Inducing Spontaneous Electric Polarizations in Double Perovskite Iodide Superlattices for New Ferroelectric Photovoltaic Materials

In this work, we use density functional theory calculations to demonstrate how spontaneous electric polarizations can be induced \textit{via} a hybrid improper ferroelectric mechanism in iodide perovskites, a family well-known to display solar-optimal band gaps, to create new materials for photoferroic applications. We first assemble three chemically distinct ($A$$A^{\prime}$)($B$$B^{\prime}$)I$_6$ double perovskites using centrosymmetric $AB$I$_3$ perovskite iodides (where $A$ = Cs, Rb, K and $B$ = Sn, Ge) as building units. In each superlattice, we investigate the effects of three types of $A$- and $B$-site cation ordering schemes and three different $B$I$_6$ octahedral rotation patterns. Out of these 27 combinations, we find that 15 produce polar space groups and display spontaneous electric polarizations ranging from 0.26 to 23.33 $\mu$C/cm$^2$. Furthermore, we find that a layered $A$-site/rock salt $B$-site ordering, in the presence of an $a^0a^0c^+$ rotation pattern, produces a chiral "vortex-like" $A$-site displacement pattern. We then investigate the effect of epitaxial strain on one of these systems, (CsRb)(SnGe)I$_6$, in layered and rock salt ordered configurations. In both phases, we find strong competition between the cation ordering schemes as well as an enhancement of the spontaneous polarization magnitude under tensile strain. Finally, using advanced functionals, we demonstrate that these compounds display low band gaps ranging from 0.2 to 1.3 eV. These results demonstrate that cation ordering and epitaxial strain are powerful ways to induce and control new functionalities in technologically-useful families of materials.