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

Polycrystalline Li4MgReO6 was prepared by means of solid-state synthesis. The crystallographic structure was determined from high-resolution powder-neutron-diffraction data. It was found that Li4MgReO6 crystallizes in space group C2/m with lattice parameters, a=5.0978(3) Å, b=8.8163(5) Å, c=5.0815(3) Å, and β=109.835(2)°. This structure is an ordered variant of Li2SnO3 in which Li+, Mg2+, and Re6+ ions occupy the Sn4+ sites, which can be desribed as honeycomb layers. Due to the large differences in formal charge, the Re6+ ions order on half of the Sn sites forming a distorted face-shared tetrahedral sublattice. As Re6+ is an S=1/2 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 K.