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-degrees-C, respectively, through the use of molten alkali-metal selenides as reactive fluxes. K2CuNbSe4 crystallizes in space group D2h24-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) angstrom. 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 am 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 M-OMEGA-cm at room temperature. K3CuNb2Se12 Crystallizes in space group C2h5-P2(1)/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) angstrom and beta = 96.09 (4)-degrees. The structure consists of an infinite Cu/Nb/Se chain separated from K+ cations. The infinite chain can be formulated as infinity 1[CuNb2(Se)2(Se2)3(Se4)3-] or alternatively as infinity 1[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.