Initial decomposition mechanisms and the inverse effects of temperature and PH2 on the thermodynamic stability of UH3

Abstract

The thermodynamic stability of uranium hydrides is of broad interest and fundamental importance for understanding the hydriding corrosion of uranium, and the storage and isotope separation of hydrogen. Based on the first-principles calculations, we reveal the initial decomposition mechanism, interpret the experimental pyrolysis results, and discuss the inverse effects of temperature and hydrogen pressure (P_H2) on the thermodynamic stability of β-UH₃. The decomposition mechanism of β-UH₃ is found to be closely related to the changes of U–H bonding properties in UH₁₂ cages. Specifically, at the beginning it is difficult to break the first U–H covalent bond in each UH₁₂ cage, which brings in the existence of a concave region in the experimental P_H2–C–T curve; however, it boosts the itinerant character of U-5f electrons. Thereafter, the formation energy of H-vacancies in the degraded UH₁₁ cages is almost changeless when the H/U atom ratio decreases, resulting in the van’t Hoff plateau of the P_H2–C–T curve. Based on the above mechanisms, we propose a theoretical method to evaluate the thermodynamic stability of β-UH₃. The calculated P_H2–C–T curve is consistent with experiment, showing that temperature promotes β-UH₃ decomposition and P_H2 plays an opposite role. Moreover, this method is independent of experimental calibration and is applied to discuss the isotope effect of hydrogen in β-UH₃. This work provides new insight and a practical method for the scientific studies of uranium hydride, which is also essential to industrial applications in hydrogen isotope separation.