DIFFUSE-SCATTERING IN K1-X(ND4)XI MIXED-CRYSTALS

We have used thermal neutron scattering and x-ray diffraction to inves tigate the short range order correlations in the orientational glassy phase of K1-x(ND4)xI. Single crystals with ammonium concentrations x = 0.42 and x = 0.62 have been studied between ambient temperature and 1 0 K. Both experiments exhibit diffuse scattering patterns characterist ic of the low-temperature phase of K1-x(ND4)xI. The diffuse signal pea ks at the Brillouin zone-boundary and is strongly anisotropic. Its int ensity does not diverge as T --> 0 K, indicating that only on a local state, the f.c.c. symmetry is broken through the short-range antiferro electric correlations. As a result of the comparison between the diffe rent cross-sections obtained by x-ray and neutron scattering, it is co ncluded that static displacements of one or several atoms away from th e lattice sites (most probably the iodide ions) are involved in the di polar freezing. The dependences of the diffuse signal as functions of wave-vector, temperature and concentration are finally discussed.