“…28 Recently, researchers have proposed design strategies to improve the birefringence and NLO properties of non-π-conjugated systems. 29 One of these strategies involves the partial substitution of anions in tetrahedral units to form heteroanion groups like BO x F 4− x ( x = 1, 2, 3), 30 PO x F 4− x ( x = 1, 2, 3), 31 SO x F 4− x ( x = 1, 2, 3), 32 and SiO x F 6− x ( x = 1, 2, 3, 4, 5). 33 Indeed, these non-π-conjugated heteroanion groups result in large birefringence and NLO coefficients of compounds like NH 4 B 4 O 6 F, 34 (NH 4 ) 2 PO 3 F, 31,35 NaNH 4 PO 3 F·H 2 O, 35,36 C(NH 2 ) 3 SO 3 F 37 and so on.…”