Ultrasonic attenuation and critical relaxation rates in K${\mathrm{H}}_3$${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$ and K${\mathrm{D}}_3$${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$

The ultrasonic attenuation associated with $c_{44}$ shear waves has been investigated in the disordered phase of the ferroelastic crystals K${\mathrm{H}}_3$ ${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$ and K${\mathrm{D}}_3$ ${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$. Analysis of these data yields critical relaxation times at constant stress that diverge at the second-order transition temperature $T_c:{\tau }_{\mathrm{H}}=\frac{6.1\times {10}^{-11\mathrm{}}}{(T-T_c)}$ for the normal salt and ${\tau }_{\mathrm{D}}=\frac{46\times {10}^{-11\mathrm{}}}{(T-T_c)}$ for the isomorphous deuterated salt. This effect of deuteration on the critical relaxation time is very similar to that observed in the ferroelectric salt K${\mathrm{H}}_2$P${\mathrm{O}}_4$.