Colossal Thermoelectric Power in Charge-Ordered Li-doped La_(0.75)Li_(0.25)MnO_3 manganite system

摘要

A detail study of transport and magnetic properties of La_(1-x)Li_xMnO_(3+δ) (0.05 ≤ x ≤ 0.3) system synthesized by wet- chemical mixing route has been done. The room temperature x-ray powder diffraction (XRD) data show single phase behavior of all samples except x = 0.3. Rietveld refinement of XRD data shows structural transition from rhombohedral (R3-C) to orthorhombic (Pnma) symmetry occurs at the Li-doping level x > 0.2 with both the lattice parameter and unit-cell volume decrease with increase of 'x'. All the samples show ferromagnetic (FM) behavior while metallic behavior are shown by the samples up to Li-concentration x = 0.2. With further Li doping i.e. for x = 0.25, the sample shows insulating behavior accompanied by charge-order transition around T ～ 225K. Metallic part of the resistivity data of the samples is best fitted with an expression ρ(T) = ρ_0 + ρ_(4.5)T~(4.5) + C/sinh~2(hv_s/2k_BT) containing small-polaron contribution (last term). Most interesting finding in the present study is the observation of large anomalous decrease in thermoelectric power (S) below 100K shown by the sample with x = 0.25. Probable mechanisms responsible for the observed colossal thermoelectric power have been discussed.