K39NMR study of the paraelectric-to-incommensurate phase transition inK2ZnCl4

The temperature dependence of the quadrupole-perturbed ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}$ 1/2→-1/2 NMR line shapes has been measured in the paraelectric (PE), incommensurate (I), and commensurate (C) phases of ${\mathrm{K}}_2$ ${\mathrm{ZnCl}}_4$ between 600 and 390 K. In the PE phase and the high-temperature part of the I phase, fast chemical exchange of the ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}$ nuclei as well as a diffusive floating of the incommensurate modulation wave take place. In the low-temperature part of the I phase the modulation wave becomes solitonlike. The soliton density does not vanish at the I-C transition but persists well into the C phase.