Phase transitions and relaxation dynamics in (NH_4I)_x(KI)_(1-x) mixed crystals

摘要

Dielectric studies, as well as elastic and quasielastic neutron scattering experiments were performed on the mixed molecular system (NH_4I)_x(KI)_(1-x) for ammonium concentrations 0.55 <= x <= 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 #epsilon#-phase which is characterized by the appearance of a superstructure reflection at the (300) reciprocal lattice point. In constant-Q 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.