@misc{BreternitzLehmannBarnettetal.2019,
  author    = {Breternitz, Joachim and Lehmann, Frederike and Barnett, Sarah A. and Nowell, Harriott and Schorr, Susan},
  title     = {Role of the Iodide-methylammonium interaction in the ferroelectricity of CH3NH3PbI3},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {1},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-51822},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-518227},
  pages     = {7},
  year      = {2019},
  abstract  = {Excellent conversion efficiencies of over 20\% and facile cell production have placed hybrid perovskites at the forefront of novel solar cell materials, with CH3NH3PbI3 being an archetypal compound. The question why CH3NH3PbI3 has such extraordinary characteristics, particularly a very efficient power conversion from absorbed light to electrical power, is hotly debated, with ferroelectricity being a promising candidate. This does, however, require the crystal structure to be non-centrosymmetric and we herein present crystallographic evidence as to how the symmetry breaking occurs on a crystallographic and, therefore, long-range level. Although the molecular cation CH3NH3+ is intrinsically polar, it is heavily disordered and this cannot be the sole reason for the ferroelectricity. We show that it, nonetheless, plays an important role, as it distorts the neighboring iodide positions from their centrosymmetric positions.},
  language  = {en}
}
@article{FranzToebbensSteckhanetal.2018,
  author    = {Franz, Alexandra and T{\"o}bbens, Daniel M. and Steckhan, Julia and Schorr, Susan},
  title     = {Determination of the miscibility gap in the solid solutions series of methylammonium lead iodide/chloride},
  series = {Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials},
  volume    = {74},
  journal   = {Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials},
  publisher = {International Union of Crystallography},
  address   = {Chester},
  issn      = {2052-5206},
  doi       = {10.1107/S2052520618010764},
  pages     = {445 -- 449},
  year      = {2018},
  abstract  = {Perovskites are widely known for their enormous possibility of elemental substitution, which leads to a large variety of physical properties. Hybrid perovskites such as CH3NH3PbI3 (MAPbI(3)) and CH3NH3PbCl3 (MAPbCl(3)) are perovskites with an A([XII])B([VI)X(-)([II)(])structure, where A is an organic molecule, B is a lead(II) cation and X is a halide anion of iodine or chlorine. Whereas MAPbCl(3) crystallizes in the cubic space group Pm (3) over barm, MAPbI(3) is in the tetragonal space group I4/mcm. The substitution of I by Cl leads to an increased tolerance against humidity but is challenging or even impossible due to their large difference in ionic radii. Here, the influence of an increasing Cl content in the reaction solution on the miscibility of the solid solution members is examined systematically. Powders were synthesized by two different routes depending on the I:Cl ratio. High-resolution synchrotron X-ray data are used to establish values for the limits of the miscibility gap which are 3.1 (1.1) mol\% MAPbCl(3) in MAPI(3) and 1.0 (1) mol\% MAPbI(3) in MAPCl. The establishment of relations between average pseudo-cubic lattice parameters for both phases allows a determination of the degree of substitution from the observed lattice parameters.},
  language  = {en}
}