Design of novel proton-transfer acid-base ferroelectrics: Interplay between structure, competing synthons, and protonation states.

Organic ferroelectrics based on proton-transfer mechanisms hold promise for low-power and sustainable electronic applications. Acid-base proton-transfer (PT) salts, in particular, allow systematic adjustment of spontaneous polarization and coercive fields through functionalization and substitution of the organic species. However, such modifications can also affect crystal structure, synthon formation, and protonation states, posing challenges for crystal engineering. Acid-base PT salts are relatively scarce in the Cambridge Crystallographic Database, but organic crystal structure prediction (CSP) stands out as an effective tool to design new PT materials by enabling virtual screening of various molecular packings and their energetic rankings. We systematically assessed 30 different acid-base combinations using CSP with DFT using the vdW-DF2 functional. We identified this vdW-DF functional as effective for PT ferroelectrics due to its accurate ranking of the crystal structure of an existing PT ferroelectric and a competing cocrystal, as well as its reliable performance for PT barriers. For the different combinations, several favored cocrystal or divalent salt forms incompatible with PT ferroelectricity. In other cases, competing synthons or molecular packings were favored over those supporting PT. However, we among the 30 we also predicted 3 novel acid-base PT ferroelectrics with higher polarization than existing PT ferroelectrics and 2 new antiferroelectrics, highlighting CSP’s potential for engineering novel organic functional materials.