STRUCTURE AND STABILITY OF NONSTOICHIOMETRIC CUBIC PHASE DELTA-NBN1.2(O,C)

The nonstoichiometric delta-niobium nitride with surplus content of ni trogen atoms and the NaCl-type structure (a = 0.439 nm), i.e. delta-Nb N1.2(O, C), is stabilized in epitaxial deposited films. The diffractio n patterns of these films display intensive diffuse scattering with re gular intensity vanishings in the form of plane regions in the vicinit y of structural and superstructural reciprocal space points of the del ta-phase and in the form of spherical surfaces in the neighbourhood of structural points. The analysis performed shows that this scattering can be associated with the presence of mixed-nature short-range order regions in the nonstoichiometric delta-NbN1.2(O, C) phase which are ch aracterized by longitudinal uncorrelated atomic displacement waves, as well as by concentration-type waves. The ordered oxycarbonitride phas e (X-phase) described in the first approximation by the cubic lattice with parameter a = 0.392 nm is found to precipitate when annealing the films at T = 873 K. It has been established that the diffuse scatteri ng occurring in delta-Nbn(1.2)(O, C) and the structure of short-range order regions exhibit certain correlation with the structure of the pr ecipitated ordered phase - G100(x), approximate to 1.1G100(delta), = K -1; G010(x), approximate to 1.1G010(delta), = K-2 (where K-1 and K-2 a re wave vectors of longitudinal atomic displacement waves characterizi ng short-range order).