THERMOCHEMISTRY OF THE FORMATION OF NITROGEN-RICH SURFACE-LAYERS ON TRANSITION-METAL NITRIDES - A STUDY OF THE VN1-X-N2 COUPLE

The formation of nitrogen-rich surface layers on substoichiometric transition metal nitrides upon cooling from their equilibrium state at high temperatures down to room temperature is described in terms of thermodynamics and diffusion equations. The formalism is applied to the VN1-x-N2 couple. Using a surface composition-dependent diffusion coefficient for nitrogen in VN1-x and a simplified superposition algorithm, the diffusion profiles can be approximated. The frequency factor introduced for the diffusion coefficient (Arrhenius-type equation) is log (D0) = -14.6 x (1 - x) + 7.8 for Q = 125.5 kJ mol-1. The compositional shift effected by nitrogen uptake at the surface of compact plates of VN1-x was investigated by X-ray diffraction and measurements of the superconducting transition temperature. Powders show a shift of the bulk composition towards stoichiometric values. The nitrogen uptake can significantly affect the results of measurements of solid-state and thermodynamic properties vs composition. With the applied formalism the diffusion profiles in the solid of a gas-solid couple (nitrogen-nitride, oxygen-oxide, hydrogen-hydride) can be predicted for given experimental conditions, provided that diffusion and p, T, (1 - x) data are known.