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

A series of La2−xSrxCoO4.5−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 K2NiF4-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 K2NiF4-type solid solution corresponding to the reaction La2−xSrxCo3+O4.5−x/2⇔La2−xSrxCo3+1−δCo3±1δO4.5−x/2±δ±δ/2O2. The equilibrium values of the electrical resistivity of LaSrCoO4±δ (x=1) were investigated as function of temperature (400–1050°C) and of oxygen partial pressure (pO2=2.9–286 Pa; 21 000 Pa). The log σ−log pO2 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 LaSrCoO4±δ (x=1) changed to a negative one with a maximum conductivity of 160 S cm−1.