Structural and magnetic properties of Ba2LuMoO6: a valence bond glass

We report here the synthesis of the site ordered double perovskite Ba2LuMoO6. Rietveld refinement of room temperature powder X-ray diffraction measurements indicate that it crystallises in the cubic space group Fm3̅m, with a = 8.3265(1) Å. Powder neutron diffraction data indicate that unusually, this cubic symmetry is maintained down to 2 K, with S = ½ Mo5+ ions situated on the frustrated face-centred cubic lattice. Despite d.c.-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.