The phase diagram elucidates structural changes and phase separation effects, induced by halide substitution in hybrid perovskite MAPb(I,Br)3 solid solution.

This paper discusses the full structural solution of the hybrid perovskite formamidinium lead tribromide (FAPbBr3) and its temperature‐dependent phase transitions in the range from 3 K to 300 K using neutron powder diffraction and synchrotron X‐ray diffraction. Special emphasis is put on the influence of deuteration on formamidinium, its position in the unit cell and disordering in comparison to fully hydrogenated FAPbBr3. The temperature‐dependent measurements show that deuteration critically influences the crystal structures, i.e. results in partially‐ordered temperature‐dependent structural modifications in which two symmetry‐independent molecule positions with additional dislocation of the molecular centre atom and molecular angle inclinations are present.

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.