Nb2Se2I6: A new structural variation of niobium chalcogenide halides with chain structure

Single crystals of Nb2Se2I6, exhibiting a metallic lustre, (monoclinic, C2/ c; a = 7.110(1), b = 13.899(3), c = 13.688(3) Angstrom, beta = 99.58(3)degr ees; Z = 8) have been obtained by heating mixture of the elements (molar ra tio Nb:Se:I = 3:1:7) in evacuated sealed fused silica tubes at 1073 K for t wo weeks. After rapid quenching, the tube was reheated to 773 K and then co oled at a rate of 2 K h(-1). Crystals of the title compound formed at the c older part of the tube besides higher niobium iodides. Nb2Se2I6 can also be obtained by slow cooling of a stoichiometric mixture of the elements from 1073 K as a minor product besides triclinic NbSe2I2 [1]. The crystal struct ure is built up with one-dimensional infinite chains (1)(infinity)[Nb2Se2I4 I1/2]. The main structural features are a short Nb-Nb distance (2.903(1) An gstrom) and a short Se-Se distance (2.318(2) Angstrom) which is expected fo r +4 and -1 oxidation states, respectively Homeotypic phases with Nb2Y2X6 c hains (Y = Se, Te; X = Br, I) are known [2] except for the combination Y = Se, X = I. They show a primitive rod packing of the Nb2Y2X6 chains in contr ast to the title compound which shows a centered rod packing.