Vacancy-Driven Tetrahedral Distortion Leading to Exceptional Second Harmonic Generation
Cation vacancies were engineered into diamond-like (DL) chalcogenides to amplify tetrahedral distortion and second harmonic generation (SHG). Substitution of Ge4+ by P5+ in Cd4GeS6 yielded the defective DL phase Cd3.5PS6, which exhibits intrinsic Cd2+ vacancies and an 8.5-fold increase in [CdS4] tetrahedral distortion relative to Cd4GeS6. Consequently, Cd3.5PS6 achieves a 2 × AgGaS2 (AGS) SHG response at 2050 nm and a laser-induced damage threshold (LIDT) of 9.4 × AGS. Further equivalent substitution of Hg2+ concentrates Cd2+ vacancies at the Cd(2) site, producing Hg0.5Cd3PS6 with a 2.66-fold enhancement in [CdS4] distortion over Cd3.5PS6. This material delivers a 2.73 × AGS SHG response at 2050 nm, an LIDT of 5 × AGS, and a birefringence of 0.076 at 2050 nm. The results establish cation vacancies and mixed-atom radius scaling as effective levers for designing high-performance mid-infrared nonlinear optical crystals, circumventing the low LIDT of AgGaS2 and two-photon absorption of ZnGeP2.