Epitaxial light actinide oxide thin films

Epitaxial thin films of ThO₂ and (U_𝑥Th_1−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 (U_𝑥Th_1−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.