Cation ordering and order-disorder phase transition in 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, Li1.33xCo2-2x,Ti1+0.67x (0 less than or equal to x less than or equal to 1). Depending on the chemical composition the samples crystallize in two space groups (S.G.): Fd (3) over barm (0 less than or equal to x less than or equal to 0.40 and x = 1) and P4(3)32 (0.50 less than or equal to x less than or equal to 0.875). Samples crystallizing in the S.G. P4(3)2 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 less than or equal to x less than or equal to 0.50 and 0.875 < x < 1 indicate changes of the crystal symmetry (Fd (3) over barm <----> P4(3)32). 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 T-C(x = 0.875) < 973 K and T-C(x = 0.75) > 1173 K. The mechanism of the phase transition is based on cation migration.