Modulation of Perovskite-Related Framework Induced by Alkaline Earth Metal in Phosphate Fluorides A2MPO4F (A= K, Rb; M = Ba, Ca)

A new phosphate fluoride, Rb2CaPO4F was synthesized from a high temperature solution. Single-crystal X-ray diffraction analysis indicates that Rb2CaPO4F belongs to Pnma space group of orthorhombic system. It exhibits an intricate three dimensional (3D) framework that consists of 1D infinite zig-zag [FRb2Ca]∞ chains interlinked by isolated PO4 tetrahedra. The 1D chains are composed of plane-sharing F-centered [FRb4Ca2] octahedra. Meanwhile, we also obtained another phosphate fluoride K2CaPO4F, which is isostructural with Rb2CaPO4F. Moreover, the effect of the cation size on the framework structures of A2CaPO4F (A= K, Rb) and K2BaPO4F has been discussed in detail. In addition, the UV-Vis-NIR diffuse reflectance spectra and infrared spectra are measured to determine their cut-off edge and anionic group, respectively. The first-principles theoretical studies are also done to aid the understanding of electronic structures and linear optical properties.