Local Structural Perturbations in HgBa2CuO4+delta

A strontium-rich 2201-type oxide, Bi1-xSr3+xCoO6-delta, with a nonmodulated structure has been synthesized. X-ray powder diffraction and high-resolution electron microscopy show a 1:1 ordering of Bi3+ and Sr2+ species within the two intermediate [Bi0.5Sr0.5O]infinity layers. The distorted [CoO2]infinity layers can also be described as built up from CoO6 octahedra and CoO5 pyramids, stacked along a alternately. In this structure, Bi3+ exhibits a tetrahedral coordination BiO3L, taking into consideration its 6s2 lone pair denoted L. The magnetic behavior of this trivalent cobalt oxide suggests that this species is not purely high spin and shows an antiferromagnetic transition below 50 K, similar to that observed in the tubular cobaltite.