Purification and Single-Crystal Growth of Potassium Cyanide

Reagent-grade KCN has been fashioned into nominally pure, low-strain, single crystals via a sequence of operations involving vacuum melting, zone refining and thermal-gradient growth of a monocrystal. Vacuum melting of KCN largely transforms oxygen-containing impurities (e.g., HCOO—, C2O4=, and CO3=) to OCN—. Zone refining removes the OCN— contamination with an effective working segregation coefficient of 0.7. Fe, Na, and Si are also readily removed. Molybdenum is a suitable container material. Single crystals of KCN were grown under vacuum in a quartz, filtering-crystallizing device and strain minimized by freeing the crystalline boule from the ampoule walls prior to annealing slowly to room temperature. Samples cleaved from such a boule proved suitable for polarized-light experiments. They typically contained 10 ppm Na and trace quantities of OCN— impurity. A pure KCN crystal has no absorption bands in the visible or near-ultraviolet regions and begins to absorp strongly at 190 mμ. As the purity of KCN crystals decreases, the tail of the fundamental absorption shifts to lower energies. This shift is related to an increase in cyanate-ion contamination.