K 2 Sr 4 (PO 3 ) 10 : A Polyphosphate with Deep-UV Cutoff Edge and Enlarged Birefringence.

A new polyphosphate K ₂ Sr ₄ (PO ₃ ) ₁₀ is synthesized by a high-temperature solution method. This compound crystallizes in the triclinic space group of P 1̅, consisting of the 1D infinite [PO ₃ ] _∞ chains and K and Sr ions between the chains. Compared with AM ₂ (PO ₃ ) ₅ (A = K, Rb, Cs; M = Ba, Pb), K ₂ Sr ₄ (PO ₃ ) ₁₀ exhibits a more complex [PO ₃ ] _∞ chain structure and more diverse metal cationic coordination environment. More importantly, K ₂ Sr ₄ (PO ₃ ) ₁₀ has both a deep-UV cutoff edge (<200 nm) and a significantly enlarged birefringence. First-principles calculations indicate that the birefringence of K ₂ Sr ₄ (PO ₃ ) ₁₀ is 0.017 at 1064 nm, about 2 times that of RbBa ₂ (PO ₃ ) 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 K ₂ Sr ₄ (PO ₃ ) ₁₀ mainly originates from the [PO ₃ ] _∞ chains arranged in an inverted zigzag. This discovery introduces a new strategy for devising novel phosphate deep-UV optical crystals with a large birefringence.