PEROVSKITES

As part of an ongoing study of alternative electrolyte and electrode materials to serve as components in Solid Oxide Fuel Cells, a series of doped layered perovskites, of general formula Sr 3Ti2.xMxO 7.8 (M=Al, Ga, In, Co, Fe, Mn), were synthesized and their electrical conductivity characterized as a function of temperature and oxygen partial pressure. For fixed-valent dopants, p-type conductivity predominates at p(O2)>l 05 atm, followed by a p(0 2)-independent electrolytic regime, and n-type electronic conductivity at very low p(0 2). The electrolytic regime exhibits low magnitudes of oxygen ion conductivity (ca. 106 S/cm at 700 *C) and high activation energies (1.4-1.5 eV). Solid solutions with transition metals display significantly higher conductivities. For M = Co, a p-type conductivity region at high p(O2) is observed, followed by an ionic regime at intermediate p(O2) characterized by high ionic conductivity (10-2 S/cm at 700 'C for Sr3Ti1 2Coo.8O 7-.) and low activation energy (0.64 eV). The magnitude of the conductivity increases with increasing Co content. lodometric titration measurements demonstrate the predominance of Co(IV) in air. The identification of the electrolytic regime is