Ba4Ga2Se8: A ternary selenide containing chains and discrete Se22− units

The ternary selenide Ba4Ga2Se8 has been synthesized by reaction of BaSe, Ga2Se3, and Se at 1023 K. Single-crystal X-ray diffraction analysis revealed a monoclinic structure (space group P21/c, Z=4, a=13.2393(5) A, b=6.4305(2) A, c=20.6432(8) A, β  =104.3148(6)°) featuring one-dimensional chains of corner-sharing Ga-centered tetrahedra and discrete Se22− anionic units, with charge-compensating Ba2+ cations located between them. The UV/vis/NIR diffuse reflectance spectrum reveals an optical band gap of 1.63(2) eV, which is consistent with the black color of the crystals and agrees with a calculated gap of 1.51 eV obtained from band structure calculations. The presence of the Se22− units narrows the band gap in Ba4Ga2Se8 relative to other Ba–Ga–Se phases.