Structural transition in Mo3Sb7 probed by muon spin relaxation

Longitudinal-field μSR measurements have been made for Mo3Sb7 focusing on the nature of the structural transition recently found at TS≅50K. Taking account of a critical behavior of the relaxation rate λ at ∼TS, together with the motional narrowing of the nuclear dipolar field revealed in a zero-field experiment, and the tetragonal lattice symmetry lowering below TS, we propose long-range order of spin-singlet dimers, i.e., the formation of the valence bond crystal below TS. As a possible origin, the frustration in the interdimer antiferromagnetic interaction is suggested.