Hybrid Improper Ferroelectricity: A Theoretical, Computational, and Synthetic Perspective

We review the theoretical, computational, and synthetic literature on hybrid improper ferroelectricity in layered perovskite oxides. Different ferroelectric mechanisms are described and compared, and their elucidation using theory and first-principles calculations is discussed. We also highlight the connections between crystal chemistry and the physical mechanisms of ferroelectricity. The experimental literature on hybrid improper ferroelectrics is surveyed, with a particular emphasis on cation-ordered double perovskites, Ruddlesden–Popper and Dion–Jacobson phases. We discuss preparative routes for synthesizing hybrid improper ferroelectrics in all three families and the conditions under which different phases can be stabilized. Finally, we survey some synthetic opportunities for expanding the family of hybrid improper ferroelectrics.