Soliton density inRb2ZnBr4

The modulation wave in ${\mathrm{Rb}}_2$ ${\mathrm{ZnBr}}_4$ is pinned over most of the incommensurate phase and ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ NMR line-shape analysis cannot distinguish between a true incommensurate phase and a large-period commensurate phase with N=17. The temperature variations of the soliton density, the line shape, and the spin-lattice relaxation rate speak for the coexistence of a large number of domains pinned by impurities rather than for the existence of a devil’s staircase and pinning by the discreteness of the crystal lattice. The large thermal hysteresis in the soliton density demonstrates the presence of a metastable, impurity-induced ‘‘chaotic’’ phase with random intersoliton spacing which exists even far below the incommensurate-commensurate transition.