Three Forms of the Misfit Layered Cobaltite [Ca2CoO3] [CoO2]1.62·A 4D Structural Investigation

Three misfit layer compounds with the same chemical formula Ca₃Co_4−xO_9+δ 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.