K₂Sr₄(PO₃)₁₀: A Polyphosphate with Deep-UV Cutoff Edge and Enlarged Birefringence

A new polyphosphate K₂Sr₄(PO₃)₁₀ is synthesized by a high-temperature solution method. This compound crystallizes in the triclinic space group of P1̅, consisting of the 1D infinite [PO₃]_∞ chains and K and Sr ions between the chains. Compared with AM₂(PO₃)₅ (A = K, Rb, Cs; M = Ba, Pb), K₂Sr₄(PO₃)₁₀ exhibits a more complex [PO₃]_∞ chain structure and more diverse metal cationic coordination environment. More importantly, K₂Sr₄(PO₃)₁₀ has both a deep-UV cutoff edge (<200 nm) and a significantly enlarged birefringence. First-principles calculations indicate that the birefringence of K₂Sr₄(PO₃)₁₀ is 0.017 at 1064 nm, about 2 times that of RbBa₂(PO₃)₅ (0.008 at 1064 nm), which reaches a new height among the reported mixed alkali metal and alkaline earth metal phosphate. Theoretical calculations and structural analyses show that the enlarged birefringence of K₂Sr₄(PO₃)₁₀ mainly originates from the [PO₃]_∞ chains arranged in an inverted zigzag. This discovery introduces a new strategy for devising novel phosphate deep-UV optical crystals with a large birefringence.