NMR-NQR studies of structural phase transitions

Molar heat capacity of solid chloranil was measured between 11 and 300 °K with particular attention to the phase transition at 92 °K. The heat and entropy of the transition were 38 J mol−1 and 0.41 J °K−1 · mol−1. The transition is of a lambda shape starting at 70 °K and ends at 100 °K. The anomalous heat capacity curve was explained by the soft librational mode theory. There is another large transition at about 570 °K. The shape and magnitude of the transitions are very sensitive to the long‐range regularity of the structure and depend on the method of preparation.

Heat capacity of solid tetrachloro‐p ‐benzoquinone (chloranil) between 11 and 300 °K. Phase transition at 92 °K

Exact solution of the eigenvalue problem for a spin 32 system in the presence of a magnetic field

Direct observation of a soft phonon associated with a structural phase transition in a molecular crystal

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.

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

The nuclear quadrupole resonance of Cl35 and its Zeeman effect have been observed in chloranil between 78° and 373°K. Effects of thermal expansion appear negligible hence the Bayer theory is used with the measured temperature dependence of the pure quadrupole frequencies and the field‐gradient asymmetries to yield room‐temperature rigid‐body libration amplitudes of 〈θx2〉 = 18×10—3 rad2, 〈θy2〉 = 〈θz2〉 = 0, where x is along the molecular normal. A broad phase transition centered near 100°K is examined. The results suggest that the potential well V(θx) has a double minimum and that the transition occurs when the molecules have sufficient torsional energy to override the barrier.

C. B. Richardson

Papers

Temperature Dependence of the Zeeman Effect in the Nuclear Quadrupole Resonance in Chloranil