Layered NaBa₂M₃Q₃(Q₂) (M = Cu or Ag; Q = S or Se) Chalcogenides and Local Ordering in Their Mixed-Anion Compositions

Three new NaBa₂M₃Q₃(Q₂) (M = Ag or Cu; Q = S or Se) chalcogenides were prepared by using solid-state methods and structurally characterized by using single-crystal X-ray diffraction. NaBa₂Ag₃Se₃(Se₂) and NaBa₂Cu₃Se₃(Se₂) crystallize in monoclinic space group C2/m and have a two-dimensional structure composed of edge-sharing MSe_4/4 tetrahedra separated by Na⁺ and Ba²⁺ cations, along with (Se₂)^2- dimers at the center of the spacings between [M₃Se₃]^3- slabs. NaBa₂Ag₃S₃(S₂) adopts a related structure with space group C2/m but has additional, crystallographically distinct Ag atoms in the [Ag₃S₃]^3- layer that are linearly coordinated. NaBa₂Ag₃Se₃(Se₂) and NaBa₂Ag₃S₃(S₂) have indirect band gaps measured to be 1.2 and 1.9 eV, respectively, which is supported by band structures calculated using density functional theory. Mixed-anion NaBa₂Cu₃Se_5-S compositions were prepared to probe the presence of anion ordering and heteronuclear (S-Se)^2- dimers. Structural analyses of the sulfoselenides indicate that selenium preferentially occupies the Q-Q dimer sites, while Raman spectroscopy reveals a mixture of (S₂), (Se₂), and heteronuclear (S-Se) units in the sulfur-rich products. The local ordering of the chalcogens is rationalized using simple bonding concepts and adds to a growing framework for understanding ordering phenomena in mixed-anion systems.