The alkaline-earth metal adsorption on Si(001) has attracted much interest for finding a proper template in the growth of high-kappa and crystalline films. Up to now on the flat Si(001) surface with double domains and single-layer steps, the adsorbed Ba atoms are known to induce the 2 x 3 structure through removing two Si dimers and adding a Ba atom per unit cell in each domain. In the present investigation, the Si (001)-4 degrees off surface with D-B steps and single domains has been employed as a substrate and the reconstruction at the initial stage of Ba adsorption has been investigated by scanning tunneling microscopy and synchrotron photoemission spectroscopy. On this vicinal and single domain terrace, a novel 3 x 2 structure rotated by 90 degrees from the 2 x 3 structure has been found. Such a 3 x 2 structure turns out to be formed by adding a Ba atom and a Si dimer per unit cell. This results from the fact that the adsorbed Ba2+ ions with a larger ionic radius relieve tensile stress on the original Si dimers exerted by the rebonded atoms at the D-B step. (C) 2018 Elsevier B.V. All rights reserved.