The structure of the oxygen-induced Rh{110}(2x2)p2mg-O phase has been
determined using low-energy electron diffraction intensity-voltage (LE
ED I-V) analysis, and consists of a missing row reconstructed substrat
e with oxygen atoms in a zigzag arrangement. The oxygen atoms occupy t
he upper threefold sites on the {111} microfacets, and the nearest-nei
ghbour bonding geometry is the same as for the (2 x 1) oxygen-induced
low-temperature phase. In spite of the similar geometries, the reactiv
ities of these structures for CO oxidation are very different. When th
e oxygen is removed from the (2 x 2)p2mg surface at low temperature, t
he substrate retains the (1 x 2) missing row reconstruction. LEED anal
ysis of the (1 x 2) surface indicates that the first interlayer contra
ction is 11%, which is lower than the value determined previously by H
e scattering.