Na₆ MQ₄ (M=Zn, Cd; Q=S, Se): Promising New Ternary Infrared Nonlinear Optical Materials

Four sodium-based ternary IR nonlinear optical (NLO) materials, Na₆ MQ₄ (M=Zn, Cd; Q=S, Se), were prepared through a high-temperature flux method. The crystal structure of the compounds is built up of isolated [MQ₄ ] tetrahedra and a 3D framework formed by the NaQ_n (n=4, 5) units. The two selenides, Na₆ MSe₄ (M=Zn, Cd), as promising IR NLO materials, show moderate second-harmonic generation (SHG) responses (0.9 and 0.5×AgGaS₂ ) with good phase-matching behavior, as well as high laser damage thresholds (2 and 1.9×AgGaS₂ ). The two sulfides, Na₆ MS₄ (M=Zn, Cd), exhibit higher laser damage thresholds (13 and 4×AgGaS₂ ), but smaller SHG responses (0.3 and 0.2×AgGaS₂ ). Theoretical calculations and statistical analyses indicate that the SHG effect and band gap in the compounds originate mainly from the distorted NaQ₄ NLO-active units with a short Na-S bond length, which provides a new insight into the design of novel IR NLO materials.