The temperature dependence of the ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}$ spin-lattice relaxation time $T_1$ has been measured in the paraelectric and incommensurate phases of ${\mathrm{K}}_2$${\mathrm{SeO}}_4$. The phason and amplitudon contributions to $T_1$ have been determined from the variation of $T_1^{\mathrm{-}1}$ over the incommensurate line. We found that in ${\mathrm{K}}_2$${\mathrm{SeO}}_4$ the modulation wave is pinned to the underlying crystal lattice and that a gap of about 160 GHz exists in the phason excitation spectrum.

• Received 29 March 1989

DOI:https://doi.org/10.1103/PhysRevB.40.799