Synthesis, structure, and optical properties of K2.4Ga2.4M1.6Q8 (M = Si, Ge; Q = S, Se) crystals and glasses

Abstract

Four new compounds, K_2.4Ga_2.4M_1.6Q₈ (M = Si, Ge; Q = S, Se), were successfully synthesized via a reactive flux method. All compounds crystallize in the tetragonal system of space group I4/mcm, and belong to the Tl₂Se₂ structure type (Pearson symbol tI16). The isostructural scaffold of the compounds feature [(Ga_0.6/M_0.4)Q_4/2] one-dimensional chains separated by K⁺ ions. The chains are composed of edge-sharing [(Ga_0.6/M_0.4)Q_4/2] tetrahedrons. Powder and glass samples were synthesized from solid state reactions with different annealing processes. The transport properties of crystalline K_2.4Ga_2.4Ge_1.6S₈ and K_2.4Ga_2.4Ge_1.6Se₈ were determined from optical absorption measurements to be wide band gap semiconductors (E_g = 3.5 eV for K_2.4Ga_2.4Ge_1.6S₈ and 2.7 eV for K_2.4Ga_2.4Ge_1.6Se₈). A red shift of the band gap energy in the glass samples was observed. K_2.4Ga_2.4Ge_1.6S₈ glass showed a wide transparent range from 0.6 µm to 10 µm and a high soften-temperature of 533 °C. First-principles calculations fitted well with the optical measurements, indicating that the Ge substitution is beneficial for narrowing the band gap.