Polarization-Magnetization Coupling in Visible Light Ferroelectric Double Perovskites

The bulk photovoltaic effect (BPVE), arising from broken inversion symmetry in ferroelectrics, offers a distinct pathway toward high-efficiency next-generation photovoltaics. We propose and investigate A/A$^\prime$-ordered double perovskites KLaFeMoO$_6$ and NaLaFeMoO$_6$ as promising single-phase ferroelectric photovoltaic (FE-PV) materials. First-principles calculations reveal robust P2$_1$ symmetry with A-site layer and B-site rock-salt ordering, accompanied by hybrid improper ferroelectricity driven by $a^{-}a^{-}c^{+}$ octahedral tilts. Both compounds exhibit significant spontaneous polarization and indirect band gaps of $\sim$ 1.8 eV, well suited for visible-light absorption ($>$10$^5$ cm$^{-1}$). Low carrier effective masses along the polar axis indicate efficient charge transport. \textit{Ab initio} molecular dynamics simulations (AIMD) show that polarization-coupled magnetization switching is feasible above room temperature, making these materials suitable for room-temperature applications.