Contrasting oxide crystal chemistry of Nb and Ta:: The structures of the hexagonal bronzes BaTa2O6 and Ba0.93Nb2.03O6

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