Refinement of the crystal structures of strontium cobalt oxychlorides by neutron powder diffraction

The crystal structures of strontium cobalt oxychlorides of general formula Sr₂CoO₃Cl (Co-1L), Sr₃Co₂O₅Cl₂ (Co-2L), and Sr₄Co₃O_7.5Cl₂ (Co-3L) have been determined from neutron powder data. The compounds crystallize in tetragonal symmetry, space group P4/nmm (Co-1L) or I4/mmm (Co-2L and Co-3L). Their structures are analogous to the Ruddlesden-Popper phases of general formula Sr(n+1)M(n)O(2n+1). All three structures contain layers of highly distorted CoO₅Cl octahedron. The Co-3L structure, in addition, has a layer of regular CoO₆ octahedra. Because the in-plane oxygen position of the latter is partially occupied, some of the Co atoms adopt pyramidal coordination, with their basal planes parallel to the c-axis. The temperature dependence of the magnetic susceptibility is also reported. The moment per mole of Co atom decreases with increasing number of (CoO₂) layers.