Symmetry breaking in the metal-insulator transition of BaVS₃

It has been believed for a long time that the metal-to-insulator (MI) transition of BaVS₃ is not accompanied by any spatial order of the spin and lattice. We have carried out x-ray-diffraction measurements of BaVS₃ single crystals using a laboratory x-ray source as well as synchrotron radiation, and found that superlattice reflections which double the lattice constant c exist below the transition temperature. The most probable space group at the low-temperature insulator phase contains two inequivalent vanadium sites, and thus a charge disproportionation of the vanadium ions is considered the main cause of the MI transition.