Composition and electrical conductivity of some cobaltates of the type La2-xSrxCoO4.5-x/2 +/-delta

A series of La2-xSrxCoO4.5-x/2+/-delta (x = 0.5-1.5) compounds as powders a nd ceramic shapes were prepared by solid state reactions. All compositions show a single-phase K2NiF4-type structure within the whole temperature regi on (X-ray investigation at 20-1000 degreesC in air). Reversible oxygen deso rption-sorption behaviour in argon during heating and cooling between 300 a nd 1050 degreesC was observed. The amount of exchanged oxygen, delta, was f ound to be a function of the Sr content, x: From 0.5 less than or equal to x less than or equal to 0.8 the amount of exchanged oxygen decreased to zer o and from 0.9 less than or equal to x less than or equal to 1.5 it increas ed to a maximum value. All oxygen stoichiometry changes occured within the range of the K2NiF4-type solid solution corresponding to the reaction La2-x SrxCo3+O4.5-x/2<->La2-xSrxCo1-delta3+Codelta3+/-1O4.5-x/2+/-delta+/-delta / 2O(2). The equilibrium values of the electrical resistivity of LaSrCoO4+/-d elta (x=1) were investigated as function of temperature (400-1050 degreesC) and of oxygen partial pressure (pO(2) = 2.9-286 Pa; 21 000 Pa). The log si gma - log pO(2) dependence point at the p-type of the conductivity had a sl ope of 1 /10 in the temperature range 400-1000 degreesC. Above 850 degreesC the temperature coefficient of the conductivity of LaSrCoO4+/-delta (x=1) changed to a negative one with a maximum conductivity of 160 S cm(-1). (C) 2000 Elsevier Science B.V. All rights reserved.