Density-functional study of the chemisorption of N on and below Fe(110) and Fe(001) surfaces

We study ordered nitrogen overlayers on and below (110) and (001) surfaces of ferromagnetic bcc iron by using ultrasoft-pseudopotential density- functional calculations. With the exception of (1 x 1) N/Fe (110), the dissociative surface adsorption of N-2 is predicted to be exothermic, whereas subsurface absorption is always endothermic. The results for chemisorption at Fe (001) agree with experiments; for the (110) face, large deformations induced by N can signalize a tendency to reconstruction. For subsurface nitrogen, some symmetric positions below Fe (001) turn out to be unstable, and large local deformations are present for the stable sites. These structures cannot serve as candidates for nitridized surface films in the initial stage of growth because they are thermodynamically unstable. Below the (110) surface at concentration 0.25 of N, we find even no appropriate metastable sites for nitrogen at zero temperature. The magnetization is markedly suppressed at iron atoms close to N, the controlling parameter being the Fe- N distance and, in some cases, also the induced change of the Fe- Fe separation. In almost all cases we find a small negative magnetic moment on nitrogen atoms.