Engineering ferroelectricity and metal-to-insulator transitions in metal-oxide heterostructures*

Structurally designing materials by combining two or more different materials for realizing interesting emergent properties to target a desired functionality reduces the need for serendipity. One of the nascent strategies for pursuing new acentric compounds focuses on layering centrosymmetric materials in a particular fashion that lifts inversion symmetry while retaining other electronic, magnetic or optical functionalities found in the constituent materials, thus enabling new multiferroics or narrow bandgap polar semiconductors. Here, we formulate an approach to functionalize a polar metal i.e., (Ca₁^__xSr_xFeO₃)_m/(SrFeO₃)_n into an insulating ferroelectric constructed from metallic oxide components CaFeO₃ and SrFeO₃. We have investigated several odd and even superlattices with different periodicities (e.g. m x n = 1x1, 2x2, 1x3, 3x1 etc). We find that the trigger for such a metal-to-insulator (M-I) transition is the disproportionation of the oxidation states of the central Fe ions. We find that appropriate periodicity and cation ordering is critical for the desired transition. The present work opens a door for designing ferroelectrics with tunable energy gaps by inducing controlled M-Itransitions.