Anti-corrugation and nitrogen c(2 × 2) on Cr(100): STM on atomic scale and quantitative LEED

Abstract

We present a LEED I- V analysis of c(2 × 2)- N Cr(100) . We found nitrogen residing in fourfold hollow sites and exclude adsorption models in which nitrogne adsorbs on a metal site (on-top, substitutional or second-layer interstitial). We achieved a Pendry R-factor of 0.16 for the best-fit structure. Nitrogen resides at a vertical distance of 0.36 Å above the first chromium layer. The interlayer spacing between the first and the second chromium layer is expanded to 1.55 Å (7.5% with respect to the bulk value of 1.44 Å). The interlayer spacing between the second and the third layer is contracted to 1.41 Å. The second chromium layer is buckled (0.13 Å). The second-layer chromium atom beneath a nitrogen atom resides deeper in the bulk. The nitrogen bond length to the four first-layer chromium atoms amounts to 2.07 Å, the bond length to the second-layer chromium atom amounts to 1.97 Å. The nitrogen position in c(2 × 2)-N determined by LEED is used to identify hollow sites in scanning tunnelling microscopy images. We found that hollow sites in p(1 × 1)-Cr(100) are imaged as hillocks and chromium atoms as depressions. This is anti-corrugation of clean Cr(100). Anti-corrugation seems to be related to a surface state of clean Cr(100) and is lifted in p(1 × 1)- N Cr(100) at a (local) nitrogen coverage of 1 monolayer.