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 1000°C with a b^* multiplicity of m≈13.5 and the other at 1350°C 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 α-U₃O₈ and β-U₃O₈ types. The α-U₃O₈ slabs are wider and contain somewhat larger three-sided tunnels that appear to be more suitable for interstitial Ta atoms than the β-U₃O₈ slabs. The density measurements confirm that additional Ta atoms are present in the structure.