Structural investigations [#!MonctonPRL77!#,#!vanBodegomPRB81!#]

Since the spin Peierls transition involves a lattice deformation, lattice structures and phonons also reflect the onset of the transition. X-ray and neutron scattering measurements [130,131] observed the superlattice reflections below $T_{\rm SP}$, supporting the dimerization of the lattice. It was found, though, that the superlattice reflections do not necessarily appear along the chain direction; in TTF-CuBDT, the lattice also deforms perpendicular to the chain at the same time as the dimerization takes place [130].

Another finding of these scattering measurements is the existence of soft phonons at the superlattice Bragg point. These soft phonons were observed well above $T_{\rm SP}$, at temperatures close to the structural transition temperature $T_{\rm L}$; therefore, this mode is probably a feature of the lattice system, not being induced by the spin Peierls behavior. The existence of the soft phonon mode probably helps the spin Peierls transition, because the lattice dimerization corresponds to the freezing of this mode [132].