Composition and electrical conductivity of some cobaltates of the type La2−xSrxCoO4.5−x/2±δ

Abstract A series of La 2− x Sr x CoO 4.5− x /2± δ ( x =0.5–1.5) compounds as powders and ceramic shapes were prepared by solid state reactions. All compositions show a single-phase K 2 NiF 4 -type structure within the whole temperature region (X-ray investigation at 20–1000°C in air). Reversible oxygen desorption–sorption behaviour in argon during heating and cooling between 300 and 1050°C was observed. The amount of exchanged oxygen, δ , was found to be a function of the Sr content, x : from 0.5≤ x ≤0.8 the amount of exchanged oxygen decreased to zero and from 0.9≤ x ≤1.5 it increased to a maximum value. All oxygen stoichiometry changes occured within the range of the K 2 NiF 4 -type solid solution corresponding to the reaction La 2− x Sr x Co 3+ O 4.5− x /2 ⇔La 2− x Sr x Co 3+ 1− δ Co 3±1 δ O 4.5− x /2± δ ± δ /2O 2 . The equilibrium values of the electrical resistivity of LaSrCoO 4± δ ( x =1) were investigated as function of temperature (400–1050°C) and of oxygen partial pressure ( p O 2 =2.9–286 Pa; 21 000 Pa). The log σ −log p O 2 dependence point at the p-type of the conductivity had a slope of 1/10 in the temperature range 400–1000°C. Above 850°C the temperature coefficient of the conductivity of LaSrCoO 4± δ ( x =1) changed to a negative one with a maximum conductivity of 160 S cm −1 .