Cation ordering and order-disorder phase transition in co-substituted Li4Ti5O12 spinels

International audienceIn 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, Li 1.33x Co 2-2x Ti 1+0.67x O 4 (0 ≤ x ≤ 1). Depending on the chemical composition the samples crystallize in two space groups (S.G.) Fd3̄m (0 ≤ x ≤ 0.40 and x = 1) and P4 3 32 (0.50 ≤ x ≤ 0.875). Samples crystallizing in the S.G. P4 3 32 are ordered spinels with a cation ordering of the 13 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 andlt; x andlt; 0.50 and 0.875 andlt; x andlt; 1 indicate changes of the crystal symmetry (Fd3̄m ↔ P4 3 32). The partially ordered spinel x = 0.50 has a convergent, reversible, order-disorder phase transition at T c = (1083 ± 10) K. Samples with x = 0.875 and 0.75 have an order - disorder phase transition out of our experimental ranges with T C (x = 0.875) andlt; 973 K and T C (x = 0.75) andgt; 1173 K. The mechanism of the phase transition is based on cation migration