Surface reconstructions govern ice nucleation on silver iodide.

Silver iodide (AgI) is among the most effective ice-nucleating agents, attributed to its close lattice match with hexagonal ice. However, the atomic-level mechanism behind its efficiency remains unclear. The basal surfaces of AgI are polar and inherently unstable, necessitating a compensation mechanism, such as surface reconstruction, which may disrupt the favorable lattice match with ice. We combine noncontact atomic force microscopy with advanced computational modeling to determine the atomic structure of basal AgI surfaces in ultrahigh vacuum. The Ag-terminated (0001) surface exhibits a (2 × 2) reconstruction with ordered Ag vacancies, preserving a hexagonal arrangement of surface atoms that facilitates epitaxial ice growth. In contrast, the I-terminated (000[Formula: see text]) surface adopts a complex rectangular reconstruction, incompatible with continuous ice layer formation. These findings highlight the decisive role of surface atomic structure and indicate that the Ag-terminated basal plane is primarily responsible for efficient ice nucleation on AgI.