Li5OsO6 and Li5ReO6 polycrystalline samples were synthesized by conventional solid state methods. Employing powder neutron diffraction data, the crystal structure of Li5OsO6 was re-investigated. Li5OsO6 crystallizes in C2/m space group in an ordered NaCl structure type where a = 5.0472(1) A, b = 8.7827(2) A, c = 5.0079(1) A, Beta = 109.777(2) degree and V = 208.90(1).A^3 Magnetic susceptibility and heat capacity data indicate an AFM long range order below 40K although there is evidence for low dimensional short range order above 80K. As well, the frustration index, f = |theta|/TN ~1, in contrast to the isostructural and isoelectronic compound, Li4MgReO6, which is a spin glass below 12K and has f ~ 14. An attempt was made to rationalize these differences using spin dimer analysis. The key results are that the spin exchange interactions in the Re-based compound are stronger and are consistent with a frustrated triangular lattice model while a low dimensional short range order model is better for Li5OsO6. The main reason for this is a strong Jahn-Teller distortion in the OsO6 octahedron material which constrains the unpaired electron to occupy the dxy orbital.
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