  X-ray photoelectron spectroscopy (XPS) and resonant x-ray emission
spectroscopy (RXES) measurements of pellet and thin film forms of TiO$_2$ with
implanted Fe ions are presented and discussed. The findings indicate that
Fe-implantation in a TiO$_2$ pellet sample induces heterovalent cation
substitution (Fe$^{2+}\rightarrow$ Ti$^{4+}$) beneath the surface region. But
in thin film samples, the clustering of Fe atoms is primarily detected. In
addition to this, significant amounts of secondary phases of Fe$^{3+}$ are
detected on the surface of all doped samples due to oxygen exposure. These
experimental findings are compared with density functional theory (DFT)
calculations of formation energies for different configurations of structural
defects in the implanted TiO$_2$:Fe system. According to our calculations, the
clustering of Fe-atoms in TiO$_2$:Fe thin films can be attributed to the
formation of combined substitutional and interstitial defects. Further, the
differences due to Fe doping in pellet and thin film samples can ultimately be
attributed to different surface to volume ratios.
