Layer intermixing during metal/metal oxide adsorption: Ti/sapphire(0001)

First principles density functional calculations for adsorption of Ti on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ indicate that $\mathrm{Ti}:{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ interfaces become intermixed. Substitutional Ti replaces a surface Al atom rather than a subsurface Al, and the Al-terminated surface is unstable under Ti adsorption. Adsorbed Ti displaces the surface Al, resulting in a mixed Ti/Al interfacial layer instead of a sharp $\mathrm{Ti}:{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interface. Our results provide a coherent picture of the structural and electronic properties of this interface and are consistent with available experimental data.