Changes in local structure of lithium manganese spinels (Li:Mn=1:2) characterised by XRD, DSC, TGA, IR, and Raman spectroscopy

A series of LiMn2O4 spinels, obtained by the sol-gel method maintaining constant Li:Mn ratio, were calcined at different temperatures (300-900 °C) to create crystal lattice defects. The structure and electronic properties of the samples were studied by complementary experimental techniques (XRD with Rietveld analysis, BET-N2, DSC, MS-TG/SDTA, electrical conductivity, laser Raman and IR spectroscopy with 2D correlation analysis). The oxygen-spinel equilibrium shifts caused by various calcination in the air and in oxygen free atmosphere modulate the type and concentration of the anionic or cationic defects. The relationship between the local symmetry of MnO6 octahedrons, manifested in symmetric and asymmetric vibrations, seemed to determine the phase transition at around the room temperature. Removing of the degeneration of t2g and eg orbitals, due to Jahn-Teller effect, evolves stabilization energy causing increase of activation energy of electron migration. Distortion of the local symmetry makes impossible the Jahn-Teller effect and in consequence stops the phase transition around room temperature.