Electronic structure of ${\mathrm{Ce}}_{1-x}{\mathrm{Sr}}_x{\mathrm{TiO}}_3:$ Comparison between substitutional and vacancy doping

The change of the electronic structure across the metal-insulator transition in a Mott-Hubbard system ${\mathrm{Ce}}_{1-x}{\mathrm{Sr}}_x{\mathrm{TiO}}_3$ has been investigated by x-ray absorption and photoemission spectroscopy. The results are compared with ${\mathrm{CeTiO}}_{3+\mathrm{\delta }},$ where hole doping is achieved by excess oxygen instead of Sr substitution. It is found that additional doped-hole states are created in the insulator gap for both cases, while the density of states at the Fermi level in ${\mathrm{CeTiO}}_{3+\mathrm{\delta }},$ is nearly half of that in ${\mathrm{Ce}}_{1-x}{\mathrm{Sr}}_x{\mathrm{TiO}}_3$ at the same nominal doping. This suggests a strong reduction in the mobility of carriers due to cation vacancies produced by excess oxygen.