Phase transformations in nearly stoichiometric NbNx

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Abstract
Processes of the phase transformation in nearly stoichiometric niobium nitride, from cubic δ‐NbN to hexagonal ε‐NbN, are studied by selected‐area electron diffraction, by using NbNx films of the transition state prepared by annealing δ‐NbNx films at about 1000 °C in an atmosphere of H2 gas. The diffraction patterns taken from the annealed NbNx films suggest that the phase transformations (vacancy‐poor‐) δ‐NbNx (cub.) → (N‐vacancy‐) ordered δ‐NbN (tet.) → ordered ζ‐NbN (hex.) → ordered γ′‐NbN (mon.) → δ′‐NbN (hex.) → ε‐NbN (hex.) occur. Ordered δ‐NbN, ordered ζ‐NbN, ordered γ′‐NbN, and δ′‐NbN are transient phases which occur during the transformation δ‐NbNx → ε‐NbN, and the superlattice structures of the three ordered phases are described by the ordering of N vacancies with random Nb vacancies in their fundamental lattices. Mechanisms of the transformations from δ‐NbNx to ε‐NbN at about 1000 °C are discussed on the basis of the transient phases observed during the phase transformations.

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