LAUSR

Two rare‐earth pnictide‐based infrared (IR) nonlinear optical (NLO) crystals, LaSiP3 and LaSi2P6, are successfully synthesized by metal salt flux method, which are the first two cases for rare‐earth pnictides as IR NLO crystals. LaSiP3 (Pna21) features 2D structure consisting of alternately stacked SiP4 tetrahedra layers and isolated PP chains. LaSi2P6 (Cmc21) has a 3D structure composed of two types of SiP4 tetrahedra layers and diversiform phosphorous polyanions. Particularly, LaSi2P6 exhibits the largest second‐order NLO coefficient (d33 = 98.5 pm V−1) among the known IR NLO phosphides. Besides, LaSiP3 and LaSi2P6 both can achieve phase matching, cover wide IR transparent regions, and own large birefringence (0.24 and 0.25 @2050 nm for LaSiP3 and LaSi2P6, respectively). The studies on LaSiP3 and LaSi2P indicate that they are potentially applied in the middle‐ and far‐IR regions. In addition, more importantly, the theoretical calculations uncover the second‐harmonic‐generation enhancement mechanisms for PP bonds in different forms, which highlight that either PP chains or phosphorous polyanions will be excellent NLO‐active units for constructing high‐performance IR NLO crystals.