Synthesis and characterization of the new quaternary two-dimensional materials KCu₂NbQ₄(Q = Se, S)

Reaction of K₂Se₅ or K₂S₅ with elemental Nb, Cu, and Se or S affords KCu₂NbSe₄ or KCu₂NbS₄. The structure of KCu₂NbSe₄ has been determined by single crystal X-ray diffraction methods. KCu₂NbSe₄ crystallizes with four formula units in the space group C¹⁶_2v-C2cm of the orthorhombic system in a cell of dimensions a = 5.663(2) Å, b = 18.791(5) Å, c = 7.645(3) Å at -167°C. The structure consists of two-dimensional metal-chalcogen layers separated by K⁺ cations. The Nb atom and two independent Cu atoms are in tetrahedral sites, and there appears to be some substitutional disorder among these sites, as judged from the isotropic thermal parameters. The CuSe₄ and NbSe₄ tetrahedra share edges and corners to make up the two-dimensional layers. If we arbitrarily choose a cutoff of 4.00 Å then the K atom is coordinated by nine Se atoms. The metal tetrahedra have their expected dimensions with NbSe distances varying from 2.392(3) to 2.452(3) Å and CuSe distances from 2.427(2) to 2.440(4) Å. There are short metal-metal interactions with NbCu distances at 2.831(1) and 2.833(4) Å. From Weissenberg photography KCu₂NbS₄ is found to be isostructural: cell dimensions at room temperature are a = 5.76 Å, b = 18.46 Å, c = 7.45 Å.