The
rotational dynamics of an organic cation in hybrid halide perovskites
is intricately linked to the phase transitions that are known to occur
in these materials; however, the exact relation is not clear. We have
performed detailed model studies on methylammonium lead iodide
and formamidinium lead iodide to unravel the relation between rotational
dynamics and phase behavior. We show that the occurrence of the phase
transitions is due to a subtle interplay between dipole–dipole
interactions between the organic cations, specific (hydrogen bonding)
interactions between the organic cation and the lead iodide lattice,
and deformation of the lead iodide lattice in reaction to the reduced
rotational motion of the organic cations. This combination of factors
results in phase transitions at specific temperatures, leading to
the formation of large organized domains of dipoles. The latter can
have significant effects on the electronic structure of these materials.
We investigate the dielectric constant and the dielectric decrement of aqueous NaCl
solutions by means of molecular dynamic simulations. We thereby compare the performance
of four different force fields and focus on disentangling the origin of the dielectric
decrement and the influence of scaled ionic charges, as often used in nonpolarizable
force fields to account for the missing dynamic polarizability in the shielding of
electrostatic ion interactions. Three of the force fields showed excessive contact ion
pair formation, which correlates with a reduced dielectric decrement. In spite of the
fact that the scaling of charges only weakly influenced the average polarization of
water molecules around an ion, the rescaling of ionic charges did influence the
dielectric decrement, and a close-to-linear relation of the slope of the dielectric
constant as a function of concentration with the ionic charge was found.
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