Inverse design of compounds that have simultaneously ferroelectric and Rashba cofunctionality

Cofunctionality-the coexistence of different (possibly even contraindicated) properties in the same compound-is an exciting prospect that never fails to deliver interesting surprises, as in transparent while electrically conducting compounds, topological insulators having Rashba spin split bands, or electrically conducting while thermally insulating (thermoelectric) compounds. Here we study a cofunctionality of ferroelectricity and Rashba effect that combines the directional helical spin-polarization characteristic of the energy bands of bulk Rashba compounds with the existence of two opposite electric polarization states, characteristic of atomic displacements in displacive ferroelectrics. Flipping of the ferroelectric polarization (e.g., via an applied electric field) would result in the reversal of the Rashba spin polarization. However, thus far, only very few (essentially one) compounds have been found to be ferroelectric Rashba semiconductors (FERSCs), e.g., GeTe (R3m). In this paper, we propose a general strategy for the identification of compounds that possess cofunctionalities and apply it to perform an inverse design, finding compounds that simultaneously have ferroelectricity and Rashba spin splitting. The inverse design combining functionality f(1) with f(2) involves definition and utilization of causal factors that enable the said functionalities, and involves three steps: (1) screening materials that satisfy the enabling DPs common to the two functionalities f(1) and f(2); (2) filtering the materials according to DPs that are unique to each of the individual functionalities f(1) and f(2); and (3) by using the ensuing two compound lists of compounds C(f(1)) and C(f(2)) identifying the intersection between them, i.e., compounds C(f (1), f(2)) possessing simultaneous f(1) and f(2). Based on this process, we design 52 atomic combinations not suspected to be FERSCs that were previously synthesized. Of these 52 compounds, we find 24 FERSCs that are thermodynamically stable (i.e., reside on the convex hull) and, at the same time, feature larger spin splitting (large than 25 meV). These include BrF5 (Cmc2(1)), TlIO3 (R3m), ZnI2O6 (P2(1)), LaTaO4 (Cmc2(1)), Tl3S3Sb (R3m), Sn2P2Se6 (Pc), and Bi2SiO5 (Cmc2(1)) that have Rashba spin splitting of 31, 57, 111, 40, 90, 67, and 76 meV, respectively. Density functional theory calculations illustrate the reversal of the spin texture when the electric polarization is flipped. This paper validates a general approach for the search of other cofunctionalities based on causal enabling design principles rather than uncovering machine correlations.