Long-range antiferromagnetic ordering in theS=12ordered rocksalt oxideLi5OsO6: Comparison with the isoelectronic and isostructural spin glassLi4MgReO6

Abstract

${\mathrm{Li}}_{5}\mathrm{Os}{\mathrm{O}}_{6}$ and ${\mathrm{Li}}_{5}\mathrm{Re}{\mathrm{O}}_{6}$ polycrystalline samples were synthesized by conventional solid state methods. Employing powder neutron diffraction data, the crystal structure of ${\mathrm{Li}}_{5}\mathrm{Os}{\mathrm{O}}_{6}$ was reinvestigated. ${\mathrm{Li}}_{5}\mathrm{Os}{\mathrm{O}}_{6}$ crystallizes in $C2∕m$ in an ordered NaCl structure type, where $a=5.0472(1)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $b=8.7827(2)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $c=5.0079(1)\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, $\ensuremath{\beta}=109.777(2)\ifmmode^\circ\else\textdegree\fi{}$ and $V=208.90(1)\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{3}$. Magnetic susceptibility and heat capacity data indicate an antiferromagnetic long-range order below $40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, although there is evidence for low dimensional short-range order below $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. As well, the frustration index, $f=\ensuremath{\mid}\ensuremath{\theta}\ensuremath{\mid}∕{T}_{N}\ensuremath{\sim}1$, in contrast to the isostructural and isoelectronic compound, ${\mathrm{Li}}_{4}\mathrm{Mg}\mathrm{Re}{\mathrm{O}}_{6}$, which is a spin glass below $12\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and has $f\ensuremath{\sim}14$. 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 ${\mathrm{Li}}_{5}\mathrm{Os}{\mathrm{O}}_{6}$. The main reason for this is a strong Jahn-Teller distortion in the $\mathrm{Os}{\mathrm{O}}_{6}$ octahedron material which constrains the unpaired electron to occupy the ${d}_{xy}$ orbital.