Crystal structure solution for the A₆B₂O₁₇ (A = Zr, Hf; B = Nb, Ta) superstructure

Zr₆Ta₂O₁₇, Hf₆Nb₂O₁₇ and Hf₆Ta₂O₁₇ crystal structure solutions have been solved using synchrotron X-ray powder diffraction and neutron powder diffraction in conjunction with simulated annealing, charge flipping and Rietveld refinement. These structures have been shown to be isomorphous with the Zr₆Nb₂O₁₇ superstructure, leading to the classification of the A₆B₂O₁₇ (A = Zr, Hf; B = Nb, Ta) orthorhombic compound family with symmetry Ima2 (No. 46). The asymmetrical structural units of cation-centred oxygen polyhedra used to build the structure are as follows: (i) one set of symmetry-equivalent six-coordinated polyhedra, (ii) three sets of symmetry-equivalent seven-coordinated polyhedra and (iii) one set of symmetry-equivalent eight-coordinated polyhedra. The potential for cation order and disorder was discussed in terms of cation atomic number contrast in X-ray and neutron powder diffraction as well as the bond valence method. In addition, the structural mechanisms for experimentally observed compositional variations within the solid solution range can be attributed to the addition or removal of a set of symmetry-equivalent seven-coordinated polyhedra accompanied by corresponding oxygen tilts within the A₆B₂O₁₇ structure.