Structural and Optical Properties of Phase-Pure UO₂, α-U₃O₈, and α-UO₃ Epitaxial Thin Films Grown by Pulsed Laser Deposition

Abstract

Fundamental understanding of the electronic, chemical, and structural properties of uranium oxides requires the synthesis of high-crystalline-quality epitaxial films of different polymorphs of one material or different phases with various oxygen valence states. We report the growth of single-phase epitaxial UO2, α-U3O8, and α-UO3 thin films using pulsed laser deposition. Both oxygen partial pressure and substrate temperature play critical roles in determining the crystal structure of the uranium oxide films. X-ray diffraction and Raman spectroscopy demonstrate that the films are single phase with excellent crystallinity and epitaxially grown on a variety of substrates. Chemical valance states and optical properties of epitaxial uranium oxide films are studied by X-ray photoelectron spectroscopy and UV–vis spectroscopy, which further confirm the high-quality stoichiometric phase-pure uranium oxide thin films. Epitaxial UO2 films show a direct band gap of 2.61 eV, while epitaxial α-U3O8 and α-UO3 films exhibit indirect band gaps of 1.89 and 2.26 eV, respectively. The ability to grow high-quality epitaxy actinide oxide thin films and to access their different phases and polymorphous will have significant benefits to the future applications in nuclear science and technology.

Irradiation-Driven Restructuring of UO2 Thin Films: Amorphization and Crystallization

“…Uranium dioxide (UO 2 ) is the most commonly used fuel for nuclear energy, and its mineral analogue uraninite is the dominant ore of uranium. − UO 2 exists in a single-phase cubic fluorite structure at temperatures up to the melting point (2865 °C), − and hyperstoichiometric (UO 2+ x , 0 < x < 0.25) cubic structures are also important. ,, UO 2 is stable during reactor operation and accommodates substantial radiation damage without amorphization. − The fission products, neutrons, and large thermal gradients generate a complex network of defects within the large (5–10 μm) UO 2 grains and fission gas (Kr, Xe) bubbles at grain boundaries. ,,− Prolonged radiation exposure leads to considerable restructuring at the outer edges of UO 2 fuel pellets. , The resulting morphology, known as high-burnup structure (HBS), consists of nanoscale grains and pores. − The HBS changes the thermophysical properties of fuel pellets and increases radiation tolerance. A better understanding of HBS formation mechanisms is essential for predicting fuel performance for UO 2 and developing new nanostructured fuels for nuclear energy.…”

Advances in actinide thin films: synthesis, properties, and future directions

“…Reactive PVD (RPVD) occurs when a chemical reaction occurs between the deposited material and the atmosphere surrounding the substrate, often during an annealing step. In contrast, pulsed laser deposition (PLD) is a form of physical vapor deposition where a high power, low bandwidth laser is used to melt, evaporate, and ionize material from the surface of a target [3].…”

“…[72], and (e) an XRD diffraction pattern for UN [93]. The equivalent data for UO2 come from (f) [94], (g) and (h) [76], (i) [3], (j) [19], and (k) from [76]. Note that ARPES data do not yet exist for UN thin films, making initial electronic characterization a near-term target for any new synthesis.…”

“…They also exhibit divergent magnetic properties, from nearly magnetic (Np and Pu) to transition-metal-like behavior (Th, Pa, U) [2]. Technologically, the most widely used actinide compound to date has been UO 2 , in its role as the primary fuel for commercial nuclear energy production [3,4]. Despite these unique physical properties, their scarcity, inherent radioactivity, and challenges concerning their synthesis mean that actinide materials are still some of the least understood elements.…”

“…(a) RBS data from UN before and after radiation exposure ([70]), (b) XPS data for a UN thin film ( [45]), (c) SEM micrograph of a UN thin film ( [71]), (d) UN Raman spectrum[72], and (e) an XRD diffraction pattern for UN[93]. The equivalent data for UO2 come from (f)[94], (g) and (h)[76], (i)[3], (j)[19], and (k) from[76]. Note that ARPES data do not yet exist for UN thin films, making initial electronic characterization a near-term target for any new synthesis.…”