Gifu Prefectural Ceramics Research Institute^* Ceramics Research Laboratory, Nagoya Institute of Technology^**
○Kenji Tateishi^* Nobuo Ishizawa^**

Crystals of LiMn₂O₄ undergo a first order structural phase transition near the room temperature. The high temperature (HT) form adopts a normal spinel-type structure (cubic, Fd-3m) with the octahedral sites populated statistically with heterovalent Mn^III and Mn^IV in the equal ratio. The low temperature (LT) form adopts a 3×3×1 superstructure (orthorhombic, Fddd) with respect to the HT form, containing Mn1^III, Mn3^III, Mn4^IV, Mn5^IV and intervalent Mn2 atoms. The synchrotron X-ray single-crystal electron-density analysis revealed a large anisotropy of the atomic displacement parameters of the O9 amd O4 atoms. This suggests an occurrence of the bond-length fluctuation along the pseudo-tetragonal Jahn-Teller distortion parallel to the a axis in the heterocubane Mn2₄O9₄ cluster which presumably shares three electrons among four e-parentage orbitals of Mn2 and behaves as a core of Zener-type polaron. The heterocubane Mn2₄O9₄ clusters are isolated with each other and embedded in an ordered way in the charge-ordered matrix containing Mn1^III, Mn3^III, Mn4^IV and Mn5^IV. The transition between the LT and HT forms of LiMn₂O₄ can thus be regarded as an order-disorder transition of the Zener-type polarons.