  We report on the unconventional magnetism in the cubic B-site ordered double
perovskite Ba2YMoO6, using AC and DC magnetic susceptibility, heat capacity and
muSR. No magnetic order is observed down to 2 K while the Weiss temperature is
~ -160 K. This is ascribed to the geometric frustration in the lattice of
edge-sharing tetrahedra with orbitally degenerate Mo5+ s=1/2 spins. Our
experimental results point to a gradual freezing of the spins into a disordered
pattern of spin-singlets, quenching the orbital degeneracy while leaving the
global cubic symmetry unaffected, and providing a rare example of a valence
bond glass.
