Synthesis, Crystal Structure, and Magnetic Properties of Li3Mg2OsO6, a Geometrically Frustrated Osmium(V) Oxide with an Ordered Rock Salt Structure: Comparison with Isostructural Li3Mg2RuO6

The novel osmium-based oxide Li3Mg2OsO6 was synthesized in polycrystalline form by reducing Li5OsO6 by osmium metal and osmium(IV) oxide in the presence of stoichiometric amounts of magnesium oxide. The crystal structure was refined using powder X-ray diffraction data in the orthorhombic Fddd space group with a = 5.88982(5) angstrom, b = 8.46873(6) angstrom, and c = 17.6825(2) A. This compound is isostructural and isoelectronic with the ruthenium-based system Li3Mg2RuO6. The magnetic ion sublattice Os5+ (S = 3/2) consists of chains of interconnected corner- and edge-shared triangles, which brings about the potential for geometric magnetic frustration. The Curie Weiss law holds over the range 80-300 K with C = 1.42(3) emu center dot K/mol [mu(eff) = 3.37(2) mu(B)] and theta(C) = 105.8(2) K. Below 80 K, there are three anomalies at 75, 30, and 8 K. Those at 75 and 30 K are suggestive of short-range antiferromagnetic correlations, while that at 8 K is a somewhat sharper maximum showing a zero-field-cooled/field-cooled divergence suggestive of perhaps spin freezing. The absence of magnetic Bragg peaks at 3.9 K in the neutron diffraction pattern supports this characterization, as does the absence of a sharp peak in the heat capacity, which instead shows only a very broad maximum at similar to 12 K. A frustration index of f = 106/8 = 13 indicates a high degree of frustration. The magnetic properties of the osmium phase differ markedly from those of the isostructural ruthenium material, which shows long-range antiferromagnetic order below 17 K, f = 6, and no unusual features at higher temperatures. Estimates of the magnetic exchange interactions at the level of spin-dimer analysis for both the ruthenium and osmium materials support a more frustrated picture for the latter. Errors in the calculation and assignment of the exchange pathways in the previous report on Li3Mg2RuO6 are identified and corrected.