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 [LiOn] (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.