Edge-Sharing Tetrahedra Realizing Superior Birefringence and High Deep-Ultraviolet Transmittance in -Be₃(H₂O)₂(PO₄)₂ Crystal Synthesized by Mild Chemistry

A challenging topic for tetrahedra-based deep-ultraviolet (deep-UV, wavelength lambda < 200 nm) birefringence crystals is how to generate both large birefringence and high deep-UV transmittance. Edge-sharing tetrahedra can induce large structural distortion and thus could produce strong optical polarization anisotropy to realize the challenging topic, but edge-sharing tetrahedra frequently require extreme synthesis conditions to overcome large repulsion interaction between tetrahedra. Herein, under mild hydrothermal conditions, an alpha-Be-3(H2O)(2)(PO4)(2) crystal with edge-sharing BeO4 and its polymorph beta-Be-3(H2O)(2)(PO4)(2) with common corner-sharing BeO4 were synthesized, both featuring a three-dimensional stable structure constructed by BeO4, BeO2(H2O)(2), and PO4 tetrahedral units. Remarkably, alpha-Be-3(H2O)(2)(PO4)(2) manifests superior birefringence of 0.101 and high deep-UV transmittance of 167 nm, with the largest birefringent quality factor (BQF) of 0.75 among the known tetrahedra-based deep-UV materials. By contrast, beta-Be-3(H2O)(2)(PO4)(2) manifests a small birefringence of 0.019 and 160 nm deep-UV transmittance, with only a mediocre BQF of 0.15. Further, computational investigations reveal that edge-sharing BeO4 makes a dominant contribution to the great birefringence. This study could push the synthesis of edge-sharing-tetrahedra materials and offers an alternative strategy toward designing high-performance tetrahedra-based deep-UV birefringence materials.