Prediction, synthesis, and characterization of novel ferroelectric metal fluorides

• Applicant: Professor Dr. Florian Kraus
• Subject Area: Solid State and Surface Chemistry, Material Synthesis
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 533629173

Ferroelectrics are insulators with a spontaneous electric polarization that can be reversed by an external electric field. Their properties make them desirable functional materials with broad application potential, ranging from energy converters or sensors to random access memory in information technology. The occurrence of ferroelectricity is linked to strict structural requirements. Spontaneous polarization is only possible in polar space groups belonging to the ten pyroelectric crystal classes. In addition, the reversal of the polarization direction must occur via a paraelectric isotropy supergroup. These structural requirements were summarized by Abrahams into empirical rules and are still the basis for predicting previously unknown ferroelectrics. Historically, the focus of research has been on oxide ferroelectrics. Nowadays, a large number of oxide ferroelectric structure types are known, but only a few fluoride compounds with this property. In the present project, we aim to predict novel ferroelectric fluorides. For this purpose, we will prepare a survey of polar metal fluorides, which we will investigate for their potential as ferroelectrics using the successful Abrahams crystallographic criteria in combination with modern quantum chemical methods. We will synthesize and characterize promising candidates from this survey to demonstrate their structural prerequisites for ferroelectricity. Via an initial screening of the Pearson database, we have succeeded in identifying 105 different polar structure types for metal fluorides. Only nine of these structure types have at least one representative that is proven to be ferroelectric. Thus, a high potential for previously unknown ferroelectrics exists here. So far, we have already identified three promising candidates for new potentially ferroelectric structure types with β-MnF4 (R3c, hR360), Mn3F8 (P21, mS22) and RbCrF5 (Pmc21, oP28), with which we can start our work. Our screening further reveals that controversial structure proposals exist for about 30% of the structure types. This is because, especially for polar compounds, the assignment of the space group in the diffraction experiment is not always possible without doubt. This complicates automatic compilations of data sets needed for high-throughput DFT calculations or machine learning. This aspect highlights the great need for an unambiguous structural chemical data set for polar compounds. With the crystallographic, structural chemical and experimental experience knowledge of our working group we want to tackle this task for the material class of metal fluorides.

• DFG Programme: Research Grants
• International Connection: Austria, Finland
• Cooperation Partners: Professor Dr. Hubert Huppertz; Professor Dr. Antti Karttunen