The thermal transition in caesium chloride in relation to crystal structure

Abstract

X-Ray measurements have been made on single crystals of caesium chloride from 20 to 469° C 495° C traversing the transition CsCl (b. c. c.) ⇌ 469°C CsCl (f. c. c.). Contrary to previously published statements which were all based on CsCl powder, the lattice expansion of the b. c. c. crystal remains approximately a linear function of temperature to within 1° of the transition point. Above about 380° C the intensity of X-ray reflexions decreases more rapidly than the Debye-Waller theory predicts. Tentative explanations are discussed. As the temperature rises intensity measurements show that the mean amplitude of vibration of the Cs + ion increases faster than that of the Cl - ion. Little, if any, correlation is found between the orientation of a single crystal of b. c. c. CsCl and the assemblage of crystallites into which it breaks up above the transition point. The number of such crystallites formed is small, indicating that the rate of nucleus formation is slow compared with their rate of growth. No ‘interaction’ between the two phases can be inferred from the measurements of lattice spacings. A simple hypothesis is suggested to explain the tendency for CsCl to form distorted single crystals.