Thermoelectric power of La1-xCaxMnO3+delta: Inadequacy of the nominal Mn3+/4+ valence approach

The temperature-dependent thermoelectric power (TEP) and resistivity of 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 adiabatic 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 indicates that more holelike charge carriers and/or fewer accessible Mn transport sites are present in these compounds than expected based on the divalent doping levels employed. This result is consistent with a substantial degree of 3d(4)-3d(4) (Mn3+-Mn3+) charge disproportionation into more stable 3d(5)-3d(3) (Mn2+-Mn4+) pairs.