ANOMALOUS ROTATIONAL-DYNAMICS OF NH4 IONS IN THE PARAELECTRIC PHASE OF K-1-X(NH4)(X)H2PO4 MIXED-CRYSTALS

A study of the rotational dynamics of NH4 ions in the paraelectric phase of mixed crystals of K-1-x(NH4)(x)H2PO4 by quasielectric neutron scattering is reported. For high NH4 concentrations (x = 0.9) no anomalous behaviour was found and the model of NH4 ion jumps between equivalent orientations in the crystal gave a good description of the experimental data. In contrast the crystal with x = 0.6, which forms a protonic dipolar glass state at low temperatures, revealed an unusual temperature dependence of the elastic incoherent structure factor far above the glass formation boundary. This temperature dependence is analysed in the framework of the model of dynamical cluster formation in the paraelectric phase. The order parameter for the gradually condensing NH4 groups is obtained. The results are compared with data obtained by other methods on related compounds.