Cation ordering and order–disorder phase transitionin Co-substituted Li4Ti5O12 spinels

In this paper we present the results of the synthesis, crystal structure investigations and in situ X-ray diffraction studies of the order–disorder phase transition in cobalt substituted lithium titanate oxide spinels, Li1.33xCo2–2xTi1+0.67xO4 (0 ≤ x ≤ 1). Depending on the chemical composition the samples crystallize in two space groups (S.G.): (0 ≤ x ≤ 0.40 and x = 1) and P4332 (0.50 ≤ x ≤ 0.875). Samples crystallizing in the S.G. P4332 are ordered spinels with a cation ordering of the 1–3 type at octahedral 4b and 12d sites. The cation ordering in octahedral sites is full in the sample with x = 0.75 (Li and Ti occupy 4b and 12d sites, respectively) and decreases for samples with higher/smaller x. Changes of the extinction conditions and nonlinearities in the concentration dependence of the lattice parameter in the regions 0.40 < x < 0.50 and 0.875 < x < 1 indicate changes of the crystal symmetry ( ⟷ P4332). The partially ordered spinel x = 0.50 has a convergent, reversible, order–disorder phase transition at TC = (1083 ± 10) K. Samples with x = 0.875 and 0.75 have an order–disorder phase transition out of our experimental ranges with TC(x = 0.875) < 973 K and TC(x = 0.75) > 1173 K. The mechanism of the phase transition is based on cation migration.