PHONON SOFTENING, ORIENTATIONAL SLOWING-DOWN AND DIFFUSE-SCATTERING IN (KI)1-X(ND4I)X MIXED-CRYSTALS

Inelastic neutron scattering measurements were used to investigate the phonon dispersion curves of (KI)1-x(ND4I)x mixed crystals at x = 0.47 and x = 1 as a function of temperature. No anomaly in the lattice dynamics of pure ammonium iodide was found on approaching the disorder/order transition (T(c) congruent-to 254 K). However, for the mixed compound (KI)0.53(ND4I)0.47 an incomplete softening of the transverse acoustic modes and the growth of a quasi-elastic central peak are observed with decreasing temperatures at the Brillouin zone boundary (X-point). These results provide evidence of coupling between dipolar orientational fluctuations and acoustic phonons at the zone boundary. They are interpreted in terms of slowing-down of the ND4+ reorientations. In addition, strongly anisotropic diffuse scattering also evidenced at the X-point indicates a local antiferroelectric ordering of the ND4+ tetrahedra at low temperature.