Cubic Crystal Structure Formation and Optical Properties within the Ag-B^II-M^IV-X (B^II = Sr, Pb; M^IV = Si, Ge, Sn; X = S, Se) Family of Semiconductors.

Quaternary chalcogenide semiconductors are promising materials for energy conversion and nonlinear optical applications, with properties tunable primarily by varying the elemental composition and crystal structure. Here, we first analyze the connections among several cubic crystal structure types, as well as the orthorhombic Ag₂PbGeS₄-type structure, reported for select members within the Ag-B^II-M^IV-X (B^II = Sr, Pb; M^IV = Si, Ge, Sn; X = S, Se) compositional space. Focusing on the Ag-Pb-Si-S and Ag-Sr-Sn-S systems, we show that one structure type, with the formulas Ag₂Pb₃Si₂S₈ and Ag₂Sr₃Sn₂S₈, is favored. We have prepared powder and single-crystal samples of Ag₂Pb₃Si₂S₈ and Ag₂Sr₃Sn₂S₈, showing that each takes on the noncentrosymmetric cubic space group I4̅3d and is isostructural to the previously reported compound Ag₂Sr₃Ge₂Se₈. Through hybrid density functional theory calculations, these cubic compounds are demonstrated to be (quasi-)direct band gap semiconductors with high densities of states at the band maxima. The band-gap energies are measured by reflectance spectroscopy as 1.95(3) and 2.66(4) eV for Ag₂Pb₃Si₂S₈ and Ag₂Sr₃Sn₂S₈, respectively. We further measure the optical properties and show the electronic band structures of three other isostructural A^I-B^II-M^IV-X-type materials, i.e., Ag₂Sr₃Si₂S₈, Ag₂Sr₃Ge₂S₈, and Ag₂Sr₃Ge₂Se₈, showing that the band gaps can be predictably tuned by element substitution. Detailed visual analyses of the different structures and of their relationships with other members of the Ag-B^II-M^IV-X compositional family provide a basis for a broader understanding of the structure formation and optoelectronic properties within the quaternary chalcogenide semiconductor family.

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Author Volker Blum Thomas Lord Department of Mechanical Engineering and Materia ...

Author David Mitzi Thomas Lord Department of Mechanical Engineering and Materia ...