Using electron and synchrotron x-ray diffraction, the authors investigate the reconstructions induced on the Si (001) surface by the presence of a submonolayer of Sr atoms. These Sr surface phases on Si (001) are the first steps in the synthesis of crystalline oxide on silicon heterostructures. Through the use of in situ reflection high energy electron diffraction, the authors observe 2 x 1 --> 2 x 3 --> 1 x 2 transitions in the surface symmetry as 1/2 monolayer Sr is deposited at 650 degrees C. Anomalous synchrotron x-ray diffraction studies of the 2 x 3 structure are consistent with a model where each Sr atom replaces two silicon dimers, resulting in a change in the surface Si stoichiometry. X-ray diffraction from the surface formed when the Sr deposition occurs at room temperature shows a different dependence on x-ray energy and is consistent with a surface phase consisting of Sr adsorbed onto a stoichiometric Si surface. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3139866]