Electrical transport properties in LiMn_2O_4

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LiMn_2O_4 is one of the promising candidates for the electrode of batteries and widely used in practice. The potentialiti of this oxied as the electrode stimulates the recent investigation very much. Despite the practical utility, however, the fundamental physics of many properties in LiMn_2O_4 is still unknown. From this point of view, electrical transport properties in LiMn_2O_4 have been investigated. LiMn_2O_4 ceramics specimens were prepared by conventional solid reaction and the chemical analysis yields Li^+_5Mn^3+_1Mn^4+_8O^2-_. The bulk conductivities were obtained by complex-impedance plane analyzes as a function of temperature. The temperature dependence of the bulk conductivity consist of two thermally activated processes, one occurs at T>310K and another one at T<25K.The bulk conduction in the heating and cooling processes exhibits a hysteresis. The temperature dependence of the bulk conductivity suggests the electrical transport due to a hopping-process of small polarons in the both thermally activated processes. The relaxation process observed in dielectric properties ensures this suggestion. These experiments lead to the conclusion that the hopping motions of small polarons actually dominates the electrical conduction in LiMn_2O_4.The activation energy in the thermally activated process at lower temperatures is larger in values than that at high temperatures. This is inconsistent with the behavior in usual insulators. This anomaly and the hysteresis observed in the conductivity is ascribed to the phase transition in which the orthorhombic I4_1/amd appears in the matrix of cubic F3dm by the cooperative Jahn-Teller effect due to Mn^<3+>. This behavior is in good agreement of Wojtowicz's theory.