Strain-induced ferroelectricity and lattice coupling in BaSnO₃ and SrSnO₃

Perovskite stannates such as BaSnO₃ and SrSnO₃ exhibit promising photovoltaic properties, and hold promise for application in solar cell devices. However, the lack of ferroelectricity and the wide band gap in these materials limit their potential for photovoltaic applications. Here, by first-principles calculations, we demonstrate the realization of a primary ferroelectric polarization in non-ferroelectric BaSnO₃ and SrSnO₃ through strain engineering. In addition to the appearance of polarization, the band gaps of the materials are greatly narrowed when the paraelectric to ferroelectric phase transition takes place under compressive strain. Furthermore, an intriguing Q₂ mode triggered by lattice coupling with the polar mode is found in the stannates subjected to a sufficient tensile strain and this mode has a significant effect on the band gap, which suggests another pathway to narrow the band gap through the electric field control of the Q₂ mode. The fruitful electronic, structural, and energetic properties are discussed in detail to achieve a fundamental understanding of the strain-induced ferroelectricity, tunable band gap, and lattice couplings between the Q₂ mode and different polar/rotational distortions in the perovskite stannates.