Structure and electron density analysis of lithium manganese oxides by single-crystal x-ray diffraction

Single crystals of LiMn₂O₄ and LiMnO₂ have been successfully synthesized by a flux method. The lithium-ion deintercalated λ-MnO₂ single crystal has been prepared electrochemically using a microelectrode-based system. The single-crystal X-ray diffraction study at room temperature confirmed the cubic spinel Fd3m space group for LiMn ₂O₄ and λ-MnO₂, and the orthorhombic Pmnm space group for LiMnO₂, respectively. The crystal structure of LiMn₂O₄ was refined using two models. A new oxygen displacement model with 1/3 occupation at the 96g site gives better reliable values R = 1.70% and wR = 1.49% for 213 independent reflections. The crystal structure of LiMnO₂ has been refined to the reliable values R = 1.32% and wR = 1.45% for 676 independent reflections. In order to understand chemical bonding nature of these materials, we performed to analyze the electron density distributions by Maximum Entropy Method (MEM) using the single-crystal X-ray diffraction data. From the results of MEM analysis, the strong covalent bonding features were found between the Mn and O atoms in these manganese oxides. We also calculated electron density in LiMn₂O₄ and LiMnO ₂ by the full-potential linearized augmented-plane-wave (FLAPW) method. The electron density distribution features obtained theoretically were in good agreement with the experimental observations. A combined analysis of the MEM and FLAPW calculations clearly indicated that the covalent bonding observed between the Mn and O atoms in LiMn₂O₄ is mainly assigned to the Mn-3d and O-2p hybridization which locates in the energy range of -7.5 < E < -2.5 eV.