Observation of large thermoelectric power in charge ordered LaLiMnO (x = 0.25) manganite system.

In the present investigation detailed structural, electrical, magnetic and thermoelectric measurements on bulk samples of Li-doped LaLiMnO (0.05 ≤ x ≤ 0.3) manganites have been done. The samples are synthesized by wet-chemical mixing route and XRD analysis using Rietveld refinement revealed single phase behaviour upto Li-concentration x = 0.25. All the samples in the present series show ferromagnetic behaviour while metallic behaviour is shown by the samples upto Li concentration x = 0.2. Beyond x = 0.2 the sample (i.e., LaLiMnO) show insulating behaviour for the entire temperature range accompanied by charge-order transition around T = 225 K. The low temperature resistivity data are best fitted by ρ(T) = ρ + ρT + C/sinh(hυ/2kT), where C is a constant. Such behaviour might be an indication of the small-polaron coherent motion which involves a relaxation due to a soft optical phonon mode that is strongly coupled to the carriers. Thermoelectric power (TEP) measurements reveal interesting results showing a colossal value of −340 μV/K around 25 K for the CO sample in the present series. The probable mechanism responsible for the observed large TEP has been discussed here. [ABSTRACT FROM AUTHOR]