Ordering Sequence in Strongly Nonstoichiometric Niobium Carbide with the Formation of Nb6C5-Type Superstructures

Possible Nb 6 C 5 -type superstructures of nonstoichiometric niobium carbide NbC 0.83 are found by the evolutionary crystal structure prediction algorithm. A symmetry analysis is carried out of Nb 6 C 5 -type model superstructures, the formation of which is possible in nonstoichiometric cubic niobium carbide with B 1 structure. Channels of order–disorder transition NbC y → Nb 6 C 5 are found. It is shown that niobium and carbon atoms, which form the immediate environment of vacant sites in Nb 6 C 5 superstructures, experience the maximum displacement and are shifted away from structural vacancies (the vacant sites of the carbon sublattice). As temperature decreases, two physically admissible sequences of transformations associated with the formation of Nb 6 C 5 superstructures are possible in NbC 0.83 carbide. The transformation sequences obtained are in agreement with the calculation of the ground states of the superstructures.