Topotaxial decomposition of calcite-type KNO3 crystals

The calcite-like form of potassium nitrate, KNO₃, decomposed under the transmission electron microscope to form KNO₂ which subsequently decomposed giving the high-temperature β form of K₂O. Two closely related orientation relations were observed for KNO₂. Referred to the four-molecule cell of KNO₃, they were 111]_NNO3∥111]_KNO2, (0$\text{1}$1)_KNO3∥(0$\text{1}$1)_KNO2; and 100]_KNO3∥111]_KNO2, (001)_KNO3∥(001)_KNO2. These and a different published orientation relation for the decomposition of cadmium carbonate conform, respectively, to orientations resulting from a corresponding structural phase transformation in rubidium nitrate. β-K₂O formed with its cube axes parallel to those of KNO₂. Both nitrate and carbonate reactions can be regarded as topotaxial. Application of the crystallographic approach to orientations and accommodation of misregistry is discussed.