Realization of the bond order wave (BOW) phase of extended Hubbard models in Rb–TCNQ(II)

Rb–TCNQ(II) is shown to be a realization of the BOW phase of half-filled extended Hubbard models. The BOW phase has a regular array of sites with inversion (Ci) symmetry, a finite magnetic gap Em and broken electronic Ci symmetry. The phase is conditionally stable against dimerization for linear electron–phonon (e–ph) coupling. At 100 K, Rb–TCNQ(II) crystals have regular TCNQ− stacks at Ci centers, negligible spin susceptibility that indicates finite Em, and infrared (ir) spectra that indicate broken Ci symmetry. The 100 and 295 K crystal structures rule out a dimerization transition around 220 K that had previously been inferred from magnetic and ir data.