Li₇ Cd_4.5 Ge₄ Se₁₆ and Li_6.4 Cd_4.8 Sn₄ Se₁₆ : Strong Nonlinear Optical Response in Quaternary Diamond-Like Selenide Networks

Two new selenides with diamond-like structures, Li₇ Cd_4.5 Ge₄ Se₁₆ and Li_6.4 Cd_4.8 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 Cd_4.5 Ge₄ Se₁₆ and Li_6.4 Cd_4.8 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.