X-ray diffraction and impedance spectroscopy studies of lithium manganese oxide spinel

Abstract

Phase transition in lithium manganese oxide spinel synthesized by sol–gel technique and in samples prepared from commercially available powders of LiMn 2O 4 (Alfa-Aesar and Sigma–Aldrich) was investigated. In addition to the standard impedance measurements and the X-ray diffraction in Bragg–Brentano geometry, simultaneous measurements of impedance spectrum and X-ray pattern in non-focusing geometry were performed in the temperature range between −25 and +100 °C. Correlation between the XRD profile parameters and the electrical conductivity was evidenced. Spinel synthesized by the sol–gel technique and heat-treated at 800 °C as well as samples prepared from Alfa-Aesar powder exhibited a distinctive phase transition from cubic to orthorhombic structure upon cooling below room temperature. Structural transition was visible in the X-ray pattern as splitting of the 4 0 0 reflection of cubic structure, and was accompanied by a decrease of conductivity by a factor of about 10. In the sample prepared from powder supplied by Sigma–Aldrich the phase transition was not complete even upon cooling to −25 °C, and no stepwise change of conductivity was observed. Differences in behavior of the spinel samples can be related to the presence of additional phases (Mn 3O 4 or Li 2MnO 3), which were found in commercial powders, whereas the sol–gel synthesized samples were single phase.