Interaction of nitrogen with iron surfaces *1II. Fe(110)

Previous work on the interaction of N2 with Fe(100) and (111) single-crystal planes was extended to the most densely packed (110) face. The dissociative chemisorption proceeds even slower than with the two other planes and is associated with an activation energy of about 7 kcal/mole at low coverages. At 683 K the initial rates of adsorption have a ratio of about 60:3:1 for Fe(111):(100):(110). The activation energy for desorption is estimated to be about 50 kcal/mole and is thus slightly smaller than on Fe(100)(58 kcal/mole), but higher than on Fe(111) (51 kcal/mole). The formation of two ordered surface structures (I and II) was observed. Their unit cell vectors b1, b2 are related with the (unreconstructed) substrate lattice vectors a1, a2 through: b1 = 3a1, b1 = 2a1 (I) (which may also be designated as 2 × 3 structure), and b1 = 4a1, b1 = a1 + 3a2 (II). Both phases appear with two equivalent domain orientations with the mirror plane along the [001] direction. Structure I appears at lower surface concentrations than structure II; however their kinetics of formation is also influenced by the concentration of dissolved nitrogen. It is most probably that these large unit cells reflect the reconstruction of the topmost layer of Fe atoms [as in the case of the (111) plane]. For both surface structures quasihexagonal arrangements of Fe atoms may be constructed the nearest-neighbor distances for which agree to within ±3% with those in the (111) plane of fcc Fe4N. Thus the concept of the formation of “surface nitrides” is further supported. Dissolution of nitrogen atoms in the bulk again interferes with the surface processes.