Layered NaBa<sub>2</sub>M<sub>3</sub>Q<sub>3</sub>(Q<sub>2</sub>) (M = Cu or Ag; Q\n= S or Se) Chalcogenides and Local\nOrdering in Their Mixed-Anion Compositions

Three new NaBa<sub>2</sub>M<sub>3</sub>Q<sub>3</sub>(Q<sub>2</sub>) (M = Ag or Cu; Q = S or Se) chalcogenides were prepared\nby using\nsolid-state methods and structurally characterized by using single-crystal\nX-ray diffraction. NaBa<sub>2</sub>Ag<sub>3</sub>Se<sub>3</sub>(Se<sub>2</sub>) and NaBa<sub>2</sub>Cu<sub>3</sub>Se<sub>3</sub>(Se<sub>2</sub>) crystallize in monoclinic space group <i>C</i>2/<i>m</i> and have a two-dimensional structure composed\nof edge-sharing MSe<sub>4/4</sub> tetrahedra separated by Na<sup>+</sup> and Ba<sup>2+</sup> cations, along with (Se<sub>2</sub>)<sup>2–</sup> dimers at the center of the spacings between [M<sub>3</sub>Se<sub>3</sub>]<sup>3–</sup> slabs. NaBa<sub>2</sub>Ag<sub>3</sub>S<sub>3</sub>(S<sub>2</sub>) adopts a related structure with space\ngroup <i>C</i>2/<i>m</i> but has additional, crystallographically\ndistinct Ag atoms in the [Ag<sub>3</sub>S<sub>3</sub>]<sup>3–</sup> layer that are linearly coordinated. NaBa<sub>2</sub>Ag<sub>3</sub>Se<sub>3</sub>(Se<sub>2</sub>) and NaBa<sub>2</sub>Ag<sub>3</sub>S<sub>3</sub>(S<sub>2</sub>) have indirect band gaps measured to\nbe 1.2 and 1.9 eV, respectively, which is supported by band structures\ncalculated using density functional theory. Mixed-anion NaBa<sub>2</sub>Cu<sub>3</sub>Se<sub>5–<i>x</i></sub>S<sub><i>x</i></sub> compositions were prepared to probe the presence\nof anion ordering and heteronuclear (S–Se)<sup>2–</sup> dimers. Structural analyses of the sulfoselenides indicate that\nselenium preferentially occupies the Q–Q dimer sites, while\nRaman spectroscopy reveals a mixture of (S<sub>2</sub>), (Se<sub>2</sub>), and heteronuclear (S–Se) units in the sulfur-rich products.\nThe local ordering of the chalcogens is rationalized using simple\nbonding concepts and adds to a growing framework for understanding\nordering phenomena in mixed-anion systems.