Atomic Radius Mismatch: A Key Parameter for Design and Synthesis of High‐Entropy Physical Vapor Deposition Coatings—Review

High‐entropy metal sublattice coatings (HESCs) prepared by physical vapor deposition (PVD) have received attention due to their diverse range of properties and applications. One of the most critical requirements for their synthesis is the prediction and control of their widespread properties, and several efforts have been undertaken using various thermodynamical parameters. The majority of these predictions concentrate on high‐entropy alloys (HEAs) while high‐entropy ceramics receive little attention. One of the most important parameters to control the structure and properties of HEAs is their atomic radius mismatch (δr), which is applied to crystalline, amorphous, and composite (amorphous matrix, crystalline matrix, and multilayer) HESCs. Based on the relationships between δr and structure, mixing enthalpy (Δ H mix ), electronegativity difference (Δχ), ion bonding percentage (IBP HE ), mechanical properties (including hardness, H, and Young's modulus, E), and wear performance descriptors (H/E and H 3 /E 2 ratios), a δr‐based map to aid the design and selection of elements for HESCs is provided.