Joshua Young and Prof. Rondinelli explain in the Letter titled Octahedral Rotation Preferences in Perovskite Iodides and Bromides appearing in the Journal of Physical Chemistry Letters why halide perovskites favor in-phase octahedral tilt patterns whereas oxides prefer out-of-phase tilts.
In this work, they investigate a series of phase transitions in 12 bromide, iodide, and oxide perovskites. While similar, an unusual tetragonal phase exhibiting only in-phase rotations is stable in the halides, while one containing only out-of-phase rotations is stable is the oxides. By computing and comparing differences in electrostatic repulsion, bond valence, and charge density, they elucidate the physical and chemical mechanisms leading to this discrepancy.
These chemical insights into structural preferences may lead to a better understanding of perovskite solar cell properties and stability or even novel types of hybrid improper ferroelectrics.