Phases and optical phenomena in the system KCN-KI

At room temperature the system KCNxKI1−x shows a miscibility gap for 0.06(2) < x < 0.96(2). Large single crystals made within this range by the Czochralski technique are optically homogeneous but change their appearance from transparent to opaque in a long-time relaxation process over several weeks. Mixed crystals with x < 0.06 show no unexpected thermal and optical properties. Phase transitions in KCN rich crystals have been studied by near-infrared measurements. Doping of KCN with KI leads to an absorption spectrum in the ultraviolet region with at least two different components. Absorption of a broad band increases strongly with temperature and behaves discontinously at the phase transition. A narrow band is assigned as the exciton of KI crystals. The potential of exciton spectroscopy as a tool for analyzing phase separation processes is demonstrated.