STRUCTURE AND ELECTRICAL-PROPERTIES OF LA1-XSRXCO1-YFEYO3 .1. THE SYSTEM LA0.8SR0.2CO1-YFEYO3

Crystal structure, thermal expansion, thermogravimetry, thermoelectric ity, and electrical conductivity of compositions in the system La0.8Sr 0.2Co1-yFeyO3 with 0 less than or equal to y less than or equal to 1 w ere studied as function of Co/Fe ratio and temperature,:in air. The el ectrical conduction mechanism is attributed to the adiabatic-hopping o f p-type small polarons. At high temperatures, oxygen deficiency cause s lattice expansion and a reduction in electrical conductivity. The ob served temperature dependence of the Seebeck coefficient is attributed to changes in carrier concentration caused by a thermally excited cha rge disproportionation of Co3+ ions and by the ionic compensation of i nduced oxygen vacancies. The measured electrical conductivity and Seeb eck coefficient as a function of the Co/Fe ratio is interpreted using a two-site hopping and the site-percolation model. It is suggested tha t a preferential electronic compensation of Fe ions over Co ions may o ccur in this system.