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 pha se of mixed crystals of K-1-x(NH4)(x)H2PO4 by quasielectric neutron sc attering is reported. For high NH4 concentrations (x = 0.9) no anomalo us behaviour was found and the model of NH4 ion jumps between equivale nt orientations in the crystal gave a good description of the experime ntal data. In contrast the crystal with x = 0.6, which forms a protoni c dipolar glass state at low temperatures, revealed an unusual tempera ture dependence of the elastic incoherent structure factor far above t he glass formation boundary. This temperature dependence is analysed i n the framework of the model of dynamical cluster formation in the par aelectric phase. The order parameter for the gradually condensing NH4 groups is obtained. The results are compared with data obtained by oth er methods on related compounds.