Abstract
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.
Highlights
In the doped BCFO case, anionic electron number is reduced by the formation of positively charged oxygen-vacancy[15]
The spectra were acquired by recording the total electron yield (TEY) – details of the experimental geometry are shown in Fig. 2b
This reconstruction behavior is clearly observed when the structural effect is suppressed through Brewster geometry in RSXS measurement, leading to the additional anisotropic effect in the doped BCFO
Summary
In the doped BCFO case, anionic electron number is reduced by the formation of positively charged oxygen-vacancy[15]. According to the reported photovoltaic properties of BCFO as a function of the vacancy concentration[22], it does not show a monotonic increase even in a monotonic enhancement of the oxygen vacancy in BCFO. Such effect decreases beyond x ~ 15%22. This means that the reported diode effect[10,11] of BiFeO3 cannot be employed for explaining a change in BCFO via the oxygen vacancy. This implies that near cations (i.e., Fe in this case) chemically responds to the oxygen vacancies, leading to our attention for a role of oxygen vacancies via spectroscopic scheme such as electronic configuration
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