Crystal structure solution for the A_6B_2O_(17) (A = Zr, Hf; B = Nb, Ta) superstructure

摘要

Zr_6Ta_2O_(17), Hf_6Nb_2O_(17) and Hf_6Ta_2O_(17) 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 Zr_6Nb_2O_(17) superstructure, leading to the classification of the A_6B_2O_(17) (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 sixcoordinated 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 symmetryequivalent seven-coordinated polyhedra accompanied by corresponding oxygen tilts within the A_6B_2O_(17) structure.