Structural Distortions in Hybrid Perovskites Revisited

Tolerance factor arguments have been used to examine structural trends in inorganic perovskites for a long time. In this work, we show that for hybrid perovskites, even if one could define an ionic radius for each molecule at the A-site of a perovskite of the form ABX3 and vary the molecule so that one has a monotonic change in the tolerance factor, the structural distortions are not systematic. One finds that the symmetry of the molecule and its orientation in the inorganic cage play an important role in determining the distortions observed in these perovskites. Although the interactions between the hydrogen atoms of the molecule and the anions have been known to dictate the structural distortions of the inorganic network, those hydrogens attached to C atoms behave differently from those attached to N, emerging from the different electronegativities of the atoms that they are attached to. This leads to the hydrogens attached to N atoms, carrying an effective positive charge, allowing them to come closer to the anions, thereby significantly changing the structural distortions, in addition to determining the orientation of the molecule in the inorganic cage.