Old tree blossoms anew: Research progress on the structures and optical properties of ultraviolet selenites

Abstract

Listen

Translate article

Nonlinear optical materials and birefringent materials are essential components of modern industrial technology. Selenites, owing to the unique stereochemical activity of the lone pair electrons, have a history spanning several decades as both linear and nonlinear optical materials. Up to now (January 16, 2024), nearly 1100 selenite materials have been discovered, some of which exhibit strong second-harmonic generation effects or large birefringence. With the strategic layout of nations and the rapid advancement of modern industrial technology, the demands on the transparency range of linear and nonlinear optical crystals have become increasingly stringent. Crystals transparent only in the visible spectrum are no longer adequate to meet current requirements. Selenites, which have a long history in the field, seem to have encountered a bottleneck in their recent development, primarily due to an excessive focus on their applications in the visible and near-infrared regions while neglecting their potential uses in the ultraviolet (UV) window. To explore new development directions of selenites, our research group is actively investigating their application as UV optical materials. Through a fluorination control strategy, we successfully synthesized a UV nonlinear optical crystal Y3F(SeO3)4 and a birefringent crystal CaYF(SeO3)2, thereby developing the application of selenites in the UV region. Additionally, we found that some previously reported selenites with band gaps larger than 4.2 eV have not received sufficient attention from researchers, and their potential application in the UV region have been overlooked. To further promote the research progress of UV selenite materials, this paper comprehensively summarizes the reported selenite materials with bandgaps larger than 4.2 eV, totaling 63 compounds across 6 crystal systems and 19 space groups. We provide a detailed analysis of their structures, optical properties, and design strategies. These materials can be classified into four categories based on different anionic groups: simple selenites, fluoride selenites, selenites with tetrahedral groups, and other selenite compounds. Through a comprehensive review of UV selenites, we have identified ions or groups conducive to expanding the band gaps of selenites, proposed several reliable design methods for UV selenites, and offered useful suggestions for the development of UV selenites. Despite having a long history, selenite systems still hold significant untapped potential for further exploration. We hope this review provides valuable guidance and insights for the future development of inorganic selenites.