Structure-property relationship in spinel and rock-salt Li-Mn-Mg-O phases

A pseudo-binary phase diagram of MnOδ-MgO has been constructed using a combination of X-ray powder diffraction data on quenched samples, high-temperature X-ray powder diffraction and differential scanning calorimetry. Study on the phase diagram was carried out with general formula MgxMn1-xOδ. Samples were prepared through solid-state synthesis and quenched in liquid nitrogen over a temperature range 600 to 1200 °C. Phase characterization revealed the formation of an extensive solid solution of Mg6MnO8 and the existence of two tetragonal spinel polymorphs (T1 and T2) which contradicts the reported literature. Both tetragonal spinel polymorphs were indexed using I41/amd space group and showed significantly different lattice parameters. High-temperature X-ray powder diffraction showed a transition of low-temperature (T1) to high-temperature cubic spinel (C1), which could not be preserved on quenching. Instead, a high-temperature polymorph (T2), which appears to be metastable, was formed during quenching. It is proposed that the rapid transition of C1 to T2 involves orientational distortion and therefore assumed that the cation distributions are the same in C1 and T2. Structural analysis on T2 confirmed the partial cation site exchange between tetrahedral and octahedral sites, which are very dependent in composition x. Mg6MnO8 (C2) solid solution was observed over the entire range of 0.50 ≤ x ≤ 0.80 between 900 and 1000 °C, instead of a single line-phase as reported in the literature. Below ~ 850 °C, the solid solution is unstable and decomposed into two separate cubic compositions governed by an immiscibility dome. Neutron powder diffraction confirmed the space group for C2 to be Fm3m. Structural studies showed an increase in occupancy of an unoccupied tetrahedral Mn3 (32f) and a decreased occupancy of Mg1 (24d) and O2 (24e) with decreasing x. Since the Mn3 and Mg1 sites are partially occupied, the structure is regarded as highly disordered due to the possibility of cation site-exchange between these sites. As a result, it is not feasible to refine these sites since there are three occupancy variables exist in any one site: Mg, Mn and vacancy. Impedance measurements showed that the conductivity of T2 is several orders of magnitude higher than that of T1 (at room temperature). The conductivity of T1 is independent of composition x. The conductivity of T2 increased with increasing x up to x = 0.40. The conductivity of C2 decreased with increasing x. The highest conductivity was observed at x = 0.50, ~ 4 x 10-6 S cm-1 (at room temperature) over the entire range 0.05 ≤ x ≤ 0.80. T2 and C2 showed similar activation energy, 0.42 ~ 0.44 eV. Based on the result of the impedance measurement, it is proposed that the partial cation site-exchange in T2 may trigger disproportionation of Mn3+ to Mn2+ and Mn4+ pairs at the octahedral sites. Therefore, the proposed compensation mechanism for T2 involves partial cation site exchange and disproportionation of Mn3+ ions.