Realization of Enlarged Birefringence from BaCdBe 2 (BO 3 ) 2 F 2 to NaMgBe 2 (BO 3 ) 2 F via the Cation Size Effect as a Potential Deep-Ultraviolet Birefringent Material.

Birefringence, as one of the most important factors for birefringent materials, governs their performances in applications. In this study, two previously unreported beryllium borates, BaCdBe 2 (BO 3 ) 2 F 2 (BDBBF) and NaMgBe 2 (BO 3 ) 2 F (NMBBF), have been rationally designed by modulating interstitial cations. When smaller sizes of the cations are used, the crystal structure of NMBBF exhibits closer-packed 2D [Be 6 B 6 O 12 F 3 ] ∞ double layers rather than the 2D [Be 3 B 3 O 6 F 3 ] ∞ single layers in the crystal structure of BDBBF. The ultraviolet (UV) transmittance spectrum indicates that the short UV absorption edges of BDBBF and NMBBF are below 200 nm. The results from both theoretical calculations (theo.) and experimental characterizations (exp.) reveal enlarged birefringence from BDBBF (0.067 at 589 nm from theo. and 0.059 at 546.1 nm from exp.) to NMBBF (0.078 at 589 nm from theo. and 0.081 at 546.1 nm from exp.). Because of its excellent structure-based optical properties, NMBBF has the potential to be a deep-UV birefringent material. Our structural comparison and discussion provide a scope to aid in the design of potential deep-UV birefringent materials with large birefringence.