MS10-P04 Rotational dynamics of methyl ammonium ions and hydrogen-bonding in orthorhombic CH3NH3PbI2.94Cl0.06 by means of neutron scattering and IR investigationsPerovskites 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. 
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. 
In recent QENS investigations on MAPbI3 , 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).  Here, we reinvestigate the suggested jump-models for MAPbI3 and show the validity of the proposed QENS interpretation for MAPbI2.94Cl0.06 and MAPbCl3.
 Franz et al., submitted to Acta Crystallogr. B, 2018
 Schuck et al., J. Phys. Chem. C, 2018, 122, 5227
 Li et al., Nature Communications 2017, 8, 16086Keywords: Hybrid Perovskite, Quasielastic Neutron Scattering, Hydrogen-Bonding