A mercury-based selenite with wide bandgap and moderate birefringence via cation substitution and partial fluorination.

Exploring novel functional crystals and enhancing their optical performance through microscopic structural modulation remains a significant topic in materials chemistry. Hg2+ delivers extremely high electronic polarizability and supports diverse flexible coordination geometries, which render it an excellent structural motif for designing birefringence crystals; unfortunately, most reported Hg-containing selenites exhibit narrow optical bandgaps arising from intense hybridization between Hg 6s and O 2p orbitals, greatly limiting their short-wavelength optical applications. To obtain Hg-based selenite compounds with wide bandgaps, this work demonstrates a structural modification method involving equivalent cation substitution combined with partial fluorination. Using the parent compound HgGa2(SeO3)4 as a template, we successfully synthesized a novel fluoro-selenite, HgScF(SeO3)2, via a high-temperature hydrothermal method by replacing the highly symmetric GaO6 octahedra with partially fluorinated ScO4F2 polyhedra. Single-crystal X-ray diffraction analysis reveals that the compound features a unique three-dimensional framework constructed from Hg2O6 dimers bridging two-dimensional scandium selenite layers. Optical characterizations confirm that the introduction of F- effectively disrupts the excessive cross-linking between metal centers and SeO3 groups. Consequently, while preserving the wide bandgap characteristic of the parent compound, the birefringence of HgScF(SeO3)2 is enhanced. This study provides detailed insights into the synthesis, structure, and properties of this material, offering an experimental basis for optimizing the optical performance of crystals through precise control of the microscopic coordination environment.