Noncentrosymmetric selenide Ba4Ga4GeSe12: Synthesis, structure, and optical properties

• Ba4Ga4GeSe12 was synthesized by reaction of BaSe, Ga2Se3, and GeSe2.
• Corner-sharing tetrahedra form a 3d-network in a noncentrosymmetric structure.
• One type of tetrahedra exhibits Ga/Ge disorder; another contains only Ga.
• Optical studies indicate a 2.18 eV band gap and weak SHG signal.

The selenide Ba4Ga4GeSe12, synthesized by reaction of BaSe, Ga2Se3, and GeSe2 at 1173 K, adopts a noncentrosymmetric tetragonal structure (space group P4¯21c, 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.

Ba4Ga4GeSe12 exhibits partial disorder with Ga/Ge occupying tetrahedra forming zigzag chains and Ga occupying single tetrahedra.Figure optionsDownload as PowerPoint slide