Li7Cd4.5Ge4Se16 and Li6.4Cd4.8Sn4Se16: Strong Nonlinear Optical Response in Quaternary Diamond‐Like Selenide Networks

: Two new selenides with diamond-like structures, Li7 Cd4.5 Ge4 Se16 and Li6.4 Cd4.8 Sn4 Se16 , were synthesized by using a conventional high-temperature solid-state reaction method. They crystallize in the space group Pna21 (no. 33) of the orthorhombic system. Their three-dimensional frameworks consist of corner-sharing LiSe4 , CdSe4 , and MSe4 (M=Ge, Sn) tetrahedra. These two compounds exhibit strong powder second-harmonic generation responses that are about 1.2 and 2.5 times that of the benchmark AgGaS2 at a laser wavelength of λ=2.09 μm, and also demonstrate type I phase-matchable behavior. The optical bandgaps were determined to be 2.18 and 1.95 eV for Li7 Cd4.5 Ge4 Se16 and Li6.4 Cd4.8 Sn4 Se16 , respectively. Furthermore, these two materials exhibit congruent melting behavior at rather low temperatures of 985 and 1060 K, respectively, which makes bulk single crystal growth by using the Bridgman-Stockbarger method possible. Our study indicates that these two materials show advantages over the traditional IR NLO material CdSe and are promising for practical applications.