Synthesis, Structure, and Properties of Li2In2MQ6 (M = Si, Ge; Q = S, Se): A New Series of IR Nonlinear Optical Materials

The four isostructural compounds Li2In2MQ6 (M = Si, Ge; Q = S, Se) have been synthesized for the first time. They crystallize in the noncentrosymmetric monoclinic space group Cc with the three-dimensional framework composed of corner-sharing LiQ4, InQ4, and MQ4 tetrahedra. The second-harmonic-generation signal intensities of the two sulfides and two selenides were close to those of AgGaS2 and AgGaSe2, respectively, when probed with a laser with 2090 nm as the fundamental wavelength. They possess large band gaps of 3.61(2) eV for Li2In2SiS6, 3.45(2) eV for Li2In2GeS6, 2.54(2) eV for Li2In2SiSe6, and 2.30(2) eV for Li2In2GeSe6, respectively. Moreover, these four compounds all melt congruently at relatively low temperatures, which makes it feasible to grow bulk crystals needed for practical application by the Bridgman–Stockbarger method.