From Ca3Be(SeO3)4 to SrBe(SeO3)2: two unprecedented alkaline earth metal beryllium selenites with large band gaps and enhanced birefringence

Beryllium-based selenites have rarely been reported. Herein, the first two alkaline earth metal beryllium-based selenites, Ca3Be(SeO3)4 and SrBe(SeO3)2, have been synthesized using a mild hydrothermal method. In both crystals, Be is connected to four [SeO3]2− groups. Due to the different arrangements of [SeO3]2− groups, these two crystals exhibit significantly different optical anisotropy. From Ca3Be(SeO3)4 to SrBe(SeO3)2, the birefringence ranges from 0.005@546 nm to 0.058@546 nm. Theoretical calculations confirmed the dominant role of [SeO3]2− in the optical anisotropy of these two crystals. In addition, both of them exhibited large band gaps (5.40 and 5.28 eV) and high thermal stability (596 and 570 °C). This work demonstrates that the introduction of beryllium can effectively increase the band gap of selenites to over 5 eV and offers new ideas for the development of large-bandgap selenites.