Polar Antiperovskite Oxides: Promising Mid-Infrared Nonlinear Optical Materials with Strong Second-Harmonic Generation

Despite significant advances in nonlinear optical (NLO) materials, a systematic strategy for designing mid-infrared (mid-IR) NLO oxides that simultaneously exhibit strong second-harmonic generation (SHG) efficiency and wide optical transparency remains elusive. Herein, we report two polar antiperovskite oxide materials, (Pb1.5Cd1.5)GeO5 (PCGO) and Pb3GeO5 (PGO). Structural analyses reveal that PCGO adopts a 2H-hexagonal antiperovskite structure, whereas PGO features a three-dimensional antiperovskite framework. Both materials are thermally stable up to approximately 900 °C and exhibit wide optical transparency extending into the mid-IR region (0.3–13 μm). Powder SHG measurements indicate that PCGO and PGO exhibit strong SHG efficiencies of 0.5 and 1.3 times that of AgGaS2, respectively, along with particle-size-dependent SHG responses indicative of phase-matching behavior. Further first-principles calculations reveal moderate birefringence values of 0.053 and 0.058 at 1064 nm for PCGO and PGO, respectively, corresponding to shortest phase-matching (PM) wavelengths of 737 and 776 nm. A closer structural investigation and density functional theory calculations suggest that the pronounced distortions of OPb/Cd6 and OPb6 octahedra, together with highly polarizable cations, play a dominant role in governing the SHG responses. These results highlight polar antiperovskite oxides as a promising platform for the development of next-generation mid-IR NLO materials.