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

Abstract

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.