Hysteresis effects and mesoscopic domains: Precursors for the formation of orientational glass states

(NH4I)(x)(KI)(1 - x) single crystals were investigated using proton NMR for 0.8 < x < 1. They undergo a first-order transition from the orientationally disordered high-temperature alpha-phase into the low-temperature beta-phase. This transition exhibits a thermal hysteresis which strongly increases upon lowering x. In the alpha, beta coexistence range evidence for nanoscopic domains was obtained using proton spin diffusion. The characteristic dimensions of the domains decrease as the critical concentration x(c) is approached, below which an orientational glass state is stable at low temperature. This observation indicates that close to x(c) the (NH4I)(x)(KI)(1 - x) orientational glasses are to be characterized as nano domain states.