EPR study of central peak dynamics around the AsO44− center in KDA–KDP and ADA–ADP mixed crystals

Using the lifetime effects in the domain splitting of the ESP spectra as a detector of the polarization fluctuations around an AsO4−4 center, we have studied the dependence of the temperature T—where the local tetragonal symmetry of the high temperature paraelectric phase is broken on the time scale of the ESP experiment—on Tc in mixed KH2AsO4–KH2PO4 (KDA–KDP) and NH4H2AsO4–NH4H2PO4 (ADA–ADP) crystals. T decreases with Tc in KDA–KDP and increases with Tc in ADA–ADP crystals though the domain splitting is nearly constant. This can be understood only if the central peak width depends on the host lattice and thus varies with Tc. The width of the central peak in the polarization fluctuation spectrum, which changes in KDA from 280 MHz at 45 °C to 6 MHz at −90 °C does not depend on the concentration of the paramagnetic defect centers within the range studied. The results can be explained by a model where in the high temperature phase the mobile clusters of the low temperature phase are trapped by the defect cen...