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 3(H2O)2(PO4)2 crystal with edge-sharing BeO4 and its polymorph β-Be3(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, α-Be3(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, β-Be3(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.