Order-disorder transition in the (1-x)Li2TiO3-xMgO system (0 ≤ x ≤ 0.5)

The order-disorder phase transition of magnesium lithium titanate solid-solution (1-x)Li2TiO3-xMgO (0 <= x <= 0.5) ceramics prepared by conventional solid-state processing has been examined. The phase and structural analysis was carried out using electron diffraction, neutron diffraction and high-resolution transmission electron microscopy. Both electron and neutron diffraction results revealed the onset of an order-to-disorder transition at 0.3 < x < 0.4. Superlattice reflections found in certain regions of x = 0.2 samples and most areas of x = 0.3 samples were caused by a twin structure stabilized by Mg incorporation. Rietveld refinements of neutron diffraction data suggested a random distribution of Mg on the Li 4e sites and equal distribution of Mg on the two Ti 4e sites for x <= 0.3. As the Mg content continues to increase, the crystal symmetry transforms from monoclinic to cubic rocksalt. Consequently, the cation ordering on the 8f and 4d sites of the C2/c structure became corrupted and turned into short-range ordering on the 4a sites of a cubic structure with Fm (3) over barm symmetry, resulting in diffuse scattering in electron diffraction patterns.