Strain Decoration of Dislocations and the Static Central Peak in Ferroelastic Systems

A mechanism, based on anharmonic elasticity and the critical behavior of the strains around dislocations, is proposed for the ciritical static central peak seen in light scattering from K${\mathrm{H}}_2$P${\mathrm{O}}_4$ and K${\mathrm{H}}_3$ ${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$. It is shown that the main source of the anomaly is the ferroelasticity occurring in both crystals, though ferroelectricity in K${\mathrm{H}}_2$PO also enhances the scattering.