Defect structure of Y[sub 1-y]Ca[sub y]MnO[sub 3] and La[sub 1-y]Ca[sub y]MnO[sub 3]. II. Oxidation-reduction behavior

Electrical conductivity, Seebeck coefficient, and thermogravimetric behavior of compositions in the system Y[sub 1-y]Ca[sub y]MnO[sub 3] and La[sub 1-y]Ca[sub y]MnO[sub 3] were studied as functions of temperature and ambient oxygen activity in order to determine the mechanism of electrical transport and defect structure. Compositions in Y[sub 1-y]Ca[sub y]MnO[sub 3] exhibited very slight oxygen activity-dependent behavior as the ambient oxygen activity was reduced. A defect model which includes the thermally excited disproportionation of Mn[sup 3+] into Mn[sup 2+] and Mn[sup 4+] pairs was applied to the oxygen-activity-dependent data for La[sub 0.80]Ca[sub 0.20]MnO[sub 3] and La[sub 0.60]Ca[sub 0.40]MnO[sub 3]. For these compositions, the experimentally observed dependence of oxygen stoichiometry, electrical conductivity, and Seebeck coefficient upon ambient oxygen activity was explained using this model.