Optical properties and isotropy in anisotropic alkali tetrafluoroborates

Abstract

Ternary fluoride crystals are significant optical materials, and we present a comprehensive first-principles study of alkali tetrafluoroborates ABF₄ (A = Na, K, Rb, Cs). This includes an analysis of their electronic structure, optical properties, as well as mechanical and vibrational characteristics. The vibrational analysis reveals relatively weak ionic bonding of the alkali metal cations and a rigid (BF₄)^- polyanion structure, consistent with experimental crystal chemistry. Hybrid functional calculations of the band structure show that the valence band maximum arises primarily from F p states. In contrast, the conduction band is formed by both B p and alkali s states, with indirect band gaps. Despite the anisotropic, layered crystal structures, these materials exhibit minimal optical birefringence, which is discussed concerning their chemical groups.