Abstract: The first rare-earth borate crystal CsBa₂ScB₈O₁₆ with both zero-dimensional [B₃O₆] units and one-dimensional B—O chain structures has been synthesized by the high-temperature solution method. It crystallizes in the triclinic crystal system with the P1 space group, with lattice parameters of a=6.698 5 Å, b=7.216 Å, c=14.798 Å, α=97.563°, β=95.226°, γ=95.546°, Z=2. In this compound, [BO₃] and [B₄O₉] units are connected by sharing oxygen atoms to form a one-dimensional chain [B₅O₁₀]_∞. Octahedrally coordinated [Sc(1)O₆] groups and the one-dimensional chain [B₅O₁₀]_∞ are further connected by sharing oxygen atoms to form two-dimensional layer [Sc(1)(B₄O₉)]_∞. Interlayers are linked by octahedrally coordinated [Sc(2)O₆] groups, which share oxygen atoms to form a three-dimensional Sc—B—O framework. Isolated [B₃O₆] units are filled in the three-dimensional channels with Cs⁺ and Ba²⁺ to balance the charges. In order to further explore the novelty of the structure of CsBa₂ScB₈O₁₆, we compared it with other rare-earth borates containing alkali or alkaline-earth metals, and discuss the effect of the cation-to-boron molar ratio (n(A)/n(B)) on the degree of polymerization of B—O units, as well as the dimensionality of the B—O anion framework. Furthermore, the first-principles calculations, infrared spectrum, UV-vis-NIR diffuse reflectance spectrum and the thermal analysis of the compounds have also been performed. Property measurements show that CsBa₂ScB₈O₁₆ exhibits short UV absorption edge (＜190 nm) and moderate birefringence (0.072@1 064 nm). The optical properties of the compound are mainly contributed by the [B₃O₆], [B₅O₁₀] units, and [ScO₆] octahedra.

Key words: rare-earth borate, high-temperature solution method, crystal structure, B—O framework, first-principles calculation, birefringence