Effect of Ba substitution on structural, electrical and thermal properties of La_0.65Ca_0.35-xBa_xMnO₃(0 ≤ x ≤ 0.25) manganites

In the present communication, the effect of Ba substitution on the structural, mechanical, electrical transport and thermal properties of La_0.65Ca_0.35-xBa_xMnO₃(0.00 ≤ x ≤ 0.25) manganites have been investigated. Rietveld refinement of XRD data reveals that all samples are single phase. With increasing Ba content, a transformation of crystal structure from rhombohedral (x = 0.00-0.15) to cubic symmetry (x = 0.25) is observed. In the micro-hardness study, a correlation between the average grain size and micro-hardness is concluded. It is found that all samples exhibit a clear metal-insulator transition, and the transition temperature (T_MI) shifts monotonically towards higher temperature. The Seebeck coefficient (S) of the pristine sample is negative in the entire temperature range, whereas for all doped samples the measured S exhibits a crossover from positive to negative values. The analysis of Seebeck coefficient data indicates that the small polaron hopping model is operative in the high temperature region. The room-temperature magnitude of thermal conductivity, κ of the studied samples is in the range of 20-35 mW/cm K and such low value is presumably attributed to the Jahn Teller (J-T) effect. An analysis of κ using Wiedemann-Franz's (W-F) law confirms that the total thermal conductivity is mainly associated with the lattice phonons rather than charge carriers. The change in entropy associated with the transition evaluated from the specific heat measurements is found to decrease with increasing x, most likely due to the enhancement of magnetic inhomogeneity in these manganites.