New ferroelectric organic molecular crystals through computer design and experimental realization

When you apply an electric voltage across a material, the atoms in the material will move. This movement of atoms will cause the electrical charges in the material to also move. In conventional materials, this charge disappears when you turn off the voltage, but in ferroelectric materials, the material remembers this movement, and thus the surface charge. If you stretch or bend materials, the charge on the surface will increase or decrease, which is called piezoelectricity. This makes ferroelectric materials also well-suited for use in sensors, displays, and energy harvesting. Today's ferroelectric materials are made of ceramics that often consist of rare or toxic heavy metals. It also requires a lot of energy to make these ferroelectric ceramics; they can only be made at high temperature. An alternative is ferroelectric molecular crystals. What makes these so exciting is that there are nearly endless ways to design and optimize such materials by combining different molecules. Such molecular crystals can also give rise to entirely new and exciting properties. For example, so-called plastic crystals, which consist of molecules that are somewhat round or globular, become malleable at higher temperatures, making them much easier to integrate into electronic devices. In the project, we focus on two types of molecular ferroelectric materials: proton transfer and plastic ionic crystals. In the proton transition materials, protons switch molecules when we apply an electric voltage across the material. We have now found a theoretical method that can describe proton transfer and crystal structure quite accurately. Plastic ionic materials are plastic crystals bound together by partially ionic bonds. Since the molecules in these materials are quite round, you can rotate them and thus change direction, but they can also undergo a phase transition to a plastic ionic phase. For a number of known plastic ionic crystals, we have modeled piezoelectric properties and found that they can have a large electrical response when twisted, precisely because the molecules can rotate. Our results indicate that these materials should be further investigated for their piezoelectric properties. We have now searched through known organic crystals (approx. 500,000). Theoretically, we found about 7 new materials that we believe are likely proton-based materials, of which 3 seem attractive. We have also found about 50 possible new plastic crystals in this way. We have now been able to demonstrate experimental ferroelectric properties for 4 of the plastic ionic crystals found in the database search, but several of the materials are frail and difficult to work with, e.g., they can evaporate during measurements. At least two of the materials show such promising properties that we are working further to create ferroelectric pellets. The other two materials will also be of interest to characterize further, but it is uncertain if we will achieve this within the project due to technical difficulties. Our collaborators in Heidelberg, where Post Doc. Dr. Balagopalan had a stay abroad and learned valuable skills, have also shown great interest in the properties of the materials we have realized, and parallel measurements are now taking place in their lab. Of the proton transfer crystals, we have succeeded in demonstrating ferroelectric properties for 1 of the materials, but the properties are atypical, which can be expected due to the structure. On the theory side, we have used crystal structure prediction to find several forms of co-crystal proton transition ferroelectric crystals. Based on this, we have now theoretically designed 4 new ones. We have attempted to realize one of them, but have not yet succeeded in characterizing it thoroughly. We expect to be able to produce the crystals early next year. If we can confirm the good theoretical properties predicted, this could have a major impact. We have also succeeded in using advanced theoretical methods to characterize phase transitions in a known plastic ionic crystal. We are now thoroughly characterizing 2-4 of the plastic ionic crystals using X-ray and Raman. At the same time, we see that this requires more modeling to understand thoroughly. The project has largely been based on theory coming first to find and design interesting materials, followed by experimental testing. The weakness with this is that we have limited resources to analyze the experimental findings. There is a change notice where we want to shift resources from purchasing molecules and travel (which was less due to Covid) to personnel costs at NMBU to ensure good theoretical analysis of the experimental measurements. There is also no need for significant further purchases of materials, since many of the crystals we have found and designed consist of known and readily available molecules.

Ferroelectric organic molecular crystals have the potential to replace current inorganic materials used in ferroelectric random access memory (FeRAM) which could significantly reduce fabrications costs and reliance on toxic or scarce chemical elements. It can also extend the usage of organic ferroelectrics to other niches. However, compounds with a larger ferroelectric-paraelectric phase transition temperature is urgently needed to bring this promise into fruition. In addition, modest coercive fields and sizeable polarization is needed. However, a deeper understanding of their physical properties is required and so is also a systematic design approach. The Fox project will obtain such insight by using sophisticated modelling methods and comparison with experimental results. Moreover, we will systematically design new organic ferroelectrics in a reliable manner to improve performance. The most promising candidates will be crystallized and in turn characterized both by the project group and by collaborators, which will provide further understanding and the potential for realizing record-breaking organic ferroelectrics. Computational-guided discovery, with experimental and characterization, has in recent years proven itself as a powerful approach for engineering new materials, but organic ferroelectric molecular crystals have been largely been overlooked. We have assembled a strong team of scientists and international collaborators, with access to, and experience with, a range of different characterization methods needed for in-depth analysis of these compounds.