Synthesis, structures, and properties of two magnesium silicate fluorides Mg5(SiO4)2F2 and Mg3SiO4F2

Abstract

Two magnesium silicate fluorides, Mg₅(SiO₄)₂F₂ (compound 1) and Mg₃SiO₄F₂ (compound 2), have been successfully synthesized by solid-state reactions for the first time. Compound 1 crystallizes in the space group P2₁/c (no. 14) of the monoclinic system, whereas the latter belongs to the space group Pnma (no. 62) of the orthorhombic system. As for the crystal structure, compound 1 can be described on the basis of a “framework” made by Mg1O₄F₂ and Mg2O₅F octahedra forming many rings, in which the Mg3 and Si atoms reside. Compound 2 can also be described as the rings created by Mg2O₄F₂ octahedra, in which the Mg1 and Si atoms reside. Thermal analysis and the infrared spectrum of compound 1 were measured. In addition, the UV-Vis-NIR diffuse-reflectance spectrum exhibits that the relatively wide band gap is about 5.32 eV for compound 1, which is consistent with the theoretical value. The band structure and linear optical properties of title compounds are also theoretically calculated.