Layered T2-, O6-, O2-, and P2-type A2/3[M′2+1/3M4+2/3]O2 bronzes, A = Li, Na;: M′ = Ni, Mg;: M = Mn, Ti

Layered compounds A(2/3)[M'M-2+(1/3)4+ (2/3)]O-2 are studied for A = Li, Na, M' = Ni, Mg, and M = Mn, Ti. The metastable compounds with A = Li have been uniformly prepared from P2 structure precursors with A = Na by ion exchange. For the cases M' = Ni, Mg and M = Mn we find that the cations within a [M'M-2+(1/3)2/3(4+)]O-2, Sheet are ordered on a (root 3)a by (root 3)a superlattice that is preserved during ion exchange. The in-plane ordering is exactly the same as found for the Li and Mn atoms in the layered compound Li[Li2/3Mn2/3]O-2, better known as Li2MnO3. When M = Ti, no evidence for transition metal ordering is observed. Using the results from X-ray and neutron scattering, Li-2/3[Ni1/3Mn2/3]O-2 is shown to adopt a new orthorhombic structure that; we call T2, In the notation of Hagenmuller and Delmas, the T2 structure has a two-layer unit cell and tetrahedral sites for lithium. Li-2/3[Mg1/3Mn2/3]O-2 adopts an O6 structure. The focus of the work is the novel T2 structure, and we give crystallographic arguments for the formation of T2 versus O6 based on the detailed stacking of the ordered transition-metal layers.