Ordering Phenomena in Transition Metal Oxides

Coupling between the electronic and structural degrees of freedom in Na_xCoO₂   

Square planes of transition metals and oxygen are the basis for scientifically and technologically important materials such as perovskite ferroelectrics, superconductors and ferromagnets. Compounds based on triangular arrangements of transition metals and oxygen are much less generally studied, except in the context of frustrated magnetism in electrically insulating materials. The introduction of charge carriers into triangular magnetic lattices to yield metallic conductivity has been a goal of research for some time. Na_xCoO₂, with a crystal structure consisting of alternating layers of Na ions and triangular CoO₂ planes, is the embodiment of such a system. Square-based transition metal-oxygen lattices display a broad range of effects due to the coupling of electronic, magnetic, and structural degrees of freedom, but until now there has been no model system for investigating such effects in triangle-based electronic conductors. I will describe in this talk structural studies of the average and local crystal structure of Na_xCoO₂ over a wide range of Na content. Changes in the electron count induced by Na stoichiometry strongly affect the structure of the CoO₂ plane. The results suggest that conducting triangular lattice systems display their own class of structural and electronic coupling phenomena distinct from those previously studied in square-based systems.