Temperature Dependence of the Elastic Constants of the Alkali Halides. III. CsCl, CsBr, and CsI

Abstract
The elastic constants of CsCl, CsBr, and CsI have been measured from room temperature to 450°C. Measurements on CsCl were made up to its transition temperature at 473°C but no anomalous variations were found which would differentiate its behavior from that of CsBr and CsI. The results of the measurements are shown to be in agreement with previous experimental work. It is found that the type of temperature variation exhibited by c44 in the CsCl‐type lattice is very similar to that observed for ½(c11c12) in the NaCl‐type structure. From a consideration of the respective ionic arrangements, these elastic constants can be seen to describe shear mechanisms which could lead to instability with a static expansion of the lattice. From such a treatment, a possible mechanism is suggested to describe the CsCl‐type to NaCl‐type structure transition. This mechanism is shown to be compatible with the existing data.