LAUSR

Abstract The antiferromagnetic structures of Sr 2 CoO 2 Ag 2 Se 2 and Ba 2 CoO 2 Ag 2 Se 2 are solved using powder neutron diffraction. Both compounds adopt the same magnetic structure, based on a √2 a × √2 a × c expansion of the nuclear cell with magnetic space group P C 4 2 / n (86.72 in the Belov-Neronova-Smirnova notation). This structure is adopted as a result of nearest-neighbour antiferromagnetic interactions within the CoO 2 planes. The refined long-range-ordered magnetic moments of Sr 2 CoO 2 Ag 2 Se 2 and Ba 2 CoO 2 Ag 2 Se 2 are 3.7(1) and 3.97(3) μ B per Co ion respectively. The refined moments are significantly greater than the value predicted from just considering the spin (3 μ B ); this is attributed to a significant orbital contribution to the magnetic moment in an analogous manner to that previously observed for Sr 2 CoO 2 Cu 2 S 2 and the values conform to a relationship between the shape of the distended CoO 4 Ch 2 ( Ch = S, Se) octahedron and the size of the ordered moment established for a series of related compounds.