  121/123Sb nuclear quadrupole resonance and muon spin relaxation experiments
of Mo3Sb7 revealed symmetry breakdown to a nonmagnetic state below the
transition recently found at TS=50 K. The transition is characterized by a
distinct lattice dynamics suggested from narrowing of nuclear fields. We point
out that the Mo sublatice is a unique three-dimensional frustrated lattice
where nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions
compete, and propose that tetragonal distortion to release the frustration
stabilizes long-range order of spin-singlet dimers, i.e., valence bond crystal,
which is thermally excited to the dynamic state with cubic symmetry.
