First-Principles Investigation into Hybrid Improper Ferroelectricity in Ruddlesden–Popper Perovskite Chalcogenides Sr3B2X7 (B = Ti, Zr, Hf; X = S, Se)

Recently, hybrid improper ferroelectricity in layered perovskites is becoming an effective way to expand the compositional palette to find new ferroelectrics. In the present work, the ground state, electronic structure, optical properties, effective carrier masses, ferroelectric polarization, and polarization switching pathways of the Ruddlesden–Popper perovskite chalcogenides Sr3B2X7 (B = Ti, Zr, Hf; X = S, Se) were thoroughly investigated by first-principles calculations. Some new hybrid improper ferroelectrics with the A21am polar ground state, i.e., Sr3Zr2S7, Sr3Zr2Se7, Sr3Hf2S7, and Sr3Hf2Se7, were predicted by confirming the electric polarization induced from a trilinear coupling of octahedral rotation and tilting modes. The results showed that the ferroelectric photovoltaic-related properties such as band gaps and electric polarizations depend crucially on the amplitude of octahedral rotations and tilts of perovskite blocks. It was predicted that Sr3Zr2Se7 could be a good candidate for ferroelectric photovoltaic device applications by a systematical evaluation of ferroelectric photovoltaic properties of all hypothetical compounds.