Electrical transport properties in LiMn_2O_4

LiMn_2O_4是一种很有前途的电池电极材料，在实际中得到了广泛的应用。这种氧化物作为电极的潜力极大地激发了最近的研究。然而，尽管LiMn_2O_4具有实用性，但其许多基本物理性质仍是未知的。从这个角度出发，研究了LiMn_2O_4的电输运性质。采用传统固相反应法制备了LiMn_2O_4陶瓷样品，化学分析得到Li^+_5Mn_3 +_1Mn_4 +_8O_2-_。体电导率作为温度的函数通过复阻抗平面分析获得。体电导率的温度依赖性由两个热激活过程组成，一个发生在T> 310 K，另一个发生在T<25 K，体电导率在加热和冷却过程中表现出滞后性。体电导率的温度依赖性表明，由于在这两个热激活过程中的小极化子的跳跃过程的电输运。在介电性能中观察到的弛豫过程证实了这一建议。实验结果表明，LiMn_2O_4中小极化子的跳跃运动实际上主导了其导电过程。低温热激活过程的激活能大于高温热激活过程的激活能。这与通常绝缘体的行为不一致。这种反常现象和电导率的滞后现象是由于Mn^<3+>引起的Jahn-Teller协同效应使立方F3 dm基体中出现了正交晶I4_1/amd的相变所致。这种行为与Wojtowicz的理论是一致的。

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.