  The structure of the ultimately-thin crystalline allotrope of silicon oxide,
prepared onto a ruthenium surface, is unveiled down to atomic scale with
chemical sensitivity, thanks to high resolution scanning tunneling microscopy
and first principle calculations. An ordered oxygen lattice is imaged which
coexists with the two-dimensional monolayer oxide. This coexistence signals a
displacive transformation from an oxygen reconstructed-Ru(0001) to silicon
oxide, along which latterally-shifted domains form, each with equivalent and
degenerate epitaxial relationships with the substrate. The unavoidable
character of defects at boundaries between these domains appeals for the
development of alternative methods capable of producing single-crystalline
two-dimensional oxides.
