Epitaxial light actinide oxide thin films

Abstract

Epitaxial thin films of ThO2 and (UxTh1−x)O2 mixed oxides (MOX) have been synthesised by DC magnetron sputtering. The samples were characterised using x-ray diffraction and spectroscopic ellipsometry. Three epitaxial ThO2 samples of different crystallographic orientations have been synthesised and confirmed by x-ray diffraction analysis. The samples are [1 1 1], [1 1 0] and [0 0 1] oriented, with lattice parameters determined to be 5.714 ± 0.001 Å, 5.707 ± 0.001 Å and 5.624 ± 0.001 Å respectively. Four [0 0 1] oriented epitaxial (UxTh1−x)O2 samples have been fabricated, across 0≤x≤1, with all samples found to have a unique specular direction. The mixed oxides were found to obey Vegard’s law, with lattice parameter varying linearly with ThO2 content. Values for the optical band gap and optical constants of the epitaxial ThO2 and MOX samples have also been determined by spectroscopic ellipsometry. The band gap determined for the three ThO2 samples were in the 3.9 - 4.6 eV range, which is a slight underestimate compared to other experimental values. The optical band gap was found to vary approximately linearly with ThO2 content, across the sample series. These films can be implemented in the research of thorium-based nuclear fuels, mixed actinide oxide nuclear fuel and the long term storage of nuclear waste forms.