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Hybrid perovskites with mixed organic cations such as methylammonium andformamidinium have attracted interest due to their improved stability andcapability to tune their properties varying the composition. In this work wereport on the local variation of the structural and electronic properties inmixed A-site cation MA/FA lead iodide perovskites FAXMA1-XPBI3 evaluated fromstatic first-principles calculations in certain structures where theorientations of organic cations result from examining the energy landscape ofsome compositions. The cation replacement at the A-site to form the solidsolution causes an increase tilting of the inorganic PbI6 octahedra: in theFA-rich compounds the replacement of FA by a smaller cation like MA is tocompensate the reduced space filling offered by the smaller cation, whereas inthe MA-rich compounds is to expand the space needed for the larger cation. Infact, the effect of octahedron tiltings exceeds that of unit-cell size indetermining the band gap for these organic cation mixtures. Our calculationsindicate that the key role played by hydrogen bonds with iodine anions in purecompounds is preserved in the cation mixed perovskites. It is found that MA-Ibonds remain stronger than FA-I bonds throughout the composition rangeregardless of the unit-cell expansion as the FA content increases. Ourcalculations reveal how the hydrogen bonds stabilize the no-bonding I-5porbitals, spatially perpendicular to the Pb-I-Pb bond axis, lowering theirenergy when the H-I interaction occurs, which would explain the well-known roleof hydrogen bonding in the structural stabilization of hybrid perovskites.These results contribute to the understanding on the role played by cationmixing at A sites in the physics of lead halide perovskites.

Mixed Formamidinium-Methylammonium Lead Iodide perovskite from first-principles: Hydrogen-bonding impact on the electronic properties