ELECTRONIC-PROPERTIES OF MIXED-VALENCE MANGANATES - THE ROLE OF THE CATIONIC VACANCIES

Single-phase perovskites in the solid solution series La(1-x)a(x)MnO(3 +delta) have been obtained using a soft treatment, which makes possibl e strict stoichiometric control. Under these conditions, it becomes po ssible to systematically study; the influence of the sodium content on the electronic properties of materials in this series. As long as all the samples have practically the same Mn4+ content (33%), the number of vacancies at A and B sites of the perovskite structure depends on t he sodium content, and it decreases as x increases. Susceptibility to alternating current, magnetization, resistivity, and magnetoresistivit y measurements have allowed us to establish relevant points of the ele ctronic phase diagram of this alkali-metal-doped lanthanide manganate system. These results, together with those previously obtained for La1 -xKxMnO3+delta, reveal the existence of a correlation between the crit ical temperature for ferromagnetic ordering and the concentration of v acancies at the B sites, upsilon(B), in samples with a fixed concentra tion of Mn4+. Such a correlation can be understood in terms of a magne tic phase segregation model in which the materials are thought of as c omposed by clusters, formed by the vacancies (trapping centers for mob ile holes) and neighboring Mn cations (on which holes are trapped), an d a matrix, formed by the remaining Mn cations. Within this model, the decrease in the number of mobile holes in the matrix is the cause of the decrease in the critical temperature with upsilon(B).