Discovering low-permittivity materials: Evolutionary search for MgAl2O4 polymorphs

Low-permittivity ceramics for use as microwave band window materials must have good mechanical and optical characteristics. Unfortunately, most known ceramics are opaque to microwaves because of their high dielectric permittivity. Here, we present an effective theoretical method that may be helpful in the development of innovative low-permittivity materials. Using the material's permittivity as a global optimization function, we performed a crystal structure search for MgAl2O4 using an ab initio evolutionary algorithm implemented in the USPEX code. We identified four predicted MgAl2O4 polymorphs ( P4¯m2, P4¯2m, Cc, and Pc) that had lower permittivities than MgAl2O4 spinel (Fd3¯m). Our results indicate that the density is not the only factor that affects permittivity. Further analysis of permittivity from the viewpoint of the underlying structures shows that reduced permittivity can be effectively achieved by reducing the cation coordination number. This insight will help in the discovery of materials with...