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

Three new NaBaMQ(Q) (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. NaBaAgSe(Se) and NaBaCuSe(Se) crystallize in monoclinic space group 2/ and have a two-dimensional structure composed of edge-sharing MSe tetrahedra separated by Na and Ba cations, along with (Se) dimers at the center of the spacings between [MSe] slabs. NaBaAgS(S) adopts a related structure with space group 2/ but has additional, crystallographically distinct Ag atoms in the [AgS] layer that are linearly coordinated. NaBaAgSe(Se) and NaBaAgS(S) 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 NaBaCuSeS compositions were prepared to probe the presence of anion ordering and heteronuclear (S-Se) dimers. Structural analyses of the sulfoselenides indicate that selenium preferentially occupies the Q-Q dimer sites, while Raman spectroscopy reveals a mixture of (S), (Se), 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.