PHYSICAL-PROPERTIES OF THE NONSTOICHIOMETRIC PEROVSKITE DY1-XSRXCOO3-Y SYSTEM

Solid solutions of the nonstoichiometric Dy1-xSrxCoO3-y system with th e compositions of x=0.00, 0.25, 0.50, 0.75, and 1.00 have been synthes ized by the solid state reaction at 1000 degrees C under atmospheric a ir pressure. The crystallographic structures of the solid solutions ar e analyzed by the powder X-ray diffraction patterns at room temperatur e. The analyses assign the compositions of x=0.00 and 0.25 to the orth orhombic system with space group of Pbnm/D-2h(16) the compositions of x=0.50 and 0.75 to the tetragonal system like a typical SrCoO2.86, and the composition of x=1.00 or SrCoO250, to the brownmillerite type sys tem with space group of I(*)a. The reduced lattice volumes increas wi th x value due to the larger radius of Sr2+ ion than that of Dy3+ ion. The mole ratio of Co4+ ion to total Co ion with mixed valence state b etween Co3+ and Co4+ ions at B sites or tau value has been determined by an iodometric titration. All the samples except for the DyCoO3; com pound show the mixed valnce state and thus the composition of x=0.50 h as the maximum tau value in the system. The oxygen vacancies increasin g with x value are randomly distributed over the crystal lattice excep t for the composition of x=1.00 which have the ordering of the oxygen vacancies. The nonstoichiometric chemical formulas of the Dy1-xSrxCo1- tau3-Cotau4+O3-(x-tau/2) system are formulated from the x, tau, and y values. The electrical conductivity in the temperature range of 100 to 900 K increases with the tau value linearly because of positive holes of the Co4+ ions in pi band as a conducting carrier. The activation energy of the x=0.50 as E(n) congruent to 0.17 eV is minimum among oth er compouds. Broad and high order transition due to the overlap betwee n sigma and pi* bands broadened by the thermal activation is observed near 1000 K and shows a low temperature-semiconducting behavior. Magn etic properties following the Currie-Weiss law show the low to high sp in transition in the cobaltate perovskite. Especially, the composition of x=0.75 presents weak ferromagnetic behavior due to the Co3--O2--Co 4- indirect superexchange interaction.