Correlation of the size effect with the thermoelectric power for the Pr-based manganites Pr0.7Ca0.3-xSrxMnO3

The thermoelectric power (TEP) S versus temperature has been investigated for the manganite Pr0.7Ca0.3-xSrxMnO3 series. Although these materials exhibit the same hole concentration considering their Mn valency of 3.3, their S(T) curves are dramatically affected when x varies from 0 to 0.3, i.e., when the mean size of the interpolated cation increases. Particularly the S-290K values go from positive to negative when x increases; The remarkable feature is that TEP data for samples which exhibit colossal magnetoresistive (CMR) properties are found to be similar and can be well described by the paramagnetic (P)-antiferromagnetic (AF) and AF-ferromagnetic (F) transitions which exist in these samples. On the other hand, for the samples with higher x values, the CMR effect is smaller and the S(T) curves change shape. Clearly, the electronic properties of these materials are very sensitive to the atomic orbitals overlapping which can be varied via cationic size variations as demonstrated by the present TEP measurements. Moreover, the signatures of both P-->AF and AF-->F transitions on the TEP curves seem to be a good indicator to detect samples with CMR properties.