Formation and Decomposition of Nitrides on Iron Surfaces

The formation (by interaction with ammonia) and decomposition of nitrides on clean Fe surfaces was studied by means of Auger electron spectroscopy, x-ray photoelectron spectroscopy, thermal desorption spectroscopy, and scanning electron microscopy. The N atoms may exist in various forms with quite similar electronic properties, viz. as chemisorbed layer (= "surface nitride"), dissolved in α-Fe or γ-Fe, as γ′-nitride (= Fe4N) or as e-nitride, depending on temperature as well as pressure and duration of interaction with NH3. There is no noticeable chemical shift of the ionization energies of the Fe core levels, indicating that the bond is essentially covalent. The activation energy for the decomposition of e-nitride into Fe4N + N2 is about 27 kcal/mole, that for the decomposition of Fe4N into Fe+N2 ranges between 51 and 57 kcal/mole, depending on the mode of preparation. The latter values are identical to those found previously for the desorption of N2 from various Fe single crystal planes and indicate that the decomposition of the chemisorbed "surface nitrides" is the rate-limiting step which prevents the spontaneous decom-position of the metastable bulk iron nitrides.