MOLECULAR DISPLACEMENTS IN THE SPIN-DIMERIZED STATE OF (TETRATHIAFULVALENE)CUS4C4(CF3)4 AT 4.2 K

We have carried out a crystallographic study of the dimerized state of the spin-Peierls compound (TTF)Cu(BDT). X-ray data taken below the dimerization transition at Tc=12 K were analyzed using a model which assumes rigid molecular displacements. We find that pairing is accomplished by a simple longitudinal translation of spin-1/2 (TTF)+ units along the c→p axis in the primitive triclinic cell. We use our quantitative measurement of the displacement magnitude (Δx=0.072) within the mean-field spin-Peierls model to show that the unrenormalized energy of the dimerization phonon is about 1 meV. This result is consistent with previous observations of preexisting soft modes in x-ray diffuse-scattering studies. Implications for the observation of this transition in other materials are discussed. © 1979 The American Physical Society.