  The Niobium surface is almost always covered by a native oxide layer which
greatly influences the performance of superconducting devices. Here we
investigate the highly stable Niobium oxide overlayer of Nb(110), which is
characterised by its distinctive nanocrystal structure as observed by scanning
tunnelling microscopy (STM). Our ab-initio density functional theory (DFT)
calculations show that a subtle reconstruction in the surface Niobium atoms
gives rise to rows of 4-fold coordinated oxygen separated by regions of 3-fold
coordinated oxygen. The 4-fold oxygen rows are determined to be the source of
the nanocrystal pattern observed in STM, and the two chemical states of oxygen
observed in core-level X-ray photoelectron spectroscopy (XPS) are ascribed to
the 3-fold and 4-fold oxygens. Furthermore, we find excellent agreement between
the DFT calculated electronic structure with scanning tunnelling spectroscopy
and valence XPS measurements.
