Layered T2-, O6-, O2-, and P2-type A(2/3)[M1/3 ' 2+M2/34+]O-2 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 be en uniformly prepared from P2 structure precursors with A = Na by ion excha nge. 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 sup erlattice that is preserved during ion exchange. The in-plane ordering is e xactly 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 tran sition 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 str ucture 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 nove l T2 structure, and we give crystallographic arguments for the formation of T2 versus O6 based on the detailed stacking of the ordered transition-meta l layers.