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

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 (Pna2(1)) features 2D structure consisting of alternately stacked SiP4 tetrahedra layers and isolated P-P chains. LaSi2P6 (Cmc2(1)) 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 (d(33) = 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.