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 Li1.75Na1.25La2(BO3)3 have been synthesized using the high-temperature solution method. Compared with Na3La2(BO3)3, Li3La2(BO3)3 and Li1.75Na1.25La2(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 Li1.75Na1.25La2(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.