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 NaQn (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.