Effect of heat treatment on the structure of L-Ta 2O 5: . a study by XRPD and HRTEM methods

The effect of heat treatment on the structure of L-Ta ₂O ₅ 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-Ta ₂O ₅ 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-Ta ₂O ₅ slabs with m values in the range m=11-14 were seen. A new model of a structure of L-Ta ₂O ₅ ( m=11) is proposed. The model can be described as an ordered intergrowth of slabs of α-U ₃O ₈ and β-U ₃O ₈ types. The α-U ₃O ₈ slabs are wider and contain somewhat larger three-sided tunnels that appear to be more suitable for interstitial Ta atoms than the β-U ₃O ₈ slabs. The density measurements confirm that additional Ta atoms are present in the structure.