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

A new polyphosphate K2Sr4(PO3)(10) is synthesized by a hightemperature solution method. This compound crystallizes in the triclinic space group of P (1) over bar, consisting of the 1D infinite [PO3](infinity) chains and K and Sr ions between the chains. Compared with AM(2)(PO3)(5) (A = K, Rb, Cs; M = Ba, Pb), K2Sr4(PO3)(10) exhibits a more complex [PO3](infinity) 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](8) chains arranged in an inverted zigzag. This discovery introduces a new strategy for devising novel phosphate deep-UV optical crystals with a large birefringence.