Polymorphic crystalline wetting layers on crystal surfaces

Analogous to surface premelting, we show that a crystal surface can undergo a pre-solid–solid transition. This means that it develops a thin polymorphic crystalline layer before reaching the solid–solid transition temperature if two crystals can form a low-energy coherent interface. We confirm this in simulations and colloid experiments at single-particle resolution. The power-law increase of surface layer thickness is analogous to premelting. Different kinetics and reversibilities of surface-crystal growth are observed in various systems. Surface crystals exist not only under thermal equilibrium but also during melting, crystallization and grain coarsening. Furthermore, the premelting and pre-solid–solid transition can coexist, resulting in double surface wetting layers. We hypothesize that such surface phenomena also exist in some atomic and molecular crystals, and this could provide a mechanism to tune the properties of the materials. © 2023, The Author(s), under exclusive licence to Springer Nature Limited.