K3Na(TaF7)(SiF6): a mixed-anion pentanary fluoride with zero-dimensional anions exhibiting a large band gap

Abstract

Metal halides have potential applications in many optics-related fields, such as birefringence materials. Here, a pentanary fluoride K₃Na(TaF₇)(SiF₆) has been obtained through a facile hydrothermal route. It crystallizes in the orthorhombic space group Immm, and its zero-dimensional framework features isolated SiF₆ octahedra and TaF₇ monocapped trigonal prisms, a new type of combination representing a new structure-type. It exhibits two wide optical band gaps of 4.46 eV and 5.67 eV which is consistent with DFT calculation. K₃Na(TaF₇)(SiF₆) exhibits a larger birefringence (0.045 at 253.7 nm) compared with the commercial MgF₂ crystal.