Evolution structurale et proprietes physiques des phases AXMO2 (A = Na,K; M = Cr,Mn, Co) (x ⩽ 1)

The structures of the A_XMO₂ (x ? 1) phases may be classified in three main groups. The layer structures are the most common, their anionic packing varies with the nature of the alkali element and its insertion rate. The tunnel structures are observed only for manganese and small values of x. Cristobalite derived structures characterize the KMO₂ oxides for small size M³⁺ cations. Whereas the A_xCrO₂ and A_xMnO₂ phases are semi-conductors, the A_xCoO₂ oxides have a metallic behaviour, the small Co-Co distances allowing t_2g orbital overlaping through the common edges of adjacent octahedra. The influence of the structure on magnetic properties is discussed.