LiLa(BO) and LiNaLa(BO): A Great Enhancement in Birefringence Induced by Optimal Arrangement of π-Conjugated [BO] Units.

In virtue of the essential role in controlling polarized light, outstanding ultraviolet (UV) and deep-ultraviolet (DUV) birefringent crystals are imperative in many advanced optical instruments. To design the UV and DUV crystals with large birefringence, we paid more attention to combining the excellent gene, π-conjugated [BO] unit and metal cations beneficial to blue-shift the cutoff edge; finally, two rare-earth borates LiLa(BO) and LiNaLa(BO) have been synthesized using the high-temperature solution method. Compared with NaLa(BO), LiLa(BO) and LiNaLa(BO) are isostructural, and the isolated [BO] units are arranged nearly parallel to each other in the structure, which is conducive to generating a larger birefringence. The structural comparison between the two crystals and NaLa(BO) indicates that the various coordination environments of alkali metal cations play an important role in the evolution of the crystal structure from LiLa(BO) and LiNaLa(BO) to NaLa(BO). This work can contribute to a better understanding of the enhancement in birefringence from NaLa(BO) (0.023 @ 1064 nm) to LiLa(BO) (0.078 @ 1064 nm) with the perspective of structure-property relationships. Meanwhile, the two title crystals possess the DUV cutoff edge (<190 nm), suggesting that they can be applied as the DUV birefringent crystals.