Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.269
Publication date: 18th July 2019
Hybrid halide perovskites have clearly made a spectacular appearance in the field of solar absorber materials. [1] With cell efficiencies steeply rising, the fundamental understanding of these materials is not always commensurate. A thorough understanding of the structural features of perovskites is most important, as it is closely linked to the electronic structure of the respective materials.
One prominent example are the reports of piezoelectric and ferroelectric effects in MAPbI3 at room temperature, [2] which are contradictory to the generally accepted crystal structure in the space group I4/mcm. While the latter is centrosymmetric, a polar space group, i.e. the breaking of inversion symmetry in the structure, would be a necessary prerequisite for ferroelectricity. A convenient explanation for this apparent discrepancy would be a symmetry breaking due to the non-symmetric molecular cation orientation and dynamics, as has been suggested in the past. [3] Since the molecule is heavily disordered, [4] a non-linear effect would probably disappear on a macroscopic scale if it is only caused by this.
We will open the discussion with the question how to define perovskites [5] and which structural features are crucial for the categorisations. Further, we will set out to categorize the different structures of halide perovskites in the form of a Bärnighausen tree and will further elucidate the importance of understanding group-subgroup relationships between the different crystal structures observed in halide perovskites. With this fundamental crystallographic basis, we set out to elucidate the question of ferroelectricity in MAPbI3 at ambient conditions.
For this, we present a detailed crystallographic investigation of the possible structural origin of ferroelectricity in MAPbI3. In short, the breaking of inversion symmetry is a consequence of maximizing hydrogen bonding between the molecular cation CH3NH3+ and the surrounding I- anions. This highlights how the organic cation plays a major role on the structural properties of this material. We will elucidate how the disordered molecular cation influences its surrounding and finally deliver a conclusive explanation for ferroelectricity from a crystallographic point-of-view.
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