A structural study by quantitative low-energy electron diffraction ͑LEED͒ and density functional theory ͑DFT͒ has been performed on the Ir͑100͒-͑5 ϫ 1͒-hex surface with 0.6 ML of hydrogen adsorbed at low temperature ͑Ͻ180 K͒. The theory-experiment fit of LEED intensities based on calculations applying the two mirror symmetry planes of the uncovered surface is not satisfactory. Instead, the real structure has a unit cell with only one mirror plane yielding an excellent R factor ͑R P = 0.16͒. The

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... DFT investigation of the energetics also confirms this spontaneous symmetry breaking, yields quantitatively the same structure for the substrate as the LEED analysis and, moreover, retrieves the hydrogen positions. It was found that adsorption of hydrogen leads to an asymmetric increase of the buckling of the top hexagonal layer, with one of the previously protruding surface Ir atoms extruded even more by about 0.1 Å. This may be regarded as a precursor of the temperature-activated phase transition above 180 K to a structure in which the most protruding atom of the ͑5 ϫ 1͒ surface unit cell is ejected from the quasihexagonal top layer. These ejected atoms form single-atomwide iridium wires on the remaining atoms, which rearrange to form a bulklike fcc͑100͒ layer, leading to a complete lifting of the quasihexagonal reconstruction.