QUASI-ELASTIC NEUTRON-SCATTERING STUDY OF NH4 DYNAMICS IN K1-X(NH4)XH2PO4 MIXED-CRYSTALS

The dynamics of NH4 groups in K1-x(NH4)xH2PO4 mixed crystals have been studied using quasi-elastic neutron scattering on powders for x = 0.6 and 0.9. It was found that the rotational dynamics of NH4 is quite di fferent of these two concentrations. In the sample with x = 0.9, which undergoes a strongly first-order phase transition into an antiferroel ectric phase, the activation energy for NH4 reorientations was found t o be 130 meV. The elastic incoherent structure factor (EISF), which is a measure of the time-averaged spatial distribution of protons in the lattice, was temperature independent. In contrast, the sample with x = 0.6 forms a dipolar glass state at low temperatures and the activati on energy for NH4 reorientations is found to be considerably lower (74 meV). The EISF for the lower concentration sample is strongly tempera ture dependent. The data obtained are discussed on the basis of mechan isms of glass formation.