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)O-3 and (La,Ca)(Cr,Al,Co)03 compositions at 1000 degreesC. 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 p(o2) 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)O-3 and (La,Ca)(Cr,Al,Co)O-3 systems. (C) 2002 Elsevier Science B.V All rights reserved.