Cs3OsH9 -: Synthesis, structure, and magnetic properties

Cs3OsD9 was synthesized by the reaction of cesium deuteride with osmium powder at 870 K and a deuterium pressure above 1500 bar. Elastic neutron diffraction experiments on a powder sample at room temperature led to an atomic arrangement nearly describable in a cubic unit cell (Pm (3) over barm; a = 6.128(2) A, Z = 1). According to the formula CS3D[OsD8] the structure corresponds to that of the perovskite type. The coordination polyhedron of the deuterium atoms surrounding each osmium atom can be described crystallographically as a statistical occupation of two 24-fold positions with deuterium. A detailed investigation on the gaussian profiles of the neutron diffraction pattern leads to the result that the compound crystallizes in a tetragonal unit cell with a = 8.602(1) angstrom c = 12.250(1) angstrom and Z = 4. Below a phase transition at 110K an orthorhombic unit cell was found with a = 8.6983(4) angstrom, b = 8.6046(4) angstrom, c = 11.9437(6) angstrom and Z = 4. A provisional structure model is presented. Magnetic susceptibility measurements in the temperature range between 1.8 and 400 K reveal a weak paramagnetism. Quantum mechanical calculations confirm the experimental result and show in detail that spin momentum and orbital momentum cancel each other nearly complete with regard to the ground state.