Parent of Misfit-Layered Cobalt Oxides: [Sr2O2]qCoO2

Misfit-layered (ML) cobalt oxides of the general formula of [ M m A 2 O m +2 ] q CoO 2 have been proven to be efficient thermoelectric materials because the structure is capable in accommodating the two seemingly contradictory characteristics of high electrical conductivity and large thermoelectric power. They are also potential hosts for other oxymoron -like functions. The known phases all contain one or two square-planar M O ( M = Co, Bi, Pb, Tl, etc.) layers sandwiched together with two A O ( A = Ca, Sr, Ba, etc.) planes of square symmetry between two adjacent CoO 2 layers of hexagonal symmetry. Here, we report the realization of the simplest ( m = 0) ML phase forming in the Sr−Co−O system with the cation ratio Sr/Co = 1. Atomic-resolution transmission electron microscopy (TEM) imaging confirms for the new phase the parent three-layer crystal structure, SrO−SrO−CoO 2, which is compatible with the formula of [Sr 2 O 2 ] q CoO 2 . Electron diffraction reveals that the phase is rather commensurate, i.e. the “misfit parameter” q is 0.5. Nevertheless, in terms of the transport-property characteristics, the new ML parent is comparable to its earlier-established and more-complex derivatives.