Cl35nuclear-quadrupole-resonance investigations of the lock-in phase and of the new low-temperature phase inK2ZnCl4

The ferroelectric lock-in phase III and the new low-temperature phase IV of ${\mathrm{K}}_2$Zn${\mathrm{Cl}}_4$ are analyzed by means of ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ NQR measurements combined with group-theoretical considerations. It is shown that the eigenvector of the frozen-in soft mode in phase III, which consists mainly of rotations of the Zn${\mathrm{Cl}}_4$ tetrahedra around the pseudohexagonal $a$ axis, is very big compared to ${\mathrm{Rb}}_2$Zn${\mathrm{Cl}}_4$. The amplitude of the frozen-in rotation wave is measured to be 79° at 200 K. In the new low-temperature phase, a quadruplication of the NQR lines is observed which can be explained by a zone-boundary transition leading to a monoclinic phase. A possible influence of the domain pattern upon the NQR-signal amplitude, originating from phase solitons which survived the lock-in transition, is discussed.