Study of the Transition Metal Ordering in Layered NaxNix/2Mn1-x/2O2 (2/3 ≤ x ≤ 1) and Consequences of Na/Li Exchange

A series of layered oxides within the NaxNix/2Mn1-x/2O2 (2/3 <= x <= 1) system were synthesized by classical solid-state methodologies. A study of their long and short-range structure was undertaken by combining X-ray diffraction and NMR spectroscopy. A transition from P2 to O-3 stacking was observed at x > 0.8 when samples were made at 900 degrees C, which was accompanied by disordering of ions in the transition metal layer. The magnetic properties of the materials were consistent with this picture of ordering, with all samples showing antiferromagnetic character. At x = 2/3, competition between a P2 and a P3 structure, with different degrees of transition metal ordering, was found depending on the synthesis temperature. Na/Li exchange led to structures with octahedral or tetrahedral coordination of the alkali metal, and Li/Ni crystallographic exchange, in the resulting O-3 phases. The transition from alkali metal prismatic coordination to octahedral/tetrahedral coordination involves [TMO6](infinity) layer shearing that induces some structural disorder through the formation of stacking faults.