  We demonstrate the microscopic role of oxygen vacancies spatially confined
within nanometer inter-spacing (about 1 nm) in BiFeO3, using resonant soft
X-ray scattering techniques and soft X-ray spectroscopy measurements. Such
vacancy confinements and total number of vacancy are controlled by substitution
of Ca2+ for Bi3+ cation. We found that by increasing the substitution, the
in-plane orbital bands of Fe3+ cations are reconstructed without any redox
reaction. It leads to a reduction of the hopping between Fe atoms, forming a
localized valence band, in particular Fe 3d-electronic structure, around the
Fermi level. This band localization causes to decrease the conductivity of the
doped BiFeO3 system.
