Li₄Ca₂B₈O₁₆: A Borate with a Unique Fundamental Building Block and a Short Cutoff Edge

A complex alkali and alkaline-earth metal borate Li₄Ca₂B₈O₁₆ has been synthesized successfully via the high-temperature solution method. Li₄Ca₂B₈O₁₆ crystallizes into the space group P1̅ (No. 2) of the triclinic crystal system. The crystal structure exhibits a three-dimensional framework consisting of the [B₈O₁₆]_∞ chains connected by the [LiO] (n = 3, 4, 6) and [CaO₇] polyhedra. The fundamental building block [B₈O₁₈] differs from those of other anhydrous octa-borates, which can be identified as a unique one. Furthermore, Li₄Ca₂B₈O₁₆ shows three different Li-O units, which is unique among all non-disordered anhydrous borates. It shows a short UV cutoff edge less than 190 nm. To better study the relationships between the crystal structure and properties, the DFT calculations were used for the evaluations of the optical band gap and birefringence.