Normarski differential interference microscopy study of the ferroelastic domains in NaCN, RbCN and KCN

SUMMARY Photomicrographs of the ferroelastic domains present in the low temperature, intermediate phase of NaCN, RbCN and KCN are presented with their linear dimensions and angular orientations. The Normarski optical technique used to display the domain surfaces in colour and used to determine the small out‐of‐plane angular orientations is discussed in detail. Figures and the construction details are presented of the cold stage that allowed microscopic examination of the crystal surfaces by either reflected or transmitted light down to liquid nitrogen temperatures.