Superspace crystal symmetry of thermoelectric misfit cobalt oxides and predicted structural models

Single crystals of thermoelectric misfit lamellar cobalt oxide phases in the Bi-Ca-Co-O and I-Bi-Ca-Co-O systems were synthesized. They are characterized by aperiodic structures involving two partially independent sublattices: a CdI(2)-type pseudohexagonal CoO(2) layer and a rocksalt-type BiCaO(2) slab allowing the intercalation of iodine. The crystal symmetry of these structures is discussed using the four-dimensional superspace formalism. The superspace Laue classes of the iodine-free and the intercalated compounds are P2/m(0delta(1/2)) (a(1) = 4.901, b(1) = 4.730, b(2) = 2.80, c(1) = 14.66 A, beta = 93.49 degrees ) and A2/m(0delta1) (a'1 = 4.903, b'1 = 4.742, c'1 = 36.51 A, beta = 87.30 degrees ), respectively. A comparison is given with the related Bi-Sr-Co-O misfit compounds. The present structures are compatible with the presence of an intrinsic modulation with a wavelength matching the misfit aperiodicity in the b direction. Preliminary partial structure refinements confirm the layer stacking of the structure and the intercalation of I between the Bi-O layers for the second phase. A comparison with other cobalt oxide phases, as well as symmetry and metric considerations allow us to predict average structures for these new phases and to describe the common structural features assumed for all these lamellar misfit cobalt oxides.