Valence bond glass state in the 4d1 fcc antiferromagnet Ba2LuMoO6

B-site ordered 4d1 and 5d1 double perovskites have a number of potential novel ground states including multipolar order, quantum spin liquids and valence bond glass states. These arise from the complex interactions of spin-orbital entangled Jeff = 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