Reorientation Frequencies of Ferroelectric Dipoles near a Paramagnetic Center in K H 2 As O 4 and K D 2 As O 4

The frequency of exchange between the various "Slater"-type ${\mathrm{H}}_{2}$As${\mathrm{O}}_{4}$ dipolar configurations near a local paramagnetic center has been measured in the paraelectric phase of $\ensuremath{\gamma}$-irradiated K${\mathrm{H}}_{2}$As${\mathrm{O}}_{4}$. The life-time effects in the proton hyperfine structure of the EPR spectrum were used as a detector of the proton motion. Similar effects were also found in K${\mathrm{D}}_{2}$As${\mathrm{O}}_{4}$. The results show the absence of an instability of the lattice against any "ferroelectric mode" involving proton displacements, and they favor the order-disorder model of ferroelectricity in this class of crystals.