Abstract
Optimal materials to induce bulk photovoltaic effects should lack inversion symmetry and have an optical gap matching the energies of visible radiation. Ferroelectric perovskite oxides such as BaTiO3 and PbTiO3 exhibit substantial polarization and stability, but have the disadvantage of excessively large band gaps. We use both density functional theory and dynamical mean field theory calculations to design a new class of Mott multiferroics–double perovskite oxides A2VFeO6 (A = Ba, Pb, etc). While neither perovskite AVO3 nor AFeO3 is ferroelectric, in the double perovskite A2VFeO6 a ‘complete’ charge transfer from V to Fe leads to a non-bulk-like charge configuration–an empty V-d shell and a half-filled Fe-d shell, giving rise to a polarization comparable to that of ferroelectric ATiO3. Different from nonmagnetic ATiO3, the new double perovskite oxides have an antiferromagnetic ground state and around room temperatures, are paramagnetic Mott insulators. Most importantly, the V d0 state significantly reduces the band gap of A2VFeO6, making it smaller than that of ATiO3 and BiFeO3 and rendering the new multiferroics a promising candidate to induce bulk photovoltaic effects.
Highlights
Optimal materials to induce bulk photovoltaic effects should lack inversion symmetry and have an optical gap matching the energies of visible radiation
We note that based on the symmetry analysis[57] and all the available experimental data for double perovsite oxides compiled in the review[49], there are altogether seven tilting patterns which are allowed in a double perovskite structure A2BB′O6 and have been observed in experiment
We find that the paramagnetic state is insulating, with a gap only slightly smaller than that of the ground state with [001] antiferromagnetic ordering, indicating that double perovskite Ba2VFeO6 is a promising candidate for Mott multiferroics[62]
Summary
Our first-principles calculations are performed using density functional theory (DFT)[33] and dynamical mean field theory (DMFT)[34]. We first discuss the fully relaxed atomic structure of double perovskite Ba2VFeO6, obtained using DFT calculations with three different exchange correlation functionals (PBE+U+J, LDA+U+J and sPBEsol). The difference between the direct and optical gaps is a matrix element effect: the lowest back-folded conduction band state does not have a dipole allowed transition matrix element with the highest-lying valence band state (see the Supplementary Materials for more details) It is well-known that DFT with semi-local exchange correlation functionals substantially underestimate band gaps. While tetragonal double perovskite Pb2VFeO6 has similar structural properties to tetragonal PbTiO3, the fundamental gap Δ0 and optical gap Δoptical are both smaller than those of PbTiO3 by about 1 eV (all three exchange correlation functionals make qualitatively consistent predictions).
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