  Hybrid perovskites with mixed organic cations such as methylammonium and
formamidinium have attracted interest due to their improved stability and
capability to tune their properties varying the composition. In this work we
report on the local variation of the structural and electronic properties in
mixed A-site cation MA/FA lead iodide perovskites FAXMA1-XPBI3 evaluated from
static first-principles calculations in certain structures where the
orientations of organic cations result from examining the energy landscape of
some compositions. The cation replacement at the A-site to form the solid
solution causes an increase tilting of the inorganic PbI6 octahedra: in the
FA-rich compounds the replacement of FA by a smaller cation like MA is to
compensate the reduced space filling offered by the smaller cation, whereas in
the MA-rich compounds is to expand the space needed for the larger cation. In
fact, the effect of octahedron tiltings exceeds that of unit-cell size in
determining the band gap for these organic cation mixtures. Our calculations
indicate that the key role played by hydrogen bonds with iodine anions in pure
compounds is preserved in the cation mixed perovskites. It is found that MA-I
bonds remain stronger than FA-I bonds throughout the composition range
regardless of the unit-cell expansion as the FA content increases. Our
calculations reveal how the hydrogen bonds stabilize the no-bonding I-5p
orbitals, spatially perpendicular to the Pb-I-Pb bond axis, lowering their
energy when the H-I interaction occurs, which would explain the well-known role
of hydrogen bonding in the structural stabilization of hybrid perovskites.
These results contribute to the understanding on the role played by cation
mixing at A sites in the physics of lead halide perovskites.
