Cation ordering in layered O3 Li[Ni _xLi _1/3-2x/3Mn _2/3-x/3]O ₂ (0 ≤ x ≤ 1/2) compounds

Three layered compounds, Li[Ni _1/2Mn _1/2]O ₂ (x = 1/2), Li[Li _1/9Ni _1/3Mn _5/9]O ₂ (x = 1/3), and Li ₂MnO ₃ (x = 0) of the Li[Ni _xLi _1/3-2x/3Mn _2/3-x/3]O ₂ series were studied by electron diffraction in combination with synchrotron X-ray powder diffraction analyses. In-plane √3a _hex. × √3a _hex. ordering was found in all three samples, and a likely model for the occupancies of Li ⁺, Ni ²⁺, and Mn ⁴⁺ on the sublattices of the transition metal layer was proposed. Perpendicular to the Chex. axis, the ordered transition metal layers in the layered Li[Ni _1/2Mn _1/2]O ₂ and Li[Li _1/9Ni _1/3Mn _5/9]O ₂ crystals could be arranged mostly in the P3 ₁12 stacking with some C2/m stacking (such as abab...) abnormally. It is believed that the nickel ions in the lithium layer of these two samples play an important role in the stacking of transition metal layers. Moreover, electron diffraction analyses suggested that individual Li ₂MnO ₃ crystals that were obtained at 850 °C had at least two different C2/m stacking sequence variants (abab..., caca..., and cbcb...) coexisting along the chex. axis, which were associated with the presence of some P3 ₁l2 stacking abnormally.