Polaron hopping conduction and thermoelectric power in LaMnO_3+δ

Two different phases of LaMnO_3+δ [one showing a metal-insulator transition (MIT), referred to as LaMn-C, and the other not showing a MIT, referred to as LaMn-S] have been clearly observed to follow two different conduction mechanisms. Interestingly, small polaron hopping models of Mott, Schnakenberg, and Emin are found to fit the conductivity data of all the samples above the corresponding MIT temperature. The conductivity data of the insulating (semiconducting) LaMn-S followed a nonadiabatic hopping conduction mechanism while LaMn-C and the Pb doped samples viz. La_1-xPb_xMnO₃ (x = 0.05-0.5) showed a similar type of MIT and followed an adiabatic small polaron hopping conduction mechanism in the high temperature paramagnetic phase (above the respective MIT temperature). Activation energy (W), density of states at the Fermi level N(E_F), Debye temperature (θ_D), electron-phonon interaction constant (γ_P), etc. of LaMn-S showed appreciable differences from those of LaMn-C and La_1-xPb_xMnO₃, which show a MIT. Polaron hopping conduction is also supported by thermoelectric power (TEP) measurements. An observed small but appreciable magnetic field dependence of the TEP data (measured at B = 1.5 T) is considered to be associated with magnetic polarons.