Three forms of the misfit layered cobaltite [Ca2CoO3] [CoO2]1.62 -: A 4D structural investigation

Three misfit layer compounds with the same chemical formula Ca3CO4-xO9+delta were isolated in the Ca-Co-O system. They exhibit either a monoclinic or an orthorhombic symmetry. These crystals are constituted of two interpenetrating C sublattices showing incommensurate periods along the b axis. The structure of the three crystals can be described as an alternate stacking along [001] of distorted rock-salt-type slabs [Ca2CoO3] and of [CoO2] layers displaying a distorted CdI2-type structure. Relating to the symmetry and the different observed c periods, different sequences are found for the [CoO2] layers running along [001] within the three crystals. Two of them were determined by single X-ray diffraction using the 4D superspace formalism. A significant displacive modulation is implied, acting mainly on Ca and O atoms involved in the intersystem bonding scheme. This modulation leads to a noticeable distortion of the CoO6 octahedra of the [CoO2] layers. Strong interactions, via Ca-O bonds, are evidenced between the two sublattices. A systematic positional disorder is observed inside the [CoO] layer. (C) 2001 Academic Press.