Sequence of disorder-order and order-order transitions accompanying the formation of M2X superstructures

The symmetry analysis of the trigonal, cubic, and tetragonal M₂X superstructures formed in strongly nonstoichiometric compounds MX _y with the B1 structure has been performed. Disorder-order MX _y → M₂X transition channels have been revealed. It has been shown that three physically allowable sequences of transitions associated with the formation of ordered M₂X phases and order-order transitions between them are possible in nonstoichiometric carbides and nitrides MX _y of the transition metals of group IV with a decrease in the temperature.