Structural, thermal and morphological studies of magnesium substituted-lithium manganese oxide spinels

Coexistence of two phases having space groups of Fd3m and P4₃32 in the Mg-doped LiMn₂O₄ spinel is being reported for the first time in this article. Mg-doped LiMn₂O₄ powders have been synthesized by sol-gel method using citric acid as a chelating agent. X-ray powder diffraction (XRD) studies show that the crystal structure of LiMg_xMn_2−xO₄ for x<0.25 is a single-phase cubic spinel, which has space group of Fd3m. The cubic spinel structures having space group of Fd3m and P4₃32 are found to coexist in the compound for x=0.25. The structure becomes single-phase cubic spinel with space group P4₃32 for x>0.25. Field emission scanning electron microscopy (FESEM) shows that particle size of various synthesized powders ranges from 100 to 350 nm. Particle size decreases with increase in Mg content. Differential thermal analysis (DTA) and thermogravimetry (TG) studies show an exponential decay relationship between Mg-doping content and the decomposition temperature to form nonstoichiometry (LiMg_xMn_2−xO_4−δ) in air atmosphere. Fourier transform infrared spectroscopy (FTIR) analysis shows increase in the number of vibrational bands with increase in Mg content, which indicates ordering of the ions in the case of ordered spinel structure, and consequent reduction of the space group symmetry from O_h⁷ to O⁷.