Mn4+ cation localization in La-rich La1-xCaxMnOy manganites

Magnetization and electrical resistivity have been measured in La1-xCaxMnOy manganites in the ranges 0.00 less than or equal to x less than or equal t o 0.25 and 2.90 less than or equal to y less than or equal to 3. The magnet ization is found to depend primarily on the concentration of Mn4+ A small c oncentration of substitutional Ca ions (x approximate to 0.50-0.10) is foun d to suppress the onset of metallic behavior even for Mn4+ concentrations o f 0.25. A model is proposed that assumes localization of Mn4+ cations aroun d Ca substitutional atoms resulting in ferromagnetic clusters that are loca lly conducting. Percolation of these clusters, attained after increasing th e Ca concentration to x approximate to 0.25, leads to long-range metallic b ehavior. We propose that Mn4+ localization is due to differences in local J ahn-Teller splitting that is higher near the La sites than near the Ca ones .