The temperature and impurity-concentration dependence of the phason gap in mixed incommensurate $\left({\mathrm{Rb}}_{1-x}{\mathrm{K}}_x\right){}_2{}^{}{}_{}{}^{}\mathrm{Zn}{\mathrm{Cl}}_4$ has been studied via the phason-induced ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ spin-lattice-relaxation contribution. The results yield novel information on the nature of the coupling of the impurities to the incommensurate modulation wave and allow for a discrimination between random-field and random-interaction effects.

• Received 24 February 1986

DOI:https://doi.org/10.1103/PhysRevLett.56.2387