Defect structure and isothermal expansion of A-site and B-site substituted lanthanum chromites

The results of oxygen nonstoichiometry, linear expansion and electrical conductivity measurements are presented for selected (La,Sr)(Cr,V)O3 and (La,Ca)(Cr,Al,Co)O3 compositions at 1000 °C. The modelling of the defect structure of the oxygen deficient chromites is carried out by considering both donor (V) and acceptor (Co) dopant on the B-site. The equilibrium constants, required to calculate all a priori unknown equilibrium defect concentrations, are determined by fitting the derived model equation to the measured oxygen nonstoichiometry data for both cases. The two oxidation states, 4+ and 3+ for vanadium are considered simultaneously in V-doped lanthanum chromite in addition to 3+ and 4+ states for chromium in the host lattice. The cobalt reduction to the divalent state is apparently caused by the redox reaction, Co3++Cr3+=Co2++Cr4+, in Co-doped lanthanum chromite at high oxygen partial pressure. The proposed defect structure is confirmed by the measurement of the electrical conductivity as a function of oxygen partial pressure. Relative change of the average radius of the B-site ions as a function of pO2 is proposed as a criterion to reflect the influence of the defect structure alteration on the linear expansion. The criterion is in agreement with the measured isothermal expansion for the (La,Sr)(Cr,V)O3 and (La,Ca)(Cr,Al,Co)O3 systems.