Predicting and Accessing Metastable Phases

Metastable forms of matter are invaluable to our everyday lives, from advancing technology to understanding biological processes, with their unique properties often offering novel functionality. Despite their importance, synthesizing metastable phases is more art than science, often either serendipitous or trial-and-error. Insight into the amount of stored energy needed to form a metastable phase can aid in their fabrication. Here, we calculate metastable phase diagrams, from which we extract the metastability threshold – the excess energy stored in the metastable phase relative to the ground state. Using lanthanide sesquioxides (Ln₂O₃) as a case study, we demonstrate how metastable phase diagrams provide new insight into their synthesis and irradiation behavior. We find the metastability threshold above which metastable phases cannot be synthesized from thermal decomposition, explaining why non-equilibrium synthesis is required for fabrication of certain metastable phases. We also discuss a reverse relation between the metastability threshold and kinetics at increased temperatures, and their influence on the formation of metastable phases. Lastly, we successfully predict the sequence of metastable phases induced by irradiation in Lu₂O₃, forming three metastable phases with increasing irradiation fluence and displaying unique irradiation behavior not observed in other materials.