Cs 2 Mg(H 2 C 3 N 3 S 3 ) 4 ·8H 2 O: An Excellent Birefringent Material with Giant Optical Anisotropy in π-Conjugated Trithiocyanurate.

The design of giant birefringence was performed by adjusting cations to make parallel and compact alignments of π-conjugated (H x C 3 N 3 S 3 ) x -3 , where x = 1 and 2) groups with large polarizability anisotropy. Finally, the first mixed alkali/alkali-earth-metal trithiocyanurates, A 2 B(H 2 C 3 N 3 S 3 ) 4 · n H 2 O (A = K, Rb, Cs; B = Mg, Sr; n = 5-8, 12), were designed and synthesized successfully. Importantly, Cs 2 Mg(H 2 C 3 N 3 S 3 ) 4 ·8H 2 O ( III ) and K 2 Sr(H 2 C 3 N 3 S 3 ) 4 ·5H 2 O ( IV ) possess large birefringences of 0.580 and 0.194 at 800 nm, respectively, of III has the largest birefringence among all practical birefringent crystals, cyanurates, and hydroisocyanurates.