65,039 research outputs found
First principles study of intrinsic point defects in hexagonal barium titanate
Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both mono- and di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of vacancies in the system are presented for numerous temperatures. Comparisons are drawn with the cubic polymorph as well as with previous potential-based simulations and experimental results
Variation of magnetic properties of SrFeMoO due to oxygen vacancies
Oxygen vacancies can be of utmost importance for improving or deteriorating
physical properties of oxide materials. Here, we studied from first-principles
the electronic and magnetic properties of oxygen vacancies in the double
perovskite SrFeMoO (SFMO). We show that oxygen vacancies can increase
the Curie temperature in SFMO, although the total magnetic moment is reduced at
the same time. We found also that the experimentally observed valence change of
the Fe ions from to in the x-ray magnetic circular dichroism (XMCD)
measurements is better explained by oxygen vacancies than by the assumed mixed
valence state. The agreement of the calculated x-ray absorption spectra and
XMCD results with experimental data is considerably improved by inclusion of
oxygen vacancies.Comment: submitted to PRB but rejected, major revision, submitting to JPC
Influence of strain and oxygen vacancies on the magnetoelectric properties of multiferroic bismuth ferrite
The dependencies on strain and oxygen vacancies of the ferroelectric
polarization and the weak ferromagnetic magnetization in the multiferroic
material bismuth ferrite, BiFeO_3, are investigated using first principles
density functional theory calculations. The electric polarization is found to
be rather independent of strain, in striking contrast to most conventional
perovskite ferroelectrics. It is also not significantly affected by oxygen
vacancies, or by the combined presence of strain and oxygen vacancies. The
magnetization is also unaffected by strain, however the incorporation of oxygen
vacancies can alter the magnetization slightly, and also leads to the formation
of Fe^{2+}. These results are discussed in light of recent experiments on
epitaxial films of BiFeO_3 which reported a strong thickness dependence of both
magnetization and polarization.Comment: 9 pages, 3 figure
Controlled manipulation of oxygen vacancies using nanoscale flexoelectricity
Oxygen vacancies, especially their distribution, are directly coupled to the
electromagnetic properties of oxides and related emergent functionalities that
have implication in device applications. Here using a homoepitaxial strontium
titanate thin film, we demonstrate a controlled manipulation of the oxygen
vacancy distribution using the mechanical force from a scanning probe
microscope tip. By combining Kelvin probe force microscopy imaging and
phase-field simulations, we show that oxygen vacancies can move under a
stress-gradient-induced depolarisation field. When tailored, this nanoscale
flexoelectric effect enables a controlled spatial modulation. In motion, the
scanning probe tip thereby deterministically reconfigures the spatial
distribution of vacancies. The ability to locally manipulate oxygen vacancies
on-demand provides a tool for the exploration of mesoscale quantum phenomena,
and engineering multifunctional oxide devices.Comment: 35 pages, Main text and the supplementary information combine
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