Slow Ferroelectric Cluster Dynamics in K${\mathrm{H}}_2$As${\mathrm{O}}_4$ and K${\mathrm{H}}_2$P${\mathrm{O}}_4$ from ${\mathrm{Cr}}^{5+}$ Paramagnetic Resonance

We succeeded in substituting the paramagnetic ${\mathrm{Cr}}^{5+}$ ($S=\frac{1}{2}$) ion with the same size as ${\mathrm{As}}^{5+}$ in K${\mathrm{H}}_2$As${\mathrm{O}}_4$, for ${\mathrm{P}}^{5+}$ in K${\mathrm{H}}_2$P${\mathrm{O}}_4$, and in their deuterated isomorphs. In the crystals the high-temperature resonance spectrum showing a local tetragonal symmetry is broken on the time scale of the EPR experiment below $T^{*}>T_C$, so that orthorhombic spectra appear in the paraelectric tetragonal phase. $T^{*}$ shifts proportionally to the Curie temperature $T_C$ upon deuteration. We deduce the existence of ferroelectric clusters with lifetimes $\tau$ longer than 1.5×${10}^{-8\mathrm{}}$ sec below $T^{*}$.