Mechanisms of piezoelectricity of hybrid perovskites α-FASn <i>X</i> 3 ( <i>X</i>  = Br, I): First-principles analysis

Organic–inorganic hybrid halide perovskites lack inversion symmetry because of the polar molecular cation, which gives rise to polarization even in the cubic lattice. Due to the low bulk modulus, these materials can easily undergo deformation or strain, which typically couple with polarization resulting in piezoelectricity. Understanding the mechanisms and tunability of their piezoelectric response is essential for their applications. In this work, we analyze the role of halogen species plays in the piezoelectric response of cubic (α) formamidinium tin halide [FASnX3 (X=I/Br)] using an effective low-energy model within first-principles density functional theory. We find that the piezoelectric constant of FASnX3 is quite sensitive to the halogen species: FASnI3 has 2.5 times higher piezoelectric strain coupling (d) than FASnBr3. This originates from the contrast in how strongly the polar optical phonons couple with electric field and strain, which are readily available from first-principles calculation. This work highlights that substitution at halogen sites can engineer these phonon couplings to achieve the desired piezoelectric response.