Synthesis,Crystal Structure and Magnetic Properties of Na3OsO5

Na₃OsO₅ has been synthesized by solid state reaction of Na₂O₂ and osmium powder under a flow of oxygen. Na₃OsO₅ crystallizes trigonal in space group P3₁21 with a = 5.5529(2) Å, c = 13.4726(8) Å, V = 359.77(3) ų, Z = 3, 958 independent reflections, R₁ = 1,97 %, wR₂ = 4.27 %. The crystal structure consists of OsO₅ trigonal bipyramids separated by sodium cations, and is isostructural to Na₃ReO₅. The osmium atoms adopt a packing analogous to ccp, where Na occupies the octahedral and tetrahedral vacancies, so the cation structure is derived from the Li₃Bi structure type. Features of the synthesis of Na₃OsO₅ and related osmates are discussed. Na₃OsO₅ shows paramagnetic behaviour down to 25 K where it orders antiferromagnetically. Below 7 K a weak ferromagnetic contribution is detected. The paramagnetic region of the 5d¹ system has been successfully described in the temperature range 50 – 330 K accounting for strong spin‐orbit coupling, ligand‐field potential of D_3h symmetry (corresponding to the trigonal bipyramidal unit), and cooperative magnetic effects (molecular field parameter λ_MF).