Sequence of phase transformations in the formation of superstructures of the M6C5 type in nonstoichiometric carbides

A symmetry analysis has been made of monoclinic and trigonal superstructures of the M6C5 type, which are formed in nonstoichiometric cubic carbides MC (y) with a B1 structure. Channels of disorder-order transitions MC (y) -> M6C5 have been revealed, and the distribution functions of carbon atoms in the M6C5 superstructures have been calculated. The atomic-vacancy ordering in nonstoichiometric cubic carbides of vanadium VC (y) and niobium NbC (y) has been investigated using neutron diffraction and X-ray diffraction analyses. It has been shown that, as the temperature decreases, the Group V transition metal carbides MC (y) can undergo two physically admissible sequences of transformations associated with the formation of M6C5 phases.