Octahedral Tilting in Perovskite Polytypes

One of the most powerful strategies to control the functionality of a material is to manipulate its underlying atomic structure. This approach has found particular success in perovskite materials, where cooperative tilts of the corner-sharing octahedral network can be leveraged to influence a range of optical, magnetic, and electrical properties as well as induce entirely new functionalities such as ferroelectricity or magnetoelectric coupling. Despite these past successes, similar strategies have yet to be envisioned for simple polytypes of the perovskite structure which can exhibithighly promising functional properties as well. Here, we establish the capacity for octahedral tilting as a control parameter across a vast swathe of polytype structures. We use an innovative symmetry approach to show that the enhanced structural rigidity associated with face-sharing polyhedra does not impede co-operative tilting. Additionally, we elucidate a mechanism by which only a single zone-center tilt mode is sufficient to break global inversion symmetry in several polytypes, unlocking functionalities such as improper ferroelectricity or the bulk photovoltaic effect. We conclude by discussing some viable tilt engineering strategies, illustrating how chemical substitutions could be leveraged to tune material properties and yield useful ferroic functionalities. Our findings unveil untapped design principles which greatly enhance the prospects of perovskite polytypes as next-generation functional materials.