Nuclear quadrupole resonance study of phase transition in tetrachloro-p -benzoquinone (chloranil)

The phase transition in chloranil crystal has been studied by measurements of the 35Cl nuclear quadrupole resonance frequency and the spin-lattice relaxation time. The splitting of the resonance line in the low temperature phase was shown to behave as an order parameter of the phase transition. The experimental order parameter and the anomalous behavior of the transition probability (1/T1) near the transition point were interpreted satisfactorily in terms of the librational soft-mode theory which assumes anharmonic coupling between molecular librational modes in the crystal.