Elastic and quasielastic neutron scattering studies in KBr:KCN mixed crystals

Abstract

Neutron scattering studies in (KBr)1–x(KCN)x mixed crystals are presented utilizing powder diffraction, single crystal diffraction and time-of-flight techniques. Forx>0.6 (KBr)1–x(KCN)x crystals exhibit ferroelastic and ferroelectric low-temperature phases. Crystals withx<0.6 undergo transitions into an orientational glass state. Here we present a detailed phase diagram including new results for x=0.85 and x=0.65. For the latter system a stable rhombohedral low-temperature phase has been detected where the orientational disorder of the plastic phase is only partly removed and quadrupolar relaxations between three body diagonals are still possible. From the powder diffraction experiments we determined further the concentration dependence of the static Debye-Waller factors which can be explained by an interplay of the rotation-translation and the rotation-random strain coupling. With single crystal diffraction techniques we studied the diffuse scattered intensities which are directly related to the order parameter of the glass state. The temperature dependence of the quasielastic intensities near the critical concentration shows a strong increase forT<110K indicative for a freezing-in of shear fluctuations which is a characteristic feature of a non-ergodic instability. This phenomenon appears for ordering (x=0.65) and for non-ordering, glassy compounds. A further anomaly in (KBr)0.43(KCN)0.57 at 75 K is interpreted in terms of a residual elastic ordering process. With high-resolution time-of-flight techniques we analysed the dynamic structure factor for x=0.57. We demonstrate that the central peak consists of a static and a dynamic component. The results are compared with mode coupling theories which describe the glass transition in supercooled liquids

Relaxation dynamics in orientationally disordered molecular mixed crystal [(CH3)3CCH2OH]0.7[(CH3)2C(CH2OH)2]0.3

“…Although such a result must be cautiously considered due to the used powdered samples, previous works have shown the same conclusions in single crystal studies [24,44]. In contrast, for quadrupolar glasses the broadening of the width of the Bragg peaks has been clearly observed [45]. Nevertheless, figure 2(b) shows that the discontinuity jump in the volume expansivity takes place at a temperature close to the C p anomaly.…”

Ferroelasticity and glass-like behavior in alkali halide-alkali cyanide mixed crystals

“…Detailed diffuse and quasielastic neutron scattering experiments in (KBr), -,(KCN), close to the critical concentration have been performed by Loidl, Mullner, McIntyre, Knorr and Jex (1985) and by Loidl et al (1988Loidl et al ( , 1989. A single crystal X-ray investigation has been reported by .…”

“…For comparison and to get some insight into the statics of the orientational glass state, similar experiments have been performed in (KBr), -,(KCN), below the critical concentration (Loidl et al, 1988(Loidl et al, ,1989. Figure 7 shows transverse scans at zero energy transfer through the (060) reflection in the (001) plane of (KBr)o,43(KCN)o,5, at various temperatures (from Loidl et al, 1988).…”

“…A complete x,T-phase diagram of (KBr), -. (KCN), on the basis of X-ray, neutron and specific heat measurements has been constructed previously by , Loidl et al (1989). Figure 1 shows the phase diagram as published by Loidl et al (1989).…”

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

“…At room temperature, as well as at T"11 K the structural refinements yield the ideal cubic structure and even the thermal parameters are strongly reduced as a function of temperature. No clear experimental evidence for strong local distortions which would produce large "static" Debye-Waller factors, as observed in a number of OG [14,15], could be detected and from the neutron diffraction point of view, (NH I) (KI) displays the features of a normal mixed molecular crystal.…”