Vacancy short-range order in substoichiometric transition metal carbides and nitrides with the NaCl structure. I. Electron diffraction studies of short-range ordered compounds

The structure of cubic transition metal compounds VC_{1 - x}, NbC_{1 - x}, TiC_{1 - x}, VN_{1 - x} and TiN_{1 - x} has been studied by electron diffraction. These compounds occur with large deviations from the stoichiometric composition and the carbides have been observed with long-range ordered carbon distributions for certain compositions and temperatures. For crystals of other compositions and temperatures electron diffraction patterns frequently contain diffuse bands of intensity which have a periodic distribution with respect to the reciprocal lattice points. The diffuse intensity follows a continuous surface of complex curvature in reciprocal space, is observed for all compounds except VN_{1 - x}, and has the symmetry of the b.c.c. reciprocal lattice. These features indicate a short-range ordered non-stoichiometric vacancy distribution which is analysed in Part II of this series [Sauvage & Parthé (1972), Acta Cryst. A28, 607].