Synthesis and magnetic study on Mg-substituted Li-Mn spinel oxides

LiMn_2−xMg_xO₄ (X<0.5) cubic spinel oxide was synthesized by the sol-gel technique using Li-nitrate, Mn-acetate and Mg-acetate salts. The gel precursors were decomposed at 300 °C in air and then annealed at temperatures ranging from 500 to 850 °C in an oxygen flow. For a fixed annealing temperature of 700 °C, the lattice constant decreased with an increase in the substitution degree X and a discontinuity was found around X=0.3. With a further increase in X>0.3, the space group of the crystal structure was converted from Fd3m to P4₃32 by the cation ordering in the octahedral site. The low-temperature magnetization increased with X, and the Weiss constant moved in the positive direction and changed its sign from negative to positive around X=0.3. In the case of X=0.5, all the specimens showed P4₃32 structure and ferromagnetic character. The maximum Curie temperature (T_c=23 K) and the maximum magnetization (M_s=4.68 μ_B per the chemical formula unit) were attained simultaneously for the specimen obtained around 700 °C. The Mn valence state was sensitive not only to the substitution but also to the preparation conditions. Hence, it was possible to explain these variations by considering the magnetic interactions between transition metal ions.