The oxygen-deficient Ruddlesden-Popper (RP) phase Sr3Mn2O6 crystallizes with an ordered array of oxygen vacancies to afford a structure in which the Mn3+ ions exist in a square-pyramidal environment. The MnO5 polyhedra are linked through their corners to form a structure that is related to that observed for the single-layered material, Sr2MnO3.5. The nuclear and magnetic structures of a polycrystalline sample of Sr3Mn2O6 have been determined using Rietveld analysis of neutron powder diffraction data and electron diffraction techniques. The pure Mn3+ double-layered phase crystallizes in a superstructure of the simple RP subcell: tetragonal, P4/mbm, a = 10.8686(2) Angstrom and c = 20.2051(3) Angstrom. Magnetic susceptibility studies suggest a transition at similar to 250 K to a canted antiferromagnetic ordered structure. The magnetic unit-cell consists of ferromagnetic clusters of corner-sharing MnO5 units, which are antiferromagnetically aligned to other clusters within the layers. (C) 2003 Elsevier Inc. All rights reserved.