Displacive Phase Transitions in Malononitrile

The crystallographic phases and phase transitions of molecular crystals of malononitrile were investigated using ¹⁴N pure nuclear quadrupole resonance. The resonance frequencies, linewidths, and spin–lattice relaxation times were measured using pulse techniques. The relation between quadrupole resonance results and the order parameter of a phase transition are analyzed. Detailed measurements were carried out near one of the transitions (at 294.7°K) and the results are interpreted in terms of critical exponents. The resonance frequencies give $\text{β\hspace{0.17em}=\hspace{0.17em}0.5\hspace{0.17em}±\hspace{0.17em}0.02}$ and the linewidths $\text{γ\hspace{0.17em}=\hspace{0.17em}0.3\hspace{0.17em}±\hspace{0.17em}0.07}$ and $\text{γ′\hspace{0.17em}<\hspace{0.17em}0.1\hspace{0.17em}±\hspace{0.17em}0.1}$ . Our quadrupole resonance results are compared with specific‐heat measurement of Girdhar, Westrum, and Wulff. As two of the phase transitions show very small transition entropies, it is suggested that these are displacive phase transitions associated with instabilities in the collective modes of the crystal. The meaning of such transitions is discussed both from a thermodynamic and a microscopic point of view.