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Li 4 MgOsO 6 was synthesized by two different solid-state reaction procedures. The crystal structureswere determined byX-ray powder diffraction technique and it was revealed that Li 4 MgOsO 6 crystallizes in two different crystal symmetries in ordered rock salt structure type, namely monoclinicC2/m and orthorhombicFddd.The unit cell constants for the monoclinic system are a= 5.1074(4) Å,b = 8.8182(4) Å,c = 5.0902(2) Å, andβ = 109.845(4)º and those of the orthorhombic structure are a = 5.8485(1) Å, b = 8.3821(1) Å, andc = 17.6212(3) Å.In both systems, Os 6+ ions reside exclusively in a specific crystallographic position while Li + and Mg 2+ ions exhibitmix occupancy. The temperature dependent magnetic susceptibility data for both S = 1 osmate systems do not support the occurrence of any magnetic transition down to 2K. The Curie-Weiss fit to the paramagnetic regime of the magnetic susceptibility data reveal highly negative  θ value (-114.81 K and -121.87 K for C2/mand for Fdddsystems, respectively), which are indicative of predominant antiferromagnetic (AFM) interactions in both systems. The experimental effective magnetic moment (μ eff ) value for themonoclinic phase is 2.13 μBand that of the orthorhombic system is 2.34 μB.Due to the rather strong AFM interactions and lack of magnetic transition down to 2K, both of these novel osmates are placed in the class of highly frustrated magnets. Low temperature magnetic susceptibility (below 2K) and dynamic magnetic properties studies (µsr studies) are in order to better understand the magnetic ground states of these two polymorphs of Li 4 MgOsO 6 .

The novel osmium-based oxide Li(3)Mg(2)OsO(6) was synthesized in polycrystalline form by reducing Li(5)OsO(6) 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) Å, b = 8.46873(6) Å, and c = 17.6825(2) Å. This compound is isostructural and isoelectronic with the ruthenium-based system Li(3)Mg(2)RuO(6). The magnetic ion sublattice Os(5+) (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·K/mol [μ(eff) = 3.37(2) μ(B)] and θ(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 ∼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 Li(3)Mg(2)RuO(6) are identified and corrected.