₃ structure is formed with composition limits given by MX_2.9MX_3.14. Crystals at the lower end of this range with a composition of Nb_0.55Zr_0.45O_1.1F_1.8 (MX_2.9) have been investigated by optical and electron microscopy and by powder and single crystal X-ray diffraction in order to determine the mode by which the nonstoichiometry has been accommodated in the structure. It was found that the material does not contain crystallographic shear planes or have a perovskite bronze type of structure but contains vacancies in the anion lattice. It seems likely that these defects are associated into clusters with a definite structure rather than being isolated from each other.