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Borate has a rich chemical structure. B atoms can adopt two coordination modes of BO3 and BO4, and further aggregate into one-dimensional chains, two-dimensional layers and three-dimensional networks, so that borate has a rich crystal structure. Therefore, borate is the preferred system for the design of new synthetic optical crystal materials. Based on the theory of anionic groups, the BO3 plane element has a π-conjugated orbit of the asymmetric electron cloud distribution, with a large microscopic polarizability. The parallel arrangement of the BO3 plane elements facilitates the material to achieve good frequency-doubling and birefringence. In nature, these two parameters directly determine the laser conversion efficiency of the material and frequency range of the frequency-doubling application.
The research team of the new photoelectric functional materials of the Xinjiang Institute of Physics and Chemistry, Chinese Academy of Sciences, has systematically explored the design and synthesis of new types of ultraviolet optical crystal materials. Through a large number of experiments, a borate Li6Zn3(BO3)4 with novel structural characteristics has been synthesized. The crystals are crystallized in the triclinic P-1 space group, and the structure contains isolated BO3 groups with near-plane alignment, which makes them have a relatively large birefringence (0.065@1064 nm). The coplanar connection of LiO4 tetrahedra was first reported in this structure. Researchers have systematically summarized nearly 100 lithium borate-containing compounds, analyzed the crystal structure of Li-O polyhedrons, and summarized the Li-O coordination anionic group species and group coordination modes. It was found that there is a large repulsion between the Li-Li atoms when the LiO4 tetrahedron is coplanarly connected, resulting in a very rare case of coplanar connection of the LiO4 tetrahedron. In addition, the researchers found that Li6Zn3(BO3)4 has two reversible phase transitions in the temperature range of 290°C-360°C and 650°C-770°C. It is a potential phase change material.
The related research results were published as cover articles in the Inorganic Chemistry Frontiers.
The research was funded by the National Natural Science Foundation of China and the Xinjiang Uygur Autonomous Region International Cooperation Project.
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