The Relation between Rotational Dynamics of the Organic Cation and Phase Transitions in Hybrid Halide Perovskites

Abstract

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.

Role of the Iodide–Methylammonium Interaction in the Ferroelectricity of CH3NH3PbI3

“…Therefore, more elaborate analyses of the orientation of the molecular cation are better performed using other techniques. [7,20,33,34] As a consequence of Fourier truncation and incomplete absorption correction, one normally observes residual electron density peaks, that is, deviations from the model observed electron density, close to the heaviest atoms (here Pb), as their intensity is roughly linked to the electron count. [35,36] Having them at the iodine position instead is probably due to the missing assignment of some electron density.…”

“…We note that the C and N attributions are arbitrary, as they can hardly be distinguished by X‐ray diffraction because of their similar electron count and the time and space averaging of the diffraction method. Therefore, more elaborate analyses of the orientation of the molecular cation are better performed using other techniques . As a consequence of Fourier truncation and incomplete absorption correction, one normally observes residual electron density peaks, that is, deviations from the model observed electron density, close to the heaviest atoms (here Pb), as their intensity is roughly linked to the electron count .…”

Structural Dynamics of Two-Dimensional Ruddlesden–Popper Perovskites: A Computational Study

“…This leads to small deviations as can be seen if the results for 3D perovskite are compared to our previous work which did not use full formal charges. 40 This leads to a decrease of the temperature at which the phase transition is observed ( Figure S1 ). We have explicitly chosen to adapt the way charges are assigned to the individual ions because this leads to a straightforward transferrable force field that can be adapted easily to structures for different n and other organic cations.…”

Zur Rolle der Iodid‐Methylammonium‐Interaktion in der Ferroelektrizität in CH3NH3PbI3

“…Daher müssen zur genaueren Aufklärung dieser Frage andere Techniken hinzugezogen werden. [7,20,33,34] Durch den Abbruch der Fourier-Entwicklung und die unvollständige Absorptionskorrektur würde man normalerweise die stärksten Restelektronendichtepeaks in der Nähe der schwersten Atome (hier Pb) erwarten, da ihre Stärke grob mit der Elektronenzahl korreliert. [35,36] Die Beobachtung an der…”

“…Des Weiteren wird bei der Messung über Zeit und Raum gemittelt, wodurch dynamische Effekte das Problem zusätzlich erschweren. Daher müssen zur genaueren Aufklärung dieser Frage andere Techniken hinzugezogen werden . Durch den Abbruch der Fourier‐Entwicklung und die unvollständige Absorptionskorrektur würde man normalerweise die stärksten Restelektronendichtepeaks in der Nähe der schwersten Atome (hier Pb) erwarten, da ihre Stärke grob mit der Elektronenzahl korreliert .…”