Glassy polarization behavior in ${\mathrm{K}}_2$${\mathrm{Sr}}_4$ ${\left(\mathrm{N}\mathrm{b}{\mathrm{O}}_3\right)}_{10}$-type ferroelectrics

We extend our previous measurements of the temperature dependence of the optic indices of refraction, $n\left(T\right)\mathrm{}$, of two crystals of the ${\mathrm{K}}_2$ ${\mathrm{Sr}}_4$ ${\left(\mathrm{N}\mathrm{b}{\mathrm{O}}_3\right)}_{10}$ type with different amounts of disorder. These materials are axial ferroelectrics with the tetragonal tungsten-bronze crystal structure. As in other ferroelectrics where the translational symmetry is strongly broken, a glassy polarization $P_d$ is observed far above the ferroelectric transition temperature $T_c$. However, in these noncubic materials, angular averaging of $n\left(T\right)\mathrm{}$ need not be done, and so $P_d$ is obtained rather directly. Below $T_c$ it is in good agreement with the reversible, spontaneous polarization.