Causal Reasoning for Controlling Polarization Switching in Hybrid Improper Ferroelectrics

The potential of hybrid improper ferroelectrics (HIFs) in electronic and spintronic devices hinges on their ability to switch polarization. Although the coupling between octahedral rotation and tilt is well established, the factors that govern switching barriers remain elusive. In this study, we explore this area to demonstrate the critical role of causal reasoning in uncovering the mechanisms to control the ferroelectric switching barrier in HIFs. By combining causal discovery, causal interventions, and first-principles simulations, we identify tolerance factor, A-site cation radii mismatch, epitaxial strain, and octahedral rotation/tilt as key parameters and quantify how their interplay directly influences switching barrier. Three key insights emerge from our work: (a) the analysis identifies the structural descriptors controlling polarization reversal across a broad family of A-site-layered double perovskites and superlattices, (b) it uncovers nontrivial, material-specific rotation-tilt mechanisms, including a counterintuitive cooperative pathway where both rotation and tilt change while lowering the barrier, an effect mostly inaccessible to conventional Landau or first-principles-based approaches and (c) it maps these material-specific mechanisms to experimentally realizable parameters, showing that epitaxial strain from orthorhombic substrates (e.g., NdScO 3, NdGaO 3 ) selectively tunes octahedral distortions to achieve barrier reduction across varied compositions. These results establish actionable, materials-by-design principles linking composition, structure, and strain to polarization switching, while highlighting the potential of causal reasoning to guide intelligent, mechanism-driven strategies for engineering complex functional oxides.