Commensurate Peierls transition in a quasi-one-dimensional compound: The bromide salt of tetrathiafulvalene (TTF)

The x-ray crystal structure of the bromide salt of tetrathiafulvalene (TTF)-${\mathrm{Br}}_{0.79}$ shows incommensurate cation and anion sublattices, with a static modulation of each stack at the periodicity of the opposing stack. If the on-site Coulomb interactions were very small compared with the bandwidth, this modulation would induce an energy gap at twice the Fermi wave vector and hence a commensurate Peierls transition would be possible. If the Coulomb interactions were very large, (TTF)-${\mathrm{Br}}_x$ would be a room-temperature semiconductor. Transport, magnetic, thermal, and optical measurements are presented, which indicate that the Coulomb interactions are not very large compared with the bandwidth, estimated at $4t\sim 1.1$ eV. Hence, the system can be viewed as undergoing a commensurate Peierls transition at ∼ 170 K.