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 FABX3 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.