Borosulfates – Structural Diversity, Luminescence, and Exploration of Additional Properties

• Applicant: Professor Dr. Henning Höppe
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Synthesis and Properties of Functional Materials
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 556118528

The anions in borosulfates, occasionally also referred to as sulfato-borates, consist almost exclusively of tetrahedral units composed of boron and sulfur atoms tetrahedrally surrounded by oxygen. Typically, boron atoms are covalently bonded to sulfate tetrahedra. Accordingly, these are silicate-analogous compounds, which are chemically closely related to the already well-known borophosphates and borosilicates. Such silicate-analogous compounds are interesting from various perspectives. On the one hand, they literally show a very rich and highly interesting structural chemistry; on the other hand, they offer many approaches for studying structure-property relationships. The structural spectrum ranges from non-condensed anions like in K5[B(SO4)4] to framework structures with channels like in Bi(H3O)[B(SO4)2]4, from sulfate-rich representatives like Ba[B(S2O7)2] to sulfate-poor ones like (NH4)2[B4O6(SO4)].The unique feature of borosulfates is their very weak coordination, which scarcely influences the surrounding cations. Because tetrahedra are not centrosymmetric, the overall structures often are not either. Therefore, such materials are particularly interesting for (non-linear) optical properties. Based on the approximately 90 known compounds so far, we will synthesise and thoroughly investigate new luminescent compounds in this project, which could be suitable for thermometry, light-emitting diodes, phosphor lamps based on a xenon plasma, non-linear optical applications, and displays. Additionally, we will also focus on candidates that could at least show ion conduction and take initial steps towards porous structures. Thus, we continue the systematic and exploratory investigation of the relatively young class of compounds known as borosulfates.Regarding crystal chemistry, we expect to discover new structures with a significant structural diversity comparable to that of borophosphates or silicates. Regarding structure-property relationships, we intend to achieve fundamental insights into understanding the chemistry of borosulfates, the coordination behaviour of borosulfates, and the behaviour of weakly coordinating networks in general. We are particularly interested in their optical properties and ion conduction, for the benefit of both basic research and the evaluation of potential applications.Methodologically, we use single-crystal X-ray structure analysis for structure determination, and we prove the purity of the found compounds using X-ray powder diffraction, with which we also identify new compounds. We investigate the thermal stability employing diffraction methods, high-temperature powder diffraction, as well as classical thermal analyses such as DTA/TG. The obtained materials will be comprehensively characterized spectroscopically (luminescence and UV-Vis spectroscopy, nuclear magnetic resonance, Mössbauer spectroscopy) and regarding their ion-conducting properties.

• DFG Programme: Research Grants