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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 , 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 .
Another finding of these scattering measurements is the existence of
soft phonons at the superlattice Bragg point. These soft phonons were
observed well above , at temperatures close to the structural
transition temperature ; 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 .