The
crucial role played by the organic molecular cation in hybrid
organic–inorganic perovskite has been a challenging subject
of discussion. Effects of the organic cation on a cubic perovskite
structure of formamidinium lead iodide (FAPI) were investigated deploying
the state-of-the-art density functional theory including the spin–orbit
coupling (SOC). Equipped with Euler’s rotations, energy landscapes
corresponding to the different orientations of formamidinium (FA)
cation were calculated. From the energy landscapes, the flipping energy
barriers are interpreted to be thermal agitations required to flip
over for FA. The highest energy barrier is 24.7 meV, which is equivalent
to T ∼ 286 K, the temperature over
which the FA molecules are randomly oriented. In addition, we found
a relatively lowest energy structure when the FA is rotated by (ϕ,
θ, ψ) = (90°, 60°, 45°). From the structural
optimization, the I–Pb–I becomes angled with less than
180°. The H–I pair distribution function of this configuration
shows that the H–I distances are optimum and confined only
in the 2.70–4.25 Å shells. The resulting configuration
also breaks the inversion symmetry leading to the Rashba–Dresselhaus
effect in the electronic band structure. The largest Rashba splitting
parameter calculated along the R → M direction in k-space is ∼3.0 for the (ϕ = 90°, θ = 60°,
ψ = 45°) configuration.
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