2006 年 48 巻 1 号 p. 17-24
Crystals of LiMn2O4undergo a first order structural phase transition near the room temperature. The high temperature (HT) form adopts a normal spinel-type structure (cubic, Fd3m) with the octahedral sites populated statistically with heterovalent MnIIIand MnIVin the equal ratio. The low temperature (LT) form adopts a 3×3×1 superstructure (orthorhombic, Fddd) with respect to the HT form, containing Mn1III, Mn3III, Mn4IV, Mn5IVand intervalent Mn2 atoms. The synchrotron X-ray single-crystal electron-density analysis revealed a bond-length fluctuation along the pseudo-tetragonal Jahn-Teller distortion parallel to the a axis in the heterocubane Mn24O94cluster which presumably shares three electrons among foure-parentage orbitals of Mn2 and behaves as a core of Zener polaron. The heterocubane Mn24O94clusters are isolated with each other and embedded in an ordered way in the charge-ordered matrix containing Mn1III, Mn3III, Mn4IVand Mn5IV. The transition between the LT and HT forms of LiMn2O4can thus be regarded as an order-disorder transition of the Zener polarons. The molecular dynamics (MD) studies revealed that a periodical random switching of Mn valences, reflecting the e-parentage electron hopping model, greatly facilitates the Li-ion self-diffusion. Two distinct but coexistent processes have been proposed for the diffusion of Li atoms in the HT form of LiMn2O4.