Diffuse scattering in I mixed crystals

(Received 10 September 1992, accepted in final form 15 December 1992)

Abstract
We have used thermal neutron scattering and x-ray diffraction to investigate the short range order correlations in the orientational glassy phase of K _1-x(ND ₄) _xI. Single crystals with ammonium concentrations x = 0.42 and x = 0.62 have been studied between ambient temperature and 10 K. Both experiments exhibit diffuse scattering patterns characteristic of the low-temperature phase of K _1-x(ND ₄) _xI. The diffuse signal peaks at the Brillouin zone-boundary and is strongly anisotropic. Its intensity does not diverge as , indicating that only on a local state, the f.c.c. symmetry is broken through the short-range antiferroelectric correlations. As a result of the comparison between the different cross-sections obtained by x-ray and neutron scattering, it is concluded that static displacements of one or several atoms away from the lattice sites (most probably the iodide ions) are involved in the dipolar freezing. The dependences of the diffuse signal as functions of wave-vector, temperature and concentration are finally discussed.