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

Polycrystalline Li4MgReO6 was prepared by means of solid-state synthesis. T he crystallographic structure was determined from high-resolution powder-ne utron-diffraction data. It was found that Li4MgReO6 crystallizes in space g roup C2/m with lattice parameters, a=5.0978(3) Angstrom, b=8.8163(5) Angstr om, c=5.0815(3) Angstrom, and beta=109.835(2)degrees. 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 differ ences 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 th is material is an attractive candidate for the study of the effects of geom etric frustration on a quantum spin system. The investigation of bulk magne tic susceptibilities acid 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 interpre tation with no long-range order detectable down to T=0.5 K.