BaAl4Se7: a new infrared nonlinear optical material with a large band gap.

The new compound BaAl(4)Se(7) has been synthesized by solid-state reaction. It crystallizes in the non-centrosymmetric space group Pc and adopts a three-dimensional framework built from AlSe(4) tetrahedra and with Ba(2+) cations in the cavities. The material has a large band gap of 3.40(2) eV. It melts congruently at 901 °C and exhibits a second harmonic generation (SHG) response at 1 μm that is about half that of AgGaS(2). From a band structure calculation, BaAl(4)Se(7) is a direct-gap semiconductor with strong hybridization of the Al 3s, Al 3p, and Se 4p orbitals near the Fermi level. The calculated birefractive index is about 0.05 for wavelength longer than 1 μm and major SHG tensor elements are: d(15) = 5.2 pm V(-1) and d(13) = 4.2 pm V(-1).