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V. Huber, D. Bikaljevic, J. Redinger, N. Memmel:
"Structure of Low and High Coverage Phases of Bromine on Pd(110)";
The Journal of Physical Chemistry C, 120 (2016), 13523 - 13530.

Halogens can be used to steer the catalytic selectivity of transition metal surfaces. In the present work the structures of bromine on Pd(110) in the low-coverage c(2 × 4) and the high-coverage c(4 × 2) adlayers are investigated by density functional theory calculations and quantitative low-energy electron diffraction. Excellent agreement is achieved between theory and experiment. Both find the high-coordinated hollow site to be favored at low coverage, while in the high coverage phase also adsorption close to long-bridge sites occurs due to the Br-Br repulsion. This results in an almost uniform distribution of adatoms in the troughs of the Pd(110) surface as in the majority of halogen fcc(110) transition metal adsorption systems, but not on Pt(110). The different coordination of the hollow and quasi long-bridge adsorption sites is reflected by different distances to the top-layer Pd atoms, 271 pm for the high-coordinated hollow site and 248 pm for the low-coordinated quasi long-bridge site. With increasing bromine coverage the contraction of the first interlayer distance d12 of clean Pd(110) is reduced and inverted to an expansion of ∼3% at high bromine coverage. As shown by DFT, this change in substrate relaxation depends markedly on the adsorption site. A population analysis reveals a non-negligible ionic contribution for the Br-Pd bonding. Finally, we discuss the, at first sight, surprising DFT prediction of pure long-bridge adsorption at intermediate coverages.