The four compounds BaGa(2)MQ(6) (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)Q(4) (M = Si, Ge; Q = S, Se) tetrahedra with Ba(2+) cations in the cavities. The polar alignment of one (Ga/M)-Q2 bond for each (Ga/M)Q(4) 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 AgGaS(2) and those for the two selenides were similar as that of AgGaSe(2). The large band gaps of 3.75(2) eV, 3.23(2) eV, 2.88(2) eV, and 2.22 (2) eV for BaGa(2)SiS(6), BaGa(2)GeS(6), BaGa(2)SiSe(6), and BaGa(2)GeSe(6), respectively, will be very helpful to increase the laser damage threshold. Moreover, all the four BaGa(2)MQ(6) (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 BaGa(2)MQ(6) 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 ZnGeP(2).

The new compound LiGaGe(2)Se(6) has been synthesized. It crystallizes in the orthorhombic space group Fdd2 with a = 12.501(3) Å, b = 23.683(5) Å, c = 7.1196(14) Å, and Z = 8. The structure is a three-dimensional framework composed of corner-sharing LiSe(4), GaSe(4), and GeSe(4) tetrahedra. The compound exhibits a powder second harmonic generation signal at 2 μm that is about half that of the benchmark material AgGaSe(2) and possesses a wide band gap of about 2.64(2) eV. LiGaGe(2)Se(6) melts congruently at a rather low temperature of 710 °C, which indicates that bulk crystals can be obtained by the Bridgman-Stockbarger technique. According to a first-principles calculation, there is strong hybridization of the 4s and 4p orbitals of Ga, Ge, and Se around the Fermi level. The calculated birefractive index is Δn = 0.04 for λ ≥ 1 μm, and the calculated major SHG tensor elements are d(15) = 18.6 pm/V and d(33) = 12.8 pm/V. This new material is promising for application in IR nonlinear optics.