LACOO3 REVISITED

Homogeneous samples of LaCoO3 were prepared from coprecipitated precursors; they reveal that single-phase samples are restricted to a narrow range of the [La]/[Co] ratio of about 1.0. A first-order phase change near 1200 K only appears where Co3O4 is present as an impurity phase. Evolution with temperature from all low-spin Co(III) at T = 0 K to 50% high-spin Co3+ 50% intermediate-spin Co(III) above 650 K is monitored with magnetic susceptibility, resistance, and thermopower measurements. Evidence is found for a dynamic ordering of high-spin Co3+ and low-spin Co(III) in the intermediate-temperature interval 110 K < T < 350 K. The lifetime of a Co3+ ion decreases with increasing temperature to a tau < 10(-8) sec for T > 200 K. The intermediate-temperature phase is semiconductive with an E(g) approximate to 0.14 eV for excitations of hole-electron pairs. The holes are mobile; the electrons are trapped as Co2+ on high-spin sites until they recombine with a mobile hole. The metallic high-temperature phase for T > 650 K is nucleated and grows with increasing temperature in the interval 350 K < T < 650 K. (C) 1995 Academic Press, Inc.