Thermopower in Pr1-xCaxMnO3

We report measurements of thermopower in Pr1-xCaxMnO3 (0.33 less than or eq ual to x less than or equal to 0.48). A clear anomaly has been observed at the charge ordering temperature T-co for x = 0.33, 0.34, 0.47 and 0.48. Neg ative thermopower below T-co shows that the carriers are of electron-type i n the charge-ordering state of x = 0.47 and 0.48. This indicates that the h opping of e(g) electrons at the Mn4+ sublattice. i.e., extra Mn3+ ions, dom inates the transport properties in the charge-ordering phase. Thermopower b elow T-co changes from negative to positive around x similar to 0.35 as the e(g)-electron number (1 - x) increases, which implies a spatial ordering o f extra Mn3+ ions. The ordering of extra Mn3+ ions possibly stabilizes the 1:1 commensurate charge ordering with the wave vector of (100) with a: far from the commensurate composition, x = 0.50. Thermopower is found to be sig nificantly dependent on magnetic field. The result should be closely relate d to the stability of the charge ordering in a magnetic field.