The transition mechanism between the CsCl-type and NaCl-type structures in CsCl

In the transition between the CsCl-type and NaCl-type structures, caesium chloride retains a definite orientation relation of [110] (CsC1) || [100] (NaC1) and [001] (CsC1) || [011] (NaCl). The closest-packing planes of both structures are retained in the transition and have an approximately equal density of ions at the transition temperature. A uniaxial expansion (or shrinkage) parallel to one of the twofold axes in the CsCl-type structure (or the fourfold axes in the NaCl-type structure) attending the transition was observed under the optical microscope. The amount of the uniaxial expansion is about 15% of the size of the low- temperature form, and the volume change of about 17% in the transition is mostly due to this uniaxial expansion. The transition mechanism is explained as being due to a combination of two systematic movements of ions: intralayer rearrangement and interlayer translation.