Local environment of nitrogen in a surface nitride: A low-energy electron diffraction study of Cr(100)-(1×1)N

We present a quantitative low-energy electron diffraction study of the surface nitride produced by nitrogen segregation to the surface of the (100) face of chromium. Emphasis is put on the local environment of N atoms, in reference to surfaces of bulk nitrides, this study being one step in a longer-term investigation of the role of vacancies at substoichiometric nitride and/or carbide surfaces. The nitrogen atoms are found to occupy symmetric fourfold hollow sites. The nitrogen atoms are 0.221±0.016 Å above the chromium first layer and have four metal neighbors in the top layer with a 2.045±0.002 Å bond length and a fifth metal neighbor in the second layer at a very similar distance, 2.015±0.018 Å. This environment is made possible by having a very large expansion (24.8%) of the topmost interlayer spacing of chromium and shows great similarities with the surface structures of : (i) surface nitrides like Ni(100)-P4g-(2×2)-N (which reconstructs with a large lateral displacement of the surface metal atoms), Fe(100)-c(2×2)-N (which does not reconstruct because the metal substrate is bcc like chromium, leaving room for the N atoms to set up the maximum coordination), and (ii) the surface of a bulk nitride ${\mathrm{VN}}_{0.89}$(100) (as obtained from preliminary results).