Effect of valence fluctuations in A sites on the transport properties of La1-xRxMnO3(R = Ce, Pr)

La 1- x R x MnO 3 (R = Ce or Pr) exhibit valence fluctuations (R 3+ and R 4+ ) in the A sublattice of the ABO 3 perovskite structure. They also display a metal-insulator (M-I) transition and magnetoresistance effect even without the substitution of divalent cations. In La 1- x Ce x MnO 3 , as Ce concentration increases ( x = 0.05 to 0.6), the magnitude of resistivity decreases and the magnetic transition temperature, T c , shifts towards higher temperature. The change in transport properties is attributed to the increase in Mn 3+ -Mn 4+ pairs. The electronic and magnetic phase diagram is obtained for La 1- x Ce x MnO 3 from the resistivity and dc susceptibility measurements over a broad range of temperature and concentrations. The thermopower and magnetoresistance are also measured for La 1- x Ce x MnO 3 for two x values ( x = 0.3 and 0.6). The high temperature thermopower data fit to an adiabatic small polaron hopping mechanism.