Mechanical Properties of Formamidinium Halide Perovskites FABX_3 (FA=CH(NH_2)_2; B=Pb, Sn; X=Br, I) by First-Principles Calculations

The mechanical properties of formamidinium halide perovskites FABX_3 (FA=CH(NH_2)_2; B=Pb, Sn; X=Br, I) are systematically investigated using first-principles calculations. Our results reveal that FABX_3 perovskites possess excellent mechanical flexibility, ductility and strong anisotropy. We shows that the planar organic cation FA^+ has an important effect on the mechanical properties of FABX_3 perovskites. In addition, our results indicate that (i) the moduli (bulk modulus B, Young's modulus E, and shear modulus G) of FABBr_3 are larger than those of FABI_3 for the same B atom, and (ii) the moduli of FAPbX_3 are larger than those of FASnX_3 for the same halide atom. The reason for the two trends is demonstrated by carefully analyzing the bond strength between B and X atoms based on the projected crystal orbital Hamilton population method.