Design of new Mott multiferroics via complete charge transfer: promising candidates for bulk photovoltaics

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 BaTiO$_3$ and PbTiO$_3$ 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 $A_2$VFeO$_6$ ($A$=Ba, Pb, etc). While neither perovskite $A$VO$_3$ nor $A$FeO$_3$ is ferroelectric, in the double perovskite $A_2$VFeO$_6$ 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 $A$TiO$_3$. Different from nonmagnetic $A$TiO$_3$, the new double perovskite oxides have an antiferromagnetic ground state and around room temperatures, are paramagnetic Mott insulators. Most importantly, the V $d^0$ state significantly reduces the band gap of $A_2$VFeO$_6$, making it smaller than that of $A$TiO$_3$ and BiFeO$_3$ and rendering the new multiferroics a promising candidate to induce bulk photovoltaic effects.