BaGe₂Pn₂ (Pn = P, As): Two Congruent-Melting Non-chalcopyrite Pnictides as Mid- and Far-Infrared Nonlinear Optical Materials Exhibiting Large Second Harmonic Generation Effects

Two ternary non-chalcopyrite pnictide-based nonlinear optical (NLO) crystals, BaGe2P2 (BGP) and BaGe2As2 (BGA) with the non-centrosymmetric tetragonal space group P42mc (NO.105), were successfully synthesized by a solid-state reaction. They have a three-dimensional (3D) framework structure assembled from Ge–Ge chains connecting each other through Ge–Pn bonds. The optical measurements indicated that both compounds have wide infrared (IR) transparent windows. Powder second harmonic generation (SHG) measurements revealed that BGP and BGA not only can realize phase-matching in the IR region but also have significant SHG responses 3.3 and 4.0 times that of AgGaS2 at 2.05 μm, respectively. Furthermore, BGP and BGA are congruent-melting compounds with low melting points of 861 and 775 °C, respectively. Based on theoretical calculation, we first revealed that Ge–Ge homoatomic bonds have a contribution to SHG coefficients, which may indicate that employing homoatomic bonds including Pn–Pn bonds to construct an IR NLO crystal with a large SHG effect may be a feasible way.