Competition between vanadium tetramerization and trimerization in Ba _1-xSr_xV₁₃O₁₈

We studied the transport, magnetic, and structural properties of Ba _1-xSr_xV₁₃O₁₈, in which V and O ions form a sodium-chloride lattice with periodically missing sites. We found that the high-temperature insulating phase is characterized by spin-singlet V tetramers, whereas the low-temperature conducting phase is characterized by V trimers, and these two phases are competing in this series of compounds. Various properties substantially change at the phase transition between the two, indicating a drastic reconstruction of the electronic structure.