Brillouin-scattering study of the ferroelastic transition 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$ under uniaxial stress

The $c_{55}$ elastic constant of K${\mathrm{H}}_3$ ${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$ and K${\mathrm{D}}_3$ ${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$ has been investigated by Brillouin scattering in the vicinity of the ferroelastic transition in free crystal and under applied uniaxial stress. At zero stress, $c_{55}$ exhibits remarkable softening at the transition with no departure from classical linear temperature dependence. The effect of the ${\sigma }_5$ stress component can be very well described by a Landau-type free-energy model. On deuteration, only the temperature, at which the protons or deuterons would order without coupling to the strain, is increased; the other parameters in the free energy are not essentially affected.