Li4MgReO6: An S=1/2 antiferromagnet exhibiting spin-glass behavior

Polycrystalline ${\mathrm{Li}}_4{\mathrm{MgReO}}_6$ was prepared by means of solid-state synthesis. The crystallographic structure was determined from high-resolution powder-neutron-diffraction data. It was found that ${\mathrm{Li}}_4{\mathrm{MgReO}}_6$ crystallizes in space group $C2/m$ with lattice parameters, $a=5.0978\mathrm{}\left(3\right)\mathrm{}\hspace{0.5em}\AA ,$ $b=8.8163\mathrm{}\left(5\right)\mathrm{}\hspace{0.5em}\AA ,$ $c=5.0815\mathrm{}\left(3\right)\mathrm{}\hspace{0.5em}\AA ,$ and $\beta =109.835\mathrm{}\left(2\right)\mathrm{}°.$ This structure is an ordered variant of ${\mathrm{Li}}_2{\mathrm{SnO}}_3$ in which ${\mathrm{Li}}^{+},$ ${\mathrm{Mg}}^{2+},$ and ${\mathrm{Re}}^{6+}$ ions occupy the ${\mathrm{Sn}}^{4+}$ sites, which can be desribed as honeycomb layers. Due to the large differences in formal charge, the ${\mathrm{Re}}^{6+}$ ions order on half of the Sn sites forming a distorted face-shared tetrahedral sublattice. As ${\mathrm{Re}}^{6+}$ is an $S=1/2\mathrm{}$ ion this material is an attractive candidate for the study of the effects of geometric frustration on a quantum spin system. The investigation of bulk magnetic susceptibilities and magnetization studies as well as low-temperature neutron-diffraction experiments reveals a spin-glass behavior with a glass transition temperature of 12 K. Both heat-capacity and muon spin-relaxation measurements at low temperatures are consistent with a spin-glass interpretation with no long-range order detectable down to $T=0.5\mathrm{}\hspace{0.5em}\mathrm{K}.$