NaBa 3 [M 2 B 7 O 16 (OH) 2 ]F 2 (M = Ge, Si): Two Synthetic F Analogues of Garrelsite with [B 7 O 16 (OH) 2 ] 13- Polyanions and Deep-Ultraviolet Cutoff Edges.

Two new borogermanate/borosilicate fluorides, namely, NaBa ₃ [M ₂ B ₇ O ₁₆ (OH) ₂ ]F ₂ (M = Ge, Si), have been successfully synthesized through a conventional mild hydrothermal method. They represent the first examples of mixed alkali and alkaline-earth borogermanate/borosilicate fluorides. NaBa ₃ [M ₂ B ₇ O ₁₆ (OH) ₂ ]F ₂ (M = Ge, Si) crystallize in the space group of C 2/ c , and their structures feature a unique 3D anionic framework composed of [B ₇ O ₁₆ (OH) ₂ ] ^13- polyanions corner-sharing with SiO ₄ or GeO ₄ tetrahedra, forming 1D 10-membered-ring tunnels along the b axis, which are filled by Na ⁺ , Ba ²⁺ , and F ^- ions. UV-vis-near-IR absorption spectra identify the title compounds possessing short deep-ultraviolet absorption edges (below 200 nm), while their birefringences were calculated to be 0.021 and 0.016 at 1064 nm, respectively. Optical property, thermal stability, and theoretical calculations have also been conducted on NaBa ₃ [M ₂ B ₇ O ₁₆ (OH) ₂ ]F ₂ (M = Ge, Si).