“…9 The birefringence phenomenon comes from structural anisotropy inside crystals. 10,11 Therefore, there are many strategies to design new birefringent crystal materials, including: (1) introducing planar π-conjugated groups (e.g., [BO 3 ], [B 3 O 6 ], [C 3 N 3 O 3 ] and [NO 3 ]); typical examples are Li 6 Zn 3 (BO 3 ) 4 , 12 BaTi (BO 3 ) 2 , 13 Ba 2 Mg(BO 3 ) 2 , 14 Ba 2 Mg(B 3 O 6 ) 2 , 15 Na 3 Ba 2 (B 3 O 6 ) 2 F, 16 AZn 4 (OH) 4 19 Ba(NO 3 )Cl, 20 Cs 2 Pb(NO 3 ) 2 Br 2 , 21 23 and A(H 3 C 3 N 3 O 3 )(NO 3 ); 24 (2) incorporating heteroanionic tetrahedral groups, such as [BO x F 4−x ](x = 1-3), 25 [PO 3 F], 26,27 [PO 3 S], 28,29 and [SO 3 S]; 30,31 (3) adding d 0 metal cations with a second-order Jahn-Teller (SOJT) effect (e.g., Mo 6+ , V 5+ , and Nb 5+ ); [32][33][34] and (4) adding lone pair cations with stereoscopic activity (e.g., Sb 3+ , Pb 2+ , Sn 2+ , and Bi 3+ ). 33,[35][36][37] In recent years, it has been found that π-conjugated [C(NH 2 ) 3 ] cations exhibit greater anisotropic polarizability and larger second-order polarizability than [BO 3 ] units.…”