Mixed valence tungsten (IV,V) compounds with layered structures (Part III)[2]: Synthesis and crystal structure of Ag1-x[W2O 2Cl6] and investigations on the Ag+ ion mobility

Article abstract of DOI:10.1002/zaac.201000080

The reaction of metallic silver with tungsten tetrachlorideoxide WOCl4 or the reaction of AgCl with WOCl₃, both at 690 K, lead to Ag _1-x[W₂O₂Cl₆] as black lustrous crystal needles. The compound shows a substantial phase field width in the silver content depending on reaction temperature and reaction time and crystals with x = 0, 0.38 and 0.82 were isolated. The crystal structure determinations (monoclinic, C2/m) show the structure to be isotypic to those of Tl[W ₂O₂Cl₆] and K_0.84[W ₂O₂Cl₆] with the presence of 1D polymeric [W₂O₂Cl₆]_n strands and Ag+ ions with a variety in the occupation of the respective crystallographic site. Ag _1-x[W₂O₂Cl₆] represents a mixed valence compound with a variable ratio of W(IV) and W(V) in the W2 dumbbells with a short W-W separation of 2.85 A. In Fourier maps the electron density of the Ag ion appears smeared over a large area. By structure determinations of stoichiometric Ag_1.0[W₂O ₂Cl₆] at temperatures of123, 193, and 293 K a double minimum potential was found with the silver ion dynamically disordered over two flat basins with a thermal activation barrier of 13 meV. No long range ion mobility is present since the Ag⁺ ions are trapped within a coordinating cage consisting of ten chlorine atoms of the surrounding [W ₂O₂Cl₆] strands forming a distorted bicapped cube. By MAS-NMR spectroscopic measurements on the ¹⁰⁹Ag nuclei at different temperatures, the spin lattice relaxation times were determined giving a thermally activated barrier of 15 meV. The electronic conductivity of pressed powder samples of Ag[W₂O₂Cl₆] by a four-probe measurement applying direct current is 1Ω^-1·cm ^-1 at room temperature and 10^-3Ω ^-1·cm^-1 at 25 K. For the conduction mechanism at low temperatures a variable range hopping mechanism is suggested, whereas at higher temperatures a standard semi-conductivity with a bandgap of 60 meV is present.