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

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.