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D. Halwidl, W. Mayr-Schmölzer, D. Fobes, J. Peng, Z. Mao, M. Schmid, F. Mittendorfer, J. Redinger, U. Diebold:
"Ordered hydroxyls on Ca₃Ru₂O₇(001)";
Nature Communications, 8 (2017), 1 - 8.

As complex ternary perovskite-type oxides are increasingly used in solid oxide fuel cells,
electrolysis and catalysis, it is desirable to obtain a better understanding of their surface
chemical properties. Here we report a pronounced ordering of hydroxyls on the cleaved (001)
surface of the Ruddlesden-Popper perovskite Ca3Ru2O7 upon water adsorption at 105 K and
subsequent annealing to room temperature. Density functional theory calculations predict the
dissociative adsorption of a single water molecule (Eads = 1.64 eV), forming an (OH)ads group
adsorbed in a Ca-Ca bridge site, with an H transferred to a neighboring surface oxygen atom,
Osurf. Scanning tunneling microscopy images show a pronounced ordering of the hydroxyls
with (2 × 1), c(2 × 6), (1 × 3), and (1 × 1) periodicity. The present work demonstrates the
importance of octahedral rotation and tilt in perovskites, for influencing surface reactivity,
which here induces the ordering of the observed OH overlayers.