Long-range antiferromagnetic ordering in the S=1/2 ordered rocksalt oxide Li5OsO6:: Comparison with the isoelectronic and isostructural spin glass Li4MgReO6

Li(5)OsO(6) and Li(5)ReO(6) polycrystalline samples were synthesized by conventional solid state methods. Employing powder neutron diffraction data, the crystal structure of Li(5)OsO(6) was reinvestigated. Li(5)OsO(6) crystallizes in C2/m 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)degrees and V=208.90(1) A(3). Magnetic susceptibility and heat capacity data indicate an antiferromagnetic long-range order below 40 K, although there is evidence for low dimensional short-range order below 80 K. As well, the frustration index, f=parallel to theta parallel to/T(N)similar to 1, in contrast to the isostructural and isoelectronic compound, Li(4)MgReO(6), which is a spin glass below 12 K and has f similar to 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 Li(5)OsO(6). The main reason for this is a strong Jahn-Teller distortion in the OsO(6) octahedron material which constrains the unpaired electron to occupy the d(xy) orbital.