Structural and magnetic properties of Ba₂LuMoO₆: A valence bond glass

We report here the synthesis of the site ordered double perovskite Ba ₂LuMoO₆. Rietveld refinement of room temperature powder x-ray diffraction measurements indicates that it crystallizes in the cubic space group , with a = 8.3265(1) . Powder neutron diffraction data indicate that, unusually, this cubic symmetry is maintained down to 2 K, with , Mo⁵⁺ ions situated on the frustrated face-centred cubic lattice. Despite dc-susceptibility measurements showing Curie-Weiss behaviour with strong antiferromagnetic interactions at T ≥ 200 K, there is no evidence of long range magnetic ordering at 2 K. At T ≤ 50 K, susceptibility measurements indicate a loss in moment to ∼18% of the expected value, and there is a corresponding loss in the magnitude of the magnetic exchange. The structural and magnetic properties of this compound are compared with the related compound Ba₂YMoO₆, which is a valence bond glass.