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 (NH4I)(x)(KI)(1-x) for ammonium concentrations 0.55 less than or equal to x less than or equa l to 0.8. Based on these experiments we present a detailed and improved pha se diagram? which reveals a variety of different structural phases with dif ferent degrees of orientational order, as well as different glass states wi th 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 cons tant-e scans at zero energy transfer we followed the temperature dependenci es of these superstructure reflections and determined correlation lengths f or all concentrations investigated. We found short-range order of the epsil on-type structure for x = 0.55, while long-range order has been observed fo r all other concentrations. In addition, we performed constant-Q scans at t he (300) reciprocal lattice point for several temperatures. From a purely p henomenological analysis, two time scales were observed which we attributed (i) to structural relaxations at the phase transition from the cubic high-t emperature to the trigonal low-temperature epsilon-phase and(ii) to orienta tional relaxations of the NH4+-tetrahedra in a slightly distorted environme nt. 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.