Understanding the crystal structure of layered LiNi0.5Mn0.5O2 by electron diffraction and powder diffraction simulation

A combination of experimental techniques that probe different relevant length scales is necessary to truly understand the structure of complex solids. In LiNi0.5Mn0.5O2 electron diffraction reveals the presence of long-range ordering, previously undetected with X-ray diffraction and neutron diffraction. We propose a superstructure for this material with space group P3(1)12, and a root3alpha(hex.) root3alpha(hex.) ordering in the transition metal layers. Surprisingly, these ordered layers are stacked in abcabc sequence along the c axis, indicating the presence of long-range interactions between different transition-metal layers. Electron diffraction evidence indicates that Li, Ni, and Mn ions are not distributed randomly in the transition-metal layers, but order and form two sublattices with significantly different occupation. We further demonstrate that this ordering would be extremely difficult to detect experimentally, if not impossible, with powder diffraction by X-rays and neutrons. (C) 2004 The Electrochemical Society.