Strain-induced improper ferroelectricity in Ruddlesden-Popper perovskite halides

Activating multiple symmetry modes and promoting a strong coupling between different modes by strain are indispensable to stabilize a polar ferroelectric (FE) phase from a nonpolar perovskite. Herein, through first-principles calculations, we propose an undiscovered and general avenue to engineering ferroelectricity in photovoltaic perovskites with a Ruddlesden-Popper (RP) structure. It is demonstrated that an experimentally accessible compressive strain can induce an in-plane polarization in RP perovskite halides thin films, resulting in an unusual paraelectric to FE phase transition. The detailed analysis on structure and energy reveals that the unusual FE phase transition in the perovskite halides stems from the strong coupling between strain and antiferrodistortive (AFD) mode. Further calculations show that the strain-AFD coupling-induced ferroelectricity is not only exhibited by perovskite halides but also observed in perovskite sulfides such as ${\mathrm{Ba}}_3{\mathrm{Zr}}_2{\mathrm{S}}_7$. Moreover, it is found that the strained FE thin film possesses a suitable band gap of 1.6 eV for photovoltaic application. These findings not only unfold a general way to engineering nonpolar-to-polar transition, but also open an avenue to design optimal FE semiconductors for solar cell applications.

Figure 1

Phase diagrams of (a) ${\mathrm{Cs}}_3{\mathrm{Sn}}_2{\mathrm{I}}_7$ and (b) ${\mathrm{Cs}}_3{\mathrm{Sn}}_2{\mathrm{Br}}_7$ as a function of compressive strain; (c) $P{4}_2/mnm$ and (d) $Cmc{2}_1$ structures of ${\mathrm{Cs}}_3{\mathrm{Sn}}_2{\mathrm{I}}_7$ without strain and with 2.4% compressive strain, respectively. The arrows denote the atomic displacements of $A$-site atoms.

Figure 2

The dependence of the rotation and tilting modes for (a) ${\mathrm{Cs}}_3{\mathrm{Sn}}_2{\mathrm{I}}_7$ and (b) ${\mathrm{Cs}}_3{\mathrm{Sn}}_2{\mathrm{Br}}_7$ as a function of strain. Ferroelectric polarization of (c) ${\mathrm{Cs}}_3{\mathrm{Sn}}_2{\mathrm{I}}_7$ and (d) ${\mathrm{Cs}}_3{\mathrm{Sn}}_2{\mathrm{Br}}_7$ from Berry-phase method.

Figure 3

The energy curves of four symmetry modes ($X_1^{-}, X_2^{+}, X_3^{-},$ and ${\mathrm{\Gamma }}_5^{-}$) for ${\mathrm{Cs}}_3{\mathrm{Sn}}_2{\mathrm{I}}_7$ without strain. The energy of high-symmetry $I4/mmm$ phase is treated as the reference.

Figure 4

Evolution of energy difference with respect to the energy with zero-mode amplitude for (a) in-phase rotation mode, (b) tilting mode, (c) purely polar mode, and (d) polar mode with initial rotation and tilting modes that have the same amplitude as the ground state of ${\mathrm{Cs}}_3{\mathrm{Sn}}_2{\mathrm{I}}_7$ under 2.4% compressive strain. The high-symmetry $I4/mmm$ phase is treated as the reference structure.

Figure 5

Mechanism of the PE-FE transition and the band gap in the PE and FE region for ${\mathrm{Cs}}_3{\mathrm{Sn}}_2{\mathrm{I}}_7$. $Q_1$ denotes the tilting mode and $Q_2$ represents the in-phase rotation mode.