MS10-P04 Rotational dynamics of methyl ammonium ions and hydrogen-bonding in orthorhombic CH3NH3PbI2.94Cl0.06 by means of neutron scattering and IR investigations Götz Schuck (Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany) Monika Koch-Müller (Deutsches GeoForschungsZentrum - GFZ, Potsdam, Germany) Ilias Efthimiopoulos (Deutsches GeoForschungsZentrum - GFZ, Potsdam, Germany) Frederike Lehmann (Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany) Susan Schorr (Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany)email: goetz.schuck@helmholtz-berlin.dePerovskites with ABX3-structure show huge possibilities for element substitution on the A-, B- and X-sites, resulting in a broad variety of physical properties. One field of interest is chlorine-substituted methyl ammonium lead triiodide in which A is the organic unit [CH3NH3]+ = MA, B = Pb2+ and X = I3-xClx.
We chose to substitute 2 % of the iodine in MAPbI3 with chlorine since recent investigations with synchrotron XRD showed that only a narrow range of up to 2.5 % of the iodine in MAPbI3 can be substituted with chlorine. At the same time, only up to 1 % of the iodine in MAPbCl3 can be substituted, meaning there is a large miscibility gap. [1]
The aim of the quasi-elastic neutron scattering (QENS) investigations presented here is to understand the interrelationship of the static and the dynamic structure of MAPbX3 by comparing the temperature-dependent methyl ammonium rotational dynamics of MAPbI3, MAPbI2.94Cl0.06 and MAPbCl3. Combining the QENS results with the analysis of temperature-dependent IR vibrational spectra of MAPbI3, MAPbI2.94Cl0.06 and MAPbCl3 helps us understand the influence of the rotational dynamics of the methyl ammonium cation on the hydrogen-bonding layers in the orthorhombic low temperature phase.
Recently, the presence of layers with X...H-N hydrogen-bonds in the orthorhombic phase of MAPbX3 was discussed to explain the drastic changes of MA IR vibrational modes across the tetragonal-orthorhombic phase transition (Fig. 1). The transformation of the 2-dimensional orthorhombic hydrogen-bond layers into a more 3-dimensional arrangement in the tetragonal phase is an important feature, providing deeper insight into the mechanisms that lead to a free-rotating MA molecule in the inorganic host structure. [2]
In recent QENS investigations on MAPbI3 [3], two quasi-elastic components were identified in the tetragonal (161.5 K < T < 327 K) and cubic (T > 327 K) phase. It was interpreted that, in the cubic and tetragonal phases, the MA ion exhibits four-fold rotational symmetry perpendicular to the C-N axis (C4) along with three-fold rotation parallel to the C-N axis (C3), while only C3 rotation was present in the orthorhombic Pnma phase (T < 161.5 K). [3] Here, we reinvestigate the suggested jump-models for MAPbI3 and show the validity of the proposed QENS interpretation for MAPbI2.94Cl0.06 and MAPbCl3.

[1] Franz et al., submitted to Acta Crystallogr. B, 2018

[2] Schuck et al., J. Phys. Chem. C, 2018, 122, 5227

[3] Li et al., Nature Communications 2017, 8, 16086
Keywords: Hybrid Perovskite, Quasielastic Neutron Scattering, Hydrogen-Bonding