ESR Study of Electron-Nuclear Dipolar Relaxation for AsO4 4- Spin Probe in the Paraelectric Phase of KH2 AsO4

Saturation behavior of allowed and forbidden ESR transition of AsO4 4- paramagnetic probe in KH2 AsO4 was studied in the wide temperature interval around the paraelectric-ferroelectric phase transition, T c . The ratios between forbidden and allowed line intensities were employed to deduce information on the electron-nuclear dipolar (END) relaxation mechanism. It was shown that a proton END relaxation mechanism exhibits an extremal temperature behavior in the paraelectric phase around 230 K. The extremal temperature behavior was described by employing a model of proton hopping along the O-H...O bonds around the paramagnetic centers, and the correlation time of this hopping was estimated in the wide temperature range in the paraelectric phase (150-330 K). The temperature dependence of effective proton distance from the neighbor oxygens was obtained, and it was discussed in terms of a localization of the spin density on these oxygens caused by charge inbalance in the As-O bonds in the ferroelectric phase.