Nuclear Quadrupole Frequencies and Relaxation Times in Ferroelectric Hydrogen Chloride

The anomalous temperature dependence of the chlorine nuclear quadrupole resonance frequency in the ferroelectric solid II of HCl is interpreted by means of an attenuated 90° flip process of the molecules. Correlation times ${\tau }_f$ are calculated for the 90° flip process from ³⁵Cl spin–lattice relaxation time ${\mathrm{(T}}_1)$ data and an activation energy is derived. The calculated values of ${\tau }_f$ are in fair agreement with those previously estimated from proton rotating frame relaxation times ${\mathrm{(T}}_1\mathrm{\rho )}$ in solid II. The internal local electric field E_loc at an HCl molecule in solid II is calculated by the fourier transformation summation method. E_loc due to the electric dipole and quadrupole moments of the HCl molecule deviates by about 30° from the direction of a molecular dipole. The shift of the deuteron and chlorine‐35 nuclear quadrupole frequencies in the rigid lattice from the corresponding gas phase frequencies are explained as electrically induced quadrupole shifts by the field E_loc. Perturbation theory is used to calculate the magnitude of the electrically induced shifts to be expected which agree well with the shifts observed. It is proposed that most deuteron quadrupole frequency shifts from gas to solid may be explained by this mechanism. A mechanism for the ferroelectric phase transition in solid II of HCl is specified which involves the 90° as well as 180° flips of the molecules.