Phase transitions and relaxation dynamics in NH4I x KI 1 x mixed crystals

Dielectric studies, as well as elastic and quasielastic neutron scattering experiments were performed on the mixed molecular system NH 4 I x KI 1 x for ammonium concentrations 0.55 less than or equal to x less than or equal to 0.8. Based on these experiments we present a detailed and improved phase diagram? which reveals a variety of different structural phases with different degrees of orientational order, as well as different glass states with frozen in orientational disorder. Special attention has been devoted to the recently discovered E phase which is characterized by the appearance of a superstructure reflection at the 300 reciprocal lattice point. In constant e scans at zero energy transfer we followed the temperature dependencies of these superstructure reflections and determined correlation lengths for all concentrations investigated. We found short range order of the epsilon type structure for x 0.55, while long range order has been observed for all other concentrations. In addition, we performed constant Q scans at the 300 reciprocal lattice point for several temperatures. From a purely phenomenological analysis, two time scales were observed which we attributed i to structural relaxations at the phase transition from the cubic high temperature to the trigonal low temperature epsilon phase and ii to orientational relaxations of the NH 4 tetrahedra in a slightly distorted environment. This interpretation is in accordance with the dielectric results which also provide experimental evidence for the appearance of two time scales. C 1999 Elsevier Science B.V. All rights reserved