Polarized clusters in paraelectric KH2AsO4

We investigated the additional resonance lines in the K${\mathrm{H}}_2$As${\mathrm{O}}_4$ (KDA) ${}_{}{}^{75}{}_{}{}^{}\mathrm{As}$ NMR spectrum, whose appearance in the temperature region just above the paraelectric to ferroelectric transition temperature has been reported before. It is shown that an earlier model which assumes a bias in the fast electric-field-gradient fluctuations normally found in the paraelectric phase does not give a satisfactory account of the experimental frequencies. Agreement with experiment is attained when the formation of fully polarized clusters in the paraelectric phase is assumed. These clusters locally interrupt the fast field-gradient averaging; they have a lifetime of at least ∼ ${10}^{-3\mathrm{}}$ sec, and are highly mobile such that a given nucleus does not find itself in the cluster for more than about ${10}^{-5\mathrm{}}$ sec at a time. Such polarized clusters also agree with the low-frequency dielectric relaxation found in KDA. They may be considered the localized manifestation of the central mode in KDA.