Influence of composition and sintering temperature on density of structure defects, phase transitions, and properties of magneto-resistive strontium-doped ceramic of manganate-lanthanum perovskites

Ceramic samples of (La1-xSrx2+MnO32-)-Sr-3 (x=0, 0.1, 0.15, 0.2, 0.3, 0.4), sintered at different temperatures (1150, 1200, 1350, and 1500 degrees C) are investigated by means of methods of x-ray diffraction analysis and resistive and magnetic analysis. It is established that the perovskite structure parameter (a) and degree of its rhombohedric (R (3) over barc) distortion (a) both decrease with increasing content of Sr2+ in La1-xSrxMnO3 +/-delta (x = 0-0.4). The metal-semiconductor (T-ms) and ferromagnetic-paramagnetic (T-c) phase transition temperatures and magnetoresistive peak temperature (T-p) all increase, while the activation energy (E-a) decreases. The density of defects in the lattice which contains anion (V-(a)) cation (in A and B positions) vacancies, average valency of manganese (omega), and tolerance factor (t) also increase. It is shown that with Sr2+ doping, charge compensation is achieved by two mechanisms, first, the Mn3+ -> Mn4+ transition (similar to 80%), and second, the formation of V-(a) vacancies (similar to 20%), with the contribution of the second mechanism increasing with increasing sintering temperature. With increasing x the magnetoresistive effect decreases at T-p and the lowfield tunneling magnetoresistive effect increases slightly. It is established that the concentration dependence of the intracrystaline peak magnetoresistive effect at T-p is opposite to the concentration dependence of the intercrystalline magnetoresistive effect in the semiconductor region for T << T-p.