The impurity-concentration dependence of the Rb-87 NMR line shapes and
phason-induced spin-lattice relaxation rates has been studied over a
wide temperature range in substitutionally disordered incommensurate (
Rb1-xKx)2ZnCl4 for x = 0.00, 0.02, and 0.06. The results yield in form
ation on the impurity-concentration dependence of the phason gap and p
hase fluctuations of the modulation wave as well as on the nature of t
he coupling of the impurities to the incommensurate modulation wave.