The effect of the compositions of Mn-Mg and Li-Mn-Mg mixed oxides on the crystallite size and the lattice strain

The modification effects of lithium and manganese on the crystal properties of magnesium-based oxides were examined by the derivative thermogravimetry/differential thermal analysis (DTG/DTA) and the X-ray diffraction (XRD) technique. The Mn-containing MgO samples of various compositions were categorized two groups in terms of the predominant DTA endothermic peak accompanying the weight loss; 5-25 mol%-Mn/MgO (low-Mn) at 606-610 K and 50 and 80 mol%-Mn/MgO (high-Mn) at 648 - 655 K, respectively. These peaks were not ascribed to the decompositions of the starting materials; manganese(II) acetate(628 K) nor magnesium hydroxide(697 K). The difference of the decomposition processes under heating reflected the difference of the XRD patterns of two kinds of samples pretreated at 1073 K in a flow of N-2. Low-Mn showed the existence of three species; primarily MgO, MnO and a Mn-Mg-O solid solution while high-Mn had only the solid solution. In order to examine the effect of the chemical compositions on the crystallite size, D and the lattice strain, eta, the variations of the XRD-line widths of various Mn-containing samples were analyzed, employing the Hall's equation. The increases of Mn content increased eta, indicating that the formation of the solid solution made the crystal lattice strained, while D was independent of Mn content. In the presence of Li in low-Mn, MnO and the solid solution disappeared, suggesting that Mn was more highly-dispersed in MgO, The Hall's plots made it clear that the dose of 10 mol%-Li into high-Mn increased D not less than 10 times, as opposed to D of low-Mn with independency of Li content, These results manifested that Li played a role of the binder of the solid solution and not of Mn-dispersed MgO, The dose of Li moderated eta by 65% for low-Mn and by 10% for high-Mn, and possibly the lattice strain was moderated by the substitution of Li+ for Mg2+.