Structure and growth of three new polytypes of cadmium iodide, 60R, 72R and 32H. A new possible method of determining atomic structure of polytypes based on evaluation of theoretical stacking fault energy

Three new polytypes of cadmium iodide, two rhombohedral 60R and 72R and one hexagonal 32H have been discovered. The detailed atomic structures of 60R and 72R have been worked out; these are [(22)₃ 1223]₃ and [(22)₄ 1223]₃. These structures indicate the existence of a structure series [(22)_n 1223]₃. The growth of the polytypes 60R and 72R has been discussed in terms of creation of periodic stacking faults in the basic structure. Evidences and arguments including electron microscope observations of dislocation dissociation have been presented to show that the proposed mechanism is the most likely one through which the above polytypes have resulted. Following Hirth & Lothe, theoretical stacking fault energies of the probable structures of the two polytypes have been calculated and it has been found that the stacking fault energies for the proposed structures are minimum. Based on this criterion the structure of 32H was deduced and was found to represent the correct atomic structure. It seems possible that the minimum stacking fault energy criterion may prove to be of value in general in determining the atomic structure of polytypes.