Chemisorption and corrosion at the metal-halogen interface: Overlayer growth and compound formation by bromine on Cr(100)

Abstract

The interaction of bromine with Cr(100) has been studied in the regime 300–1300 K and over a wide range of coverage. The initial reaction results in the formation of an overlayer which exhibits a continuous series of compression structures whose end members are c(2 × 2) and c(2 × 4). The sticking probability of Br 2 remains constant at 0.9 ± 0.1 during this process. Beyond this stage, nucleation and growth of CrBr 2 commences at a rate which is ten times slower than that of the initial chemisorption reaction. The stoichiometry of the bromide phase is kinetically controlled and it grows by the Volmer-Weber mechanism to yield an epitaxial corrosion layer of a pseudohexagonal form of CrBr 2. Intensity changes and kinetic energy shifts in the Cr and Br Auger spectra indicate the occurrence of substantial valence charge transfer from metal to halogen. The structural, thermochemical and electronic properties of the system are discussed with reference to a specific model.