Electrical and thermal properties of Pr _2/3(Ba _1-xCs _x) _1/3MnO ₃ manganites

Electrical and thermal properties of Pr _2/3(Ba _1-xCs _x) _1/3MnO ₃ (0 ≤ x ≤ 0.25) manganite perovskites are reported here. Two insulator-metal (I-M) transitions (T _P1 & T _P2) are observed in the electrical resistivity (ρ) of the pristine Pr _2/3Ba _1/3MnO ₃ (PBMO) sample, and they are systematically shifted to lower temperatures with increasing Cs substitution. An upturn in ρ is noticed below 50 K in these perovskites, presumably due to the combined effect of weak localization, electron-electron and electron-phonon scattering. It is found that the absolute value of room-temperature thermoelectric power (TEP) gradually decreases with increasing Cs content, implying the annihilation of the charge carriers with doping. An analysis of the electrical resistivity and thermoelectric power data indicates that the paramagnetic insulating state above T _P1 is governed by the small polaron hopping due to a non-adiabatic process. It is argued that the electron-magnon scattering processes are responsible for low temperature metallic behavior of TEP. A distinct specific heat peak below T _P1 is observed, attributed to the magnetic ordering, and its broadening with Cs-doping corresponds to the increase of magnetic inhomogeneity. Further, the temperature variation of thermal conductivity and the low temperature plateau in κ has been associated with the delocalization of Jahn-Teller polarons and transition from Umklapp scattering to a defect-limited scattering, respectively.