Contrasting oxide crystal chemistry of Nb and Ta:: The structures of the hexagonal bronzes BaTa 2O 6 and Ba 0.93Nb 2.03O 6

Abstract

The high-temperature hexagonal forms of BaTa 2O 6 and Ba 0.93Nb 2.03O 6 have P6 /mmm symmetry with unit-cell parameters a=21.116(1) Å, c=3.9157(2) Å and a=21.0174(3) Å, c=3.9732(1) Å, respectively. Single crystal X-ray structure refinements for both phases are generally consistent with a previously proposed model, except for displacements of some Ba atoms from high-symmetry positions. The structures are based on a framework of corner- and edge-connected Nb/Ta-centred octahedra, with barium atoms occupying sites in four different types of [0 0 1] channels with hexagonal, triangular, rectangular and pentagonal cross-sections. The refinements showed that the non-stoichiometry in the niobate phase is due to barium atom vacancies in the pentagonal channels and to extra niobium atoms occupying interstitial sites with tri-capped trigonal prismatic coordination. The origin of the non-stoichiometry is attributed to minimisation of non-bonded Ba–Ba repulsions. The hexagonal structure is related to the structures of the low-temperature forms of BaNb 2O 6 and BaTa 2O 6, through a 30° rotation of the hexagonal rings of octahedra centred at the origin.