THE EFFECT OF OXYGEN STOICHIOMETRY ON THE STRUCTURE, MAGNETISM AND ELECTRON-TRANSPORT PROPERTIES OF THE RARE-EARTH MANGANATES EXHIBITING CHARGE ORDERING

The oxygen content of rare earth manganates La0.5Ca0.5MnO3, Pr0.5Ca0.5 MnO3, Pr0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 with different degrees of char ge ordering instabilities is varied by controlled annealing in N-2. Th e oxygen deficiency increases the unit cell volume and decreases the t olerance factor. The electron transport and magnetic properties are se nsitive to the deviation of the %Mn4+ variation of tolerance factor of the oxygen deficient charge ordered manganates from that of the oxyge n stoichiometric manganates with same A-site ions and different (r(A)) . Pr0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 with larger deviations show differ ent behavior while La0.5Ca0.5MnO3 and Pr0.5Ca0.5MnO3 with smaller devi ations follow the behavior expected from the decreasing Mn4+ concentra tion. Oxygen deficiency broadens the Mn4+ concentration range of charg e ordering instability in the former and suppresses in the latter. Apa rt from the crystal structure deformations the oxygen defects modify t he electronic band structure in a complex manner. (C) 1998 Elsevier Sc ience B.V. All rights reserved.