Band gap tuning from an indirect EuGa2S4 to a direct EuZnGeS4 semiconductor: syntheses, crystal and electronic structures, and optical properties

Four isostructural europium chalcogenides, EuZnGeS4 (1), EuGa2S4 (2), EuIn2S4 (3) and EuIn2Se4 (4), have been synthesized using a high-temperature solid-state method. Single-crystal X-ray diffraction analysis indicates that they crystallize in the orthorhombic space group Fddd with Z = 32. Their structures feature an MQ4 (M = Zn, Ge, Ga and In; Q = S and Se) tetrahedrally constructed 3D network, and Eu2+ ions occupy the bicapped trigonal prismatic cavities. Compounds 1–3 have optical band gaps of 2.26, 2.32 and 2.37 eV, respectively. Theory calculations indicate that 2, with an indirect band gap, can be tuned to 1, with a direct band gap, via simple chemical substitution.