Frustration-induced valence bond crystal and its melting in Mo3Sb7
  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. 
T. Koyama, H. Yamashita, Y. Takahashi, T. Kohara, I. Watanabe, Y.   Tabata, and H. Nakamura