BaGa2MQ6 (M = Si, Ge; Q = S, Se): a new series of promising IR nonlinear optical materials

Abstract

The four compounds BaGa₂MQ₆ (M = Si, Ge; Q = S, Se) have been identified as a new series of IR nonlinear optical (NLO) materials and are promising for practical applications. They are isostructural and crystallize in the noncentrosymmetric polar space group R3 of the trigonal system. Their three-dimensional framework is composed of corner-sharing (Ga/M)Q₄ (M = Si, Ge; Q = S, Se) tetrahedra with Ba²⁺ cations in the cavities. The polar alignment of one (Ga/M)–Q2 bond for each (Ga/M)Q₄ tetrahedra along the c direction is conducive to generating a large NLO response, which was confirmed by powder second-harmonic generation (SHG) using a 2090 nm laser as fundamental wavelength. The SHG signal intensities of the two sulfides were close to that of AgGaS₂ and those for the two selenides were similar as that of AgGaSe₂. The large band gaps of 3.75(2) eV, 3.23(2) eV, 2.88(2) eV, and 2.22 (2) eV for BaGa₂SiS₆, BaGa₂GeS₆, BaGa₂SiSe₆, and BaGa₂GeSe₆, respectively, will be very helpful to increase the laser damage threshold. Moreover, all the four BaGa₂MQ₆ (M = Si, Ge; Q = S, Se) compounds exhibit congruent-melting behavior, which indicates that bulk crystals needed for practical applications can be obtained by the Bridgman–Stockbarger method. The calculated birefringence indicates that these materials may be phase-matchable in the IR region and the calculated SHG coefficients agree with the experimental observations. According to our preliminary study, the BaGa₂MQ₆ compounds represent a new series of promising IR nonlinear optical (NLO) materials which do not belong to the traditional chalcopyrite-type materials such as AgGaQ2 (Q = S, Se) and ZnGeP₂.