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, 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.