Preparation, crystal structure and photoluminescence of lithium magnesium manganese borate solid solutions, LiMg_1-xMn_xBO₃

LiMg_1-xMn_xBO₃ solid solutions were prepared by solid state reaction in Ar at 800 °C. The X-ray diffraction reflections of the solid solutions with the manganese content x from 0 to 0.5 were indexed with monoclinic cells (space group C2/c (No. 15)). The solid solution of x = 0.75 has the hexagonal structure of LiMnBO₃ (space group P6 (No. 174)). Colorless transparent platelet single crystals of LiMg _0.5Mn_0.5BO₃ were prepared by heating a polycrystalline sample of x = 0.50 with a Li₂BO₂ flux and by slow cooling from 900 °C. The crystal structure was analyzed by single crystal X-ray diffraction (monoclinic, C2/c, a = 5.1820(2) , b = 8.9241(4) , c = 10.1529(6) , β = 91.5824(18)°, R1 = 0.0271, wR2 = 0.0645, and S = 1.219 for all data). Mn²⁺ and Mg²⁺ atoms statistically occupy a 5-fold coordinate site of oxygen trigonal bipyramid. Monoclinic LiMg_1-xMn_xBO₃ solid solutions emit red light with a peak wavelength λ_em = 682-696 nm due to d-d transition under visible light of 428 nm at room temperature. The radiation lifetimes decreased from 24 ms to 0.3 ms with increasing x from 0.02 to 0.30. Stronger red emission of 698 nm was observed for the sample of x = 0.10 by excitation of ultraviolet light λ_ex = 200 nm.