Metastability of Lanthanide Sesquioxide (Ln₂O₃) Polymorphs

Metastable phases can have distinct properties compared to the ground state, enhancing the utility of the material towards a particular technological application. The applicability of lanthanide sesquioxides (Ln₂O₃) as irradiation resistant materials has been attributed to the relative ease with which they transform to different polymorphs. Insights into the amount of stored energy that different metastable phases need via irradiation before they can access the respective ground states can be crucial towards a deeper understanding of their irradiation response. Here we employ density functional theory computations to examine the metastable phase diagrams of Ln₂O₃ compounds in order to quantify extent of metastability for each in five different polymorphs. We find the extent of metastability above which these phases cannot be synthesized, explaining why only certain metastable phases can be fabricated using non-equilibrium synthesis. We correlate the extent of metastability and the lowest irradiation dose that can induce metastable phase transitions, and use this information to predict the irradiation dose that can induce phase transitions in a range of Ln₂O₃ chemistries. We also discuss an inverse relation between the extent of metastability and kinetics at increased temperatures, and their influence on the formation of metastable phases. This work provides new insight into the formability of these phases, and their potential use as nuclear wasteforms materials and corrosion resistive coatings.