Noncentrosymmetric selenide Ba4Ga4GeSe12: Synthesis, structure, and optical properties

The selenide Ba₄Ga₄GeSe₁₂, synthesized by reaction of BaSe, Ga₂Se₃, and GeSe₂ at 1173 K, adopts a noncentrosymmetric tetragonal structure (space group P 4 bar2₁ c , Z=2, a=13.5468(4) Å, c=6.4915(2) Å) consisting of a three-dimensional network built from two types of corner-sharing MSe₄ tetrahedra, with Ba cations occupying the intervening voids. It is isostructural to Pb₄Ga₄GeS₁₂, Pb₄Ga₄GeSe₁₂, and Ba₄Ga₄SnSe₁₂, but differs subtly in site ordering. Structural refinements and bond valence sum analysis suggest partial disorder manifested by mixing of 0.75 Ga and 0.25 Ge within one tetrahedral site, and occupation of exclusively Ga within the other tetrahedral site. The optical band gap of 2.18(2) eV, measured from the UV/VIS/NIR diffuse reflectance spectrum, agrees with a calculated gap of 2.35 eV between valence and conduction bands and is consistent with the orange-yellow color of the crystals. Nonlinear optical measurements on powder samples revealed a weak second harmonic generation signal using 2.09 μm as the fundamental laser wavelength.