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Ba₂LuMoO₆ is a 4d¹ Mo⁵⁺ fcc antiferromagnet in the double perovskite structure. Here we show that it is a valence bond glass; similar to the well known isostructural Ba₂YMoO₆ compound. Muon spin rotation and relaxation measurements on Ba₂LuMoO₆ revealed the lack of magnetic order or spin freezing down to 60 mK, in contrast to Ba₂YMoO₆ where spin freezing occurs at 1 K. Inelastic neutron scattering measurements revealed a singlet-triplet gap of 28 meV similar to Ba₂YMoO₆. These results are interpeted as the formation of a valence bond glass, where spin singlets form in a disorganized manner. Some orphan spin do not form singlets and remain paramagnetic even at 60 mK.

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