Li3La2(BO3)3 and Li1.75Na1.25La2(BO3)3: A Great Enhancement in Birefringence Induced by Optimal Arrangement of π-Conjugated [BO3] 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 [BO₃] unit and metal cations beneficial to blue-shift the cutoff edge; finally, two rare-earth borates Li₃La₂(BO₃)₃ and Li_1.75Na_1.25La₂(BO₃)₃ have been synthesized using the high-temperature solution method. Compared with Na₃La₂(BO₃)₃, Li₃La₂(BO₃)₃ and Li_1.75Na_1.25La₂(BO₃)₃ are isostructural, and the isolated [BO₃] 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 Na₃La₂(BO₃)₃ indicates that the various coordination environments of alkali metal cations play an important role in the evolution of the crystal structure from Li₃La₂(BO₃)₃ and Li_1.75Na_1.25La₂(BO₃)₃ to Na₃La₂(BO₃)₃. This work can contribute to a better understanding of the enhancement in birefringence from Na₃La₂(BO₃)₃ (0.023 @ 1064 nm) to Li₃La₂(BO₃)₃ (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.