Xinjiang Lihua has made progress in research on large birefringence gain

[ Instrument Network Instrument Development ] When a beam of light is projected onto the crystal interface, two beams of refracted light are generally generated. This phenomenon is called birefringence. The birefringence of crystals is an important optical performance parameter of optoelectronic materials. Birefringent crystals are widely used in optical communication, optical devices and laser processing industries. Therefore, the exploration of large birefringent materials and excellent birefringent groups has always been a difficult and hot topic in international research. The main factors determining the birefringence properties of the crystal are the anionic framework and the cationic polyhedron. For alkali-free, alkaline earth metals with no stereoactivity, the anionic framework is an important factor in determining the birefringence properties of the crystal. However, cations, especially those containing stereoactivity, also have an important effect on the birefringence properties of the crystal. At present, the influence of the stereoactivity of Pb2+ and Sn2+ on the birefringence only stays at the theoretical calculation level, and no experimental evidence has been reported to confirm the large gain of the paired electrons of Sn2+.

Recently, the team led by Pan Shilie, a researcher at the Key Laboratory of Special Environmental Materials and Devices of the Xinjiang Institute of Physics and Chemistry, Chinese Academy of Sciences, has been working on exploring large birefringent materials and source mechanisms for generating large birefringence, through the first case of Sn2+ borate chloride. The birefringence test of the Sn2B5O9Cl and the isomorphous alkaline earth metal borate was experimentally demonstrated for the first time that the stereoactive Sn2+ can excite a large gain of birefringence. The team researchers obtained Sn2B5O9Cl crystals (1 × 1 × 0.5 mm3) in a closed system, and obtained isomorphic Ba2B5O9Cl crystals (5 × 4 × 1.0 mm3) by the top seed method, which was measured by a polarizing microscope and a gem refractometer. The birefringence of the crystal. Both experimental data and theoretical calculations show that Sn2B5O9Cl has an extremely large birefringence (0.168@546 nm) and its birefringence is 16.8 times (0.010@546 nm) which is the isomorphous compound Ba2B5O9Cl.

The researchers used theoretical calculations and structural comparisons to analyze the sources of large gains in birefringence. The large birefringence of Sn2B5O9Cl is mainly derived from the contribution of the highly distorted tin oxychloride polyhedron and the distorted BO3 group, however these distortions are absent in the isomorphic compound Ba2B5O9Cl. The method of solid space atomic cleavage further illustrates that the gain of the large birefringence is derived from a stereoactive tin oxychloride polyhedron.

In order to further prove that Sn2+ can stimulate the large gain of birefringence, the researchers analyzed the birefringence properties of β-SnB4O7 and isomorphic β-CaB4O7, SrB4O7 based on theoretical calculations, and found that the birefringence of β-SnB4O7 is about isomorphism. The alkaline earth metal borate is 30 times, which further confirms that the replacement of the alkaline earth metal cation-Sn2+ can excite a large gain in birefringence. More importantly, the replacement of alkaline earth metal cation-Sn2+ not only can obtain a large gain of birefringence, but also provides a new idea for exploring large birefringent materials in the future.

The results of this series of research were published by Wiry Important Papwer in "Applied Chemistry of Germany" published by Wiley (Angew. Chem. Int. Ed., 2019, DOI: 10.1002/anie.201911187). The research work was funded by the National Fund Committee, the Ministry of Science and Technology, and the Chinese Academy of Sciences.