X-ray studies of thermal cycles in the transformation of potassium cyanide

Abstract

Single crystals of pure potassium cyanide, and of potassium cyanide containing various amounts of sodium cyanide in solid solution, have been subjected to cyclic temperature changes traversing the thermaltransformation at around — 110 °C. By means of Laue and Bragg photographs, persistence of the crystal axes has been observed in a ‘normal cycle’ high → low → high, but a metastable crystal form was also produced by a particular sequence of changes. There was considerable hysteresis in the transformation of single crystals. Introducing lattice disturbances by incorporating sodium cyanide in solid solution broadened the transformation. Co-existence was observed of various domains of the low-temperature form in ‘hybrid’ single crystals; there was also evidence for the co-existence of domains of the low- and high-tem perature forms over a range of temperatures. These results are discussed in relation to theories of continuous thermodynamic transformations in solids.