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

Abstract

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.