The Crystal Chemistry of L-Ta2O5 and Related Structures
I E. Grey, W G. Mumme, Robert S. Roth
Single crystals of a new form of L-Ta2O5 with a 19 x b superstructure have been synthesised by flux growth. The phase has C-centred monoclinic symmetry with a = 6.199(1) , b = 69.55(1) (= 19 x 3.66 ), c = 3.880 (1) . The structure was refined in space group C112/m to R1 = 0.044 for 814 unique reflections with F > 4(F). The structure can be described as comprising chains of edge-shared TaO7 pentagonal bipyramids that are regularly folded at (010) planes to give sinusoidal chains along . These chains are interconnected along  and  by corner sharing, creating inter-chain regions that are occupied by isolated TaO6 octahedra and pairs of corner-shared octahedra. The structure conforms broadly to the model for a 19L-Ta2O5 related structure proposed by Roth and Stephenson (1969) but differs in two important respects the structure has a maximum alternation of chains containing 6 and 8 edge-shared TaO7 units, 6-8-6-8, rather than the sequence 6-6-8-8 originally reported, and the M2O5 stoichiometry is achieved by having Ta interstitial atoms rather than oxygen vacancies. The Roth and Stephenson model, with inclusion of the maximum alternation principle, can explain the structural chemistry variations in all known L-Ta2O5-related structures. However, to be consistent with this model, the space group symmetry of the reported 8 x b superstructure should be lowered from Pbam to Pb21m.
Journal of Solid State Chemistry
crystal structure, flux growth, single crystals, Ta<sub>2</sub>O<sub>5</sub>, tantalum oxide