K2.64Cs0.36SiF7: A New Fluorosilicate with a Trans-perovskite Structure
A new oxygen-free fluorosilicate K2.64Cs0.36SiF7 was synthesized by the high-temperature method. It crystallizes in a tetragonal system with the space group P4/mbm. The cell parameters of K2.64Cs0.36SiF7 are a = 7.768(4) Å, c = 5.560(6) Å. Single crystal X-ray diffraction shows that it has a trans-perovskite structure building by the [SiF6] and [FK4(K0.64/Cs0.36)2] octahedra. By structure analysis, the disorder place and disorder ratio of K2.64Cs0.36SiF7 is different with another M3SiF7 (M=K+, Rb+, Cs+ and NH4+) compounds. The ultraviolet cut off edge of K2.64Cs0.36SiF7 is shorter than 190 nm, and the band gap is 6.0 eV. Structure-property relationships were further illuminated via the first-principles calculation methods.