n (A

state chemistry of ternary and quaternary oxides of the 3 6 3 9layers is allowed to vary, and the general case may be platinum metals. This has been driven largely by the obser-vation that many of these compounds adopt crystal struc-tures which apparently contain 1D chains of transition metal coordination polyhedra, thus introducing the possi-bility of unusual electronic properties (1), but it has also been driven in part by recent progress in structural chem-istry, in particular the recognition that these structures are related to that of perovskite (2). The 2H hexagonal perov-skite structure (3) is often thought of as a pseudo-hcp stack of AO layers (A is usually an alkaline earth element) with written as (