K₂RbB₈PO₁₆: A Borophosphate with Moderate Birefringence and Deep-Ultraviolet Transmission

A new borophosphate, K₂RbB₈PO₁₆ (KRBPO) was synthesized. It exhibits a bilayer structure consisting of two B-O layers with an 18-membered ring (18-MR) joined by [PO₄], which is composed of the π-conjugated group [BO₃] and non-π-conjugated groups [BO₄] and [PO₄]. The UV-vis-NIR diffuse reflectance spectroscopy shows that the cutoff edge is less than 200 nm. The calculation indicates that KRBPO exhibits moderate birefringence of 0.057@1064 nm, and the source of birefringence is mainly from the [BO₃] groups.