Ferroelectricity in Ruddlesden-Popper Chalcogenide Perovskites for Photovoltaic Application: The Role of Tolerance Factor.

Chalcogenide perovskites with optimal band gap and desirable light absorption are promising for photovoltaic devices, whereas the absence of ferroelectricity limits their potential in applications. On the basis of first-principles calculations, we reveal the underlying mechanism of the paraelectric nature of Ba3 Zr2 S7 observed in experiments and demonstrate a general rule for the appearance of ferroelectricity in chalcogenide perovskites with Ruddlesden-Popper (RP) A3 B2 X7 structures. Group theoretical analysis shows that the tolerance factor is the primary factor that dominates the ferroelectricity. Both Ba3 Zr2 S7 and Ba3 Hf2 S7 with large tolerance factor are paraelectric because of the suppression of in-phase rotation that is indispensable to hybrid improper ferroelectricity. In contrast, Ca3 Zr2 S7 , Ca3 Hf2 S7 , Ca3 Zr2 Se7 , and Ca3 Hf2 S7 with small tolerance factor exhibit in-phase rotation and can be stable in the ferroelectric Cmc21 ground state with nontrivial polarization. These findings not only provide useful guidance to engineering ferroelectricity in RP chalcogenide perovskites but also suggest potential ferroelectric semiconductors for photovoltaic applications.