  Misfit-layered (ML) cobalt oxides of the general formula of [MmA2Om+2]qCoO2
have been proven to be efficient thermoelectric materials as the structure is
capable in accommodating the two seemingly contradictory characteristics of
high electrical conductivity and large thermo-electric power. They are also
potential hosts for other oxymoron-like functions. The known phases all contain
one or two square-planar MO (M = Co, Bi, Pb, Tl, etc.) layers sandwiched
together with AO (A = Ca, Sr, Ba, etc.) planes of square symmetry and CoO2
layers of hexagonal symmetry. Here we report realization of the simplest (m =
0) ML phase forming in the Sr-Co-O system with the cation ratio, Sr/Co = 1.
Atomic-resolution TEM imaging confirms for the new phase the parent three-layer
crystal structure, SrO-SrO-CoO2, which is compatible with the formula of
[Sr2O2]qCoO2. 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.
