Hidden layered structures from carbon-analog metastability in metal dichalcogenides

Carbon exhibits both a layered ground-state structure that produces two-dimensional (2D) nanosheets and a nonlayered diamond structure created under high pressure conditions. Motivated by this metastability relationship, we revisit the ground state structure of metal dichalcogenides that are known to have a nonlayered pyrite-type structure. Ultrathin films of pyrite-type ${\mathrm{ZnSe}}_{2}$ spontaneously transform into a layered phase. This phase is identified as a ground state, and the monolayer exhibits strong elastic anisotropy and a semiconducting bandgap larger than that of the pyrite phase by a factor of two. We demonstrate that a two-valued but directional potential energy surface exists along a Bain-like distortion path, hiding the layered ground state. This work implies that many 2D materials are hidden in nonlayered materials and connects 2D materials science with surface and high-pressure science.