Soft Phonon and Central Peak Scattering in Molecular Crystal System: C6CI402 (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)