Supertetragonal BaZrS₃: A promising perovskite sulphide with giant ferroelectricity and low band gap

Over the past century, ferroelectricity has offered exciting opportunities for fundamental research and device applications. However, most of the discovered excellent ferroelectrics are oxide materials with large band gaps, limiting their potential for optoelectronics applications. Here using first-principles calculations we identify a new narrow-gap ferroelectric beyond oxides, i.e., ferroelectric perovskite sulphide BaZrS₃. Under large compressive strains, BaZrS₃ can be stabilized into a unique super-tetragonal phase with an extraordinary polarization of 67.16 µC/cm², which is even stronger than that of conventional oxide ferroelectrics. Excitingly, the supertetragonal BaZrS₃ exhibits a direct narrow band gap of 1.2 eV and excellent electronic properties. Based on the chemical bonding analysis, we attribute the formation of supertetragonal phase to charge re-ordering in which the π bond overlap along the long 〈Zr-S〉 bond completely vanishes and the antibonding states of the σ bond appear below the Fermi level. Our work provides a conceptual strategy for designing new ferroelectrics for electronic and photovoltaic applications.