Effect of heat treatment on the structure of L-Ta2O5:: a study by XRPD and HRTEM methods

The effect of heat treatment on the structure of L-Ta2O5 has been studied by X-ray powder diffraction and high-resolution transmission electron microscopy, complemented by density measurements. Two stable low-temperature forms of L-Ta2O5 were found: one below about 1000degreesC with a b* multiplicity of m approximate to 13.5 and the other at 1350degreesC with m = 11. The former modification was disordered, containing defects and twins, while the latter seemed to be more ordered. At intermediate temperatures, ordered and disordered mixtures of L-Ta2O5 slabs with m values in the range m = 11 - 14 were seen. A new model of a structure of L-Ta2O5 (m = 11) is proposed. The model can be described as an ordered intergrowth of slabs of alpha-U3O8 and beta-U3O8 types. The alpha-U3O8 slabs are wider and contain somewhat larger three-sided tunnels that appear to be more suitable for interstitial Ta atoms than the beta-U3O8 slabs. The density measurements confirm that additional Ta atoms are present in the structure. (C) 2003 Elsevier Inc. All rights reserved.