Reinvestigation and Characterization of the Magnesium Borate Fluoride Mg-5(BO3)F

The magnesium borate fluoride Mg-5(BO3)(3)F was grown by spontaneous crystallization with molten flux based on the MgF2-LiF-Na2CO3-H3BO3 solvent. Structure solution from single-crystal X-ray diffraction shows that the title compound crystallizes in the orthorhombic space group Pnma (No. 62) with cell dimensions of a = 10.068(5) angstrom, b = 14.858(7) angstrom, c = 4.540(2) angstrom and Z = 4. Its structure features a three-dimensional (3D) Mg-O-F framework composed of MgO5F and MgO6 polyhedra and isolated BO3 groups. The detailed structure comparison referred to Mg-5(BO3)(3)F, Mg3BO3F3 and beta-Mg2BO3F was carried out. The infrared spectrum (IR) and the bond valence sum (BVS) calculations of Mg-5(BO3)(3)F verify the validity of the structure. The calculated band structure and the density of states of Mg-5(BO3)(3)F suggest that its indirect bandgap is 5.27 eV. The compound was additionally investigated by UV/ Vis-NIR diffuse reflectance spectroscopy and thermal analysis.