THERMOELECTRIC-POWER OF LA1-XCAXMNO3- INADEQUACY OF THE NOMINAL MN3+(DELTA )4+ VALENCE APPROACH/

The temperature-dependent thermoelectric power (TEP) and resistivity o f La1-xCaxMnO3+sigma polycrystals (0 less than or equal to x less than or equal to 0.45) are reported. The TEP data are analyzed within an a diabatic small polaron description in order to determine the number of charge carriers per active transport site in the paramagnetic state. The TEP in the small-polaron regime above Te is significantly smaller than that predicted by nominal Mn3+/4+ valence arguments. This indicat es that more holelike charge carriers and/or fewer accessible Mn trans port sites are present in these compounds than expected based on the d ivalent doping levels employed. This result is consistent with a subst antial degree of 3d(4)-3d(4) (Mn3+-Mn3+) charge disproportionation int o more stable 3d(5)-3d(3) (Mn2+-Mn4+) pairs.