Synthesis and Characterization of the New Quaternary One-Dimensional Chain Materials K₂CuNbSe₄ and K₃CuNb₂Se₁₂

The one-dimensional chain materials K₂CuNbSe₄ and K₃CuNb₂Se₁₂ have been synthesized at 800 and 870 °C, respectively, through the use of molten alkali-metal selenides as reactive fluxes. K₂CuNbSe₄ crystallizes in space group [formula omited]-Fddd of the orthorhombic system with eight formula units in a cell of dimensions a = 5.745 (1), b = 13.444 (1), and c = 23.907 (3) Å. The structure consists of infinite linear chains separated from the K⁺ ions. These chains, which are along the c axis, consist of edge sharing of alternating NbSe₄ and CuSe₄ tetrahedra. The structural motif thus represents an elaboration of that in KFeS₂. There are no short Se⋯Se interactions and so formal oxidation states of K(I), Cu(I), Nb(V), and Se(-II) are assigned. The compound is a poor conductor, having a resistance greater than 10 MΩ cm at room temperature. K₃CuNb₂Se₁₂ crystallizes in space group [formula omited]-P2₁/n of the monoclinic system with four formula units in a cell with dimensions a = 9.510 (6), b = 13.390 (9), and c = 15.334 (10) Å and β = 96.09 (4)°. The structure consists of an infinite Cu/Nb/Se chain separated from K⁺ cations. The infinite chain can be formulated as [formula omited][CuNb₂(Se)₂(Se₂)₃(Se₄)^3-] or alternatively as [formula omited][CuNb₂(Se)₃(Se₂)₃(Se₃)^3-] depending upon the choice of a cutoff for the length of an Se-Se bond. In the former instance the chain contains Cu(I) and Nb(IV) centers while in the latter instance it contains Cu(I) and Nb(V) centers. The two crystallographically distinct Nb atoms are seven-coordinate and the Cu atom is tetrahedral.