K₂PbB₅P₃O₁₇: A Mixed-Coordinated Borophosphate with Enhanced Birefringence Driven by the [B₂O₅] Unit

Mixed-coordinated borophosphates containing π-conjugated [BO₃] and non-π-conjugated [BO₄] and [PO₄] tetrahedra have been a research hot spot benefiting from their diverse structures and the presentation of pre-eminent optical performances. Through the high-temperature solution method, a new mixed-coordinated borophosphate K₂PbB₅P₃O₁₇ was synthesized. This compound exhibits an unprecedented [B₅P₃O₂₁] fundamental building block consisting of apex-sharing [B₃O₇], [BO₄], and [PO₄] units, possessing a short ultraviolet cutoff edge of 234 nm as well as moderate birefringence (0.045@1064 nm). Compared with the tetra-coordinated borophosphate KPbBP₂O₈ with the same elemental composition, the birefringence of K₂PbB₅P₃O₁₇ achieves a great enhancement induced by the introduction of the planar [B₂O₅] unit.