LaSiP3 and LaSi2P6: Two Excellent Rare‐Earth Pnictides with Strong SHG Responses as Mid‐ and Far‐Infrared Nonlinear Optical Crystals

Abstract Two rare‐earth pnictide‐based infrared (IR) nonlinear optical (NLO) crystals, LaSiP 3 and LaSi 2 P 6 , are successfully synthesized by metal salt flux method, which are the first two cases for rare‐earth pnictides as IR NLO crystals. LaSiP 3 ( Pna 2 1 ) features 2D structure consisting of alternately stacked SiP 4 tetrahedra layers and isolated PP chains. LaSi 2 P 6 ( Cmc 2 1 ) has a 3D structure composed of two types of SiP 4 tetrahedra layers and diversiform phosphorous polyanions. Particularly, LaSi 2 P 6 exhibits the largest second‐order NLO coefficient ( d 33 = 98.5 pm V −1 ) among the known IR NLO phosphides. Besides, LaSiP 3 and LaSi 2 P 6 both can achieve phase matching, cover wide IR transparent regions, and own large birefringence (0.24 and 0.25 @2050 nm for LaSiP 3 and LaSi 2 P 6 , respectively). The studies on LaSiP 3 and LaSi 2 P 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.