Contrasting Oxide Crystal Chemistry of Nb and Ta. The Structures of the Hexagonal Bronzes BaTa2O6 and Ba0.93Nb2.03^O6
W G. Mumme, I E. Grey, Robert S. Roth, Terrell A. Vanderah
The crystal structures of the title phases have been determined as part of a general study of the photoelectrochemical activities of niobates and tantalates, which are controlled by the structural details that determine electronic energy levels. The hexagonal forms of BaTa2O6 and Ba0.93Nb2.03O6 have P6/mmm symmetry with unit cell parameters a = 21.1163(9) , c = 3.9157(2) and a = 21.0474(3) , c = 3.9803(1) respectively. Single crystal X-ray structure refinements for both phases are generally consistent with a previously proposed model, except for the location of one of the barium atoms. 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  channels with hexagonal, triangular, rectangular and pentagonal cross sections. The refinements showed that the non-stoichiometry in the niobate phase was 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 BaNb2O6 and BaTa2O6, through a 30 rotation of the hexagonal rings of octahedra centred at the origin.
, Grey, I.
, Roth, R.
and Vanderah, T.
Contrasting Oxide Crystal Chemistry of Nb and Ta. The Structures of the Hexagonal Bronzes BaTa<sub>2</sub>O<sub>6</sub> and Ba<sub>0.93</sub>Nb<sub>2</sub>.03^O<sub>6</sub>, Journal of Solid State Chemistry
(Accessed September 21, 2023)