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) Å, b=6.4305(2) Å, c=20.6432(8) Å, β=104.3148(6)°) featuring one-dimensional chains of corner-sharing Ga-centered tetrahedra and discrete Se₂^2- 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 Se₂^2- units narrows the band gap in Ba₄Ga₂Se₈ relative to other Ba-Ga-Se phases.