A comparative study of transport behaviour of monovalent (Li~(1+)) and trivalent (Bi~(3+)) doped La_(1-x)Li_xMnO_(3+δ) and La_(0.67-y)Bi_yCa_(0.33)MnO_3 CMR materials

摘要

A comparative study of the field-dependent resistivity (ρ) and thermoelectric-power (TEP) of Li-doped La_(1-x)Li_xMnO_(3+δ) and Bi-doped La_(0.67-y)Bi_yCa_(0.3)MnO_3 is made. The perovskite structure of the La_(1-x)Li_xMnO_(3+δ) is rhombohedrally distorted in contrast to the La_(0.67-y)Bi_yCa_(0.3)MnO_3 showing orthorhombic symmetry. The metallic part of resistivity of La_(1-x)Li_xMnO_(3+δ) is best fitted with an expression ρ(T) = ρ_0 + ρ_(4.5)T~(4.5) + C/sinh~2 (hν_s/2k_BT) containing small-polaron contribution (last term), involving a relaxation time due to soft opticalphonon mode. This is in sharp contrast to the resistivity of La_(0.67-y)Bi_yCa_(0.3)MnO_3 where, the ρ(T) data are best fitted with the electron-magnon scattering model ρ = ρ_O + ρ_T T~(2.5). These results suggest the optimum requirements for the formation of optical-polaron inmanganites in the metallic phase, which depends on the characteristics of the constituent A-site ion (valence-state, ionic-radii, etc.) of manganites. Unlike Lidoping, the Bi-doping does not induce any hopping site indicated from the critical analysis of the TEP data. resistivity; magnetoresistance; Manganites; thermoelectric power; polarons; metalinsulator transition; A-site