Pb2(SeO3)(SiF6): the first selenite fluorosilicate with a wide bandgap and large birefringence achieved through perfluorinated group modification

Birefringent crystals serve as the core elements of polarizing optical devices. However, the inherent challenge of balancing bandgap and birefringence poses a significant hurdle in designing crystals with excellent overall performance. In this study, we propose a novel approach, namely modification with perfluorinated groups, to achieve dual enhancement of the bandgap and birefringence of selenite materials. We have successfully synthesized the first selenite fluorosilicate, namely, Pb₂(SeO₃)(SiF₆). This compound exhibits a three-dimensional structure composed of two-dimensional lead selenite layers bridged by SiF₆ octahedrons. Notably, by introducing a perfluorinated SiF₆ group, the bandgap of the lead selenite compound has been expanded to 4.4 eV. Furthermore, Pb₂(SeO₃)(SiF₆) demonstrates a large birefringence (0.161 @ 546 nm), surpassing most of the selenite compounds with a bandgap larger than 4.2 eV. Theoretical calculations suggest that the large birefringence of Pb₂(SeO₃)(SiF₆) can be attributed to the synergistic effects of SeO₃, PbO₄ and PbO₃F₄ polyhedrons. Our research not only pioneers a new system for selenite materials, enriching the diversity of selenite structures, but also provides a design methodology for obtaining wide bandgap birefringent selenite.