Electrical transport properties in LiMn_2O_4

LiMn2O4是一种很有前途的电池电极候选材料，在实际中得到了广泛的应用。这种氧化电极的电势极大地刺激了最近的研究。然而，尽管LiMn2O4具有实用价值，但其许多性质的基本物理仍不清楚。从这一角度出发，研究了LiMn2O4的电输运性质。用常规固相反应法制备了LiMn2O4陶瓷样品，化学分析得到Li^+_5Mn^3+_1Mn^4+_8O^2-_。通过复阻抗平面分析得到了体电导率随温度的变化关系。体电导率的温度依赖关系包括两个热激活过程，一个发生在T&gT；310K，另一个发生在T&lt；25K。体电导在加热和冷却过程中表现出滞后。体电导率与温度的关系表明，在这两个热激活过程中，由于小极化子的跳跃过程而产生的电输运。在介电性质中观察到的松弛过程证实了这一观点。这些实验结果表明，LiMn2O4中的电导主要是由小极化子的跳跃运动决定的。在较低温度下的热激活过程中的活化能大于高温下的活化能。这与通常绝缘子的行为不一致。这种反常现象和电导率的滞后现象归因于相变，在此相变中，由于Mn~(2+)和Lt；~(3+)和Gt~(3+)的协同Jahn-Teller效应，立方F_3DM的基质中出现了正交的I4_1/AMD。这一行为与沃伊托维奇的理论非常一致。

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.