  TiO$_2$ and WO$_3$ are two of the most important earth-abundant electronic
materials with applications in countless industries. Recently alloys of WO$_3$
and TiO$_2$ have been investigated leading to improvements of key performance
indicators for a variety of applications ranging from photo-electrochemical
water splitting to electrochromic smart windows. These positive reports and the
complexity of the ternary W-Ti-O phase diagram motivate a comprehensive
experimental screening of this phase space. Using combinatorial thermal
oxidation of solid solution W$_{1-x}$Ti$_{x}$ precursors combined with bulk and
surface analysis mapping we investigate the oxide phase formation and surface
passivation of tungsten titanium oxide in the entire compositional range from
pure WO$_3$ to TiO$_2$. The system shows a remarkable structural transition
from monoclinic over cubic to tetragonal symmetry with increasing Ti
concentration. In addition, a strong Ti surface enrichment is observed for
precursor Ti-concentrations in excess of 55 at.%, resulting in the formation of
a protective rutile-structured TiO$_2$ surface layer. Despite the structural
transitions, the optical properties of the oxide alloys remain largely
unaltered demonstrating an independent control of multiple functional
properties in W$_{1-x}$Ti$_{x}$O$_{n}$. The results from this study provide
valuable guidelines for future development of W$_{1-x}$Ti$_{x}$O$_{n}$ for
electronic and energy applications, but also novel engineering approaches for
surface functionalization and additive manufacturing of Ti-based alloys.
