Valence bond glass state in the 4d ¹ fcc antiferromagnet Ba₂LuMoO₆

B-site ordered 4d¹ and 5d¹ double perovskites have a number of potential exotic ground states including multipolar order, quantum spin liquids and valence bond glass states. These arise from the complex interactions of spin-orbital entangled J_eff = 3/2 pseudospins on the geometrically frustrated fcc lattice. The 4d¹ Mo⁵⁺ perovskite Ba₂YMoO₆ has been suggested to have a valence bond glass ground state. Here we report on the low temperature properties of powder samples of isostructural Ba₂LuMoO₆: the only other known cubic 4d¹ perovskite with one magnetic cation. Our muon spectroscopy experiments show that magnetism in this material remains dynamic down to 60 mK without any spin freezing or magnetic order. A singlet-triplet excitation with a gap of Δ = 28 meV is observed in inelastic neutron scattering. These results are interpreted as a disordered valence bond glass ground state similar to Ba₂YMoO₆. Our results highlight the differences of the 4d¹ double perovskites in comparison to cubic 5d¹ analogues, which have both magnetic and multipolar order.