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

A challenging topic for tetrahedra-based deep-ultraviolet (deep-UV, wavelength λ < 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 α-Be 3 (H 2 O) 2 (PO 4 ) 2 crystal with edge-sharing BeO 4 and its polymorph β-Be 3 (H 2 O) 2 (PO 4 ) 2 with common corner-sharing BeO 4 were synthesized, both featuring a three-dimensional stable structure constructed by BeO 4, BeO 2 (H 2 O) 2, and PO 4 tetrahedral units. Remarkably, α-Be 3 (H 2 O) 2 (PO 4 ) 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, β-Be 3 (H 2 O) 2 (PO 4 ) 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 BeO 4 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.