LiNa₄B₁₅O₂₅: Featuring Unprecedented B₁₅O₃₀ Fundamental Building Block and Deep-UV Cutoff Edge

In the Li₂O-Na₂O-B₂O₃ system, LiNa₄B₁₅O₂₅ has been successfully synthesized, which crystallizes in monoclinic space group C2 /c (No. 15), a = 14.153(3) Å, b = 12.122(2) Å, c = 12.719(2) Å, β = 105.237(13)°, and Z = 4. The isolated LiO₅ polyhedra and the Na₄O₁₈ tetramers are located in the void space of the 3D ^∞[B₁₅O₂₅] network of LiNa₄B₁₅O₂₅. The symmetry of B₁₅O₃₀ is a new fundamental building block, which has never been reported before. Moreover, the whole framework can be defined as a new topology based on the TOPOS database. Interestingly, for the crystal structure LiB₃O₅, as 4/5 Li⁺ ions are substituted by the Na⁺ ions, a new compound LiNa₄B₁₅O₂₅ has been successfully prepared. The structural comparison between the structures of LiB₃O₅ and LiNa₄B₁₅O₂₅ and the changes caused by the cation substitution are discussed in this paper.