Soft phonon and central peak scattering in a molecular crystal system: C6Cl4O2 (chloranil)

Elastic and inelastic neutron scattering has been used to study the continuous structural transition in chloranil at Tc=90.3 K. The transition results in a doubling of the monoclinic unit cell along the c axis and the molecular displacements correspond to a staggered rotation, φ, about axes perpendicular to the molecular planes. The temperature dependence of the order parameter, φ, has been determined from superlattice Bragg intensities with the result φ∝ (Tc−T)0.33±0.02 for 0.005< (1−T/Tc) <0.5. The response from the zone boundary phonon, whose eigenvector corresponds to the distortions below Tc, shows a pronounced softening in the temperature range 100–300 K. At temperatures below 140 K the spectrum also contains a central component diverging at Tc which is resolution limited in energy, but shows considerable width and anisotropy in momentum space.