Stabilizing Metastable Phases in Thin Films—Approaches and Benefits

Many metastable phases are known to possess superior functional or mechanical properties, but they may be overlooked in high-throughput searches for new materials if their energy is high above the convex hull of ground state energies. Examples of metastable materials, particularly structurally frustrated, will be introduced as motivation for searching for and cultivating metastable phases. While frustration is more studied in magnetic systems, structural frustration can give rise to incommensurate phases and flat energy landscapes for short-range displacements of ions as well as low migration energy barriers. Strategies for stabilizing metastable phases in thin films will be discussed with examples and limitations, including non-epitaxial interfaces such as surfaces, precipitates, and ferroelectric and ferroelastic domain walls.

Stabilizing approaches can be classified as i) thermodynamic, enthalpy- or entropy-based, or b) kinetic, where e.g. crystallization pathways may favour metastable over stable phases. Epitaxial strain is the dominating energy contribution in thin films and the importance will be compared with other energetic contributions like heat capacity, configurational entropy, and in situ and ex situ chemical conversion to metastable phases. An extended outlook will be presented at the end where a classification of displacive and reconstructive borders for metastable phases will be suggested along with promising material systems for further studies.