M1−x[W 2O 2X6] with M=K +, Tl +, Ag +, Hg 2+, Pb 2+; X=Cl, Br—A class of mixed valence tungsten (IV,V) compounds with layered structures, W–W bonds and high conductivity

Abstract

The crystal structure of WOCl 3, determined on the basis of powder diffraction data (tetragonal, P4 2/ mnm, a = 10.6856 ( 6 ) , c = 3.8537 ( 2 ) ), is isotypic to WOI 3 and contains one-dimensional strands of edge-sharing double-octahedral W 2O 4/2Cl 6 groups connected via common corners in trans position. A W–W bond of 2.99 Å is present within the planar W 2Cl 6 groups. A series of non-stochiometric, mixed valence W(IV,V) compounds M 1− x [W 2O 2Cl 6] can be obtained from WOCl 3 by reaction with metal halides (TlCl, KCl, PbCl 2) or by reaction of elemental Hg with WOCl 4. All were characterized by single crystal structure determinations and EDX measurements (Tl 0.981(2)[W 2O 2Cl 6]: monoclinic, C2/ m, a = 12.7050 ( 4 ) , b = 3.7797 ( 1 ) , c = 10.5651 ( 3 ) Å , β = 107.656 ( 1 ) ° ; K 0.84(2)[W 2O 2Cl 6]: monoclinic, C2/ m, a = 12.812 ( 3 ) , b = 3.7779 ( 6 ) , c = 10.196 ( 3 ) Å , β = 107.422 ( 8 ) ° ; Pb 0.549(3)[W 2O 2Cl 6]: orthorhombic, Immm, a = 3.7659 ( 1 ) , b = 9.8975 ( 4 ) , c = 12.1332 ( 6 ) Å ; Hg 0.554(6)[W 2O 2Cl 6]: monoclinic, C2/ m, a = 12.8361 ( 8 ) , b = 3.7622 ( 3 ) , c = 10.2581 ( 9 ) Å , β = 113.645 ( 3 ) ° ). Two representatives of this family of compounds have already been reported: Na[W 2O 2Br 6] [Y.-Q. Zhang, K. Peters, H.G. von Schnering, Z. Anorg. Allg. Chem. 624 (1998) 1415–1418] and Ag 0.74[W 2O 2Br 6] [S. Imhaïne, C. Perrin, M. Sergent, Mat. Res. Bull. 33 (1998) 927–933]. The Ag containing compound can be obtained from elemental Ag and WOBr 3. The crystal structure, originally reported in the triclinic system, was redetermined and shown to be monoclinic with space group C2/ m ( a = 13.7338 ( 10 ) , b = 3.7769 ( 3 ) , c = 10.7954 ( 9 ) Å , β = 112.401 ( 3 ) ° ). The crystal structures of these compounds are in close relationship to the structure of WOCl 3 and all contain W 2O 4/2 X 6 ( X=Cl, Br) double strands with the mono and divalent cations coordinated by the terminal halogen atoms of the W 2 X 6 groups and a short W–W bond (2.85 Å for X=Cl). A cube-shaped coordination environment is present for M=Tl, K and a trigonal-prismatic coordination for M=Ag, Hg. Hg 0.55[W 2O 2Cl 6] is a semiconductor with a non-Arrhenius behaviour, high specific conductivity of 0.05 Ω –1 cm −1 and a very small activation energy of 0.03 eV. Hg 0.55[W 2O 2Cl 6] and Ag 0.8[W 2O 2Br 6] show a temperature independent paramagnetism with a magnetic moment around 300×10 –6 cm 3 mol –1.