Mixed Valence Tungsten (IV,V) Compounds with Layered Structures (Part III*): Synthesis and Crystal Structure of Ag1-x[W2O2Cl6] and Investigations on the Ag+ Ion Mobility

Johannes Beck (1) , Wilfried Hoffbauer , Christian Kusterer , Marcel Schieweling

1 Inorganic Chemistry

The reactions of metallic silver with tungsten tetrachlorideoxide WOCl4 or the reaction of AgCl with WOCl3, both at 690 K, lead to Ag1-x[W2O2Cl6] as black lustrous crystal needles. The compound shows a substantial phase field width in the silver content depending on reaction tem-perature 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[W2O2Cl6] and K0.84[W2O2Cl6] with the presence of 1-D polymeric [W2O2Cl6]n strands and Ag+ ions with a variety in the occupation of the respective crystallographic site. Ag1-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 separa-tion of 2.85 Å. In Fourier maps the electron density of the Ag ion appears smeared over a large area. By structure determinations of stochiometric Ag1.0[W2O2Cl6] at temperatures of 123, 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 Cl atoms of the surrounding [W2O2Cl6] strands forming a distorted bicapped cube. By MAS-NMR measurements on the 109Ag nuclei at different temperature 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 1 Ω–1 cm–1 at room tempera-ture and 10-3 Ω 1 cm-1 at 25 K. For the conduction mechanism at low temperatures a variable range hopping mechanism is suggested while at higher temperatures a standard semi-conductivity with a band gap of 60 meV is present.

Mots clés: Physical Sciences

Johannes Beck, Wilfried Hoffbauer, Christian Kusterer, Marcel Schieweling. Mixed Valence Tungsten (IV,V) Compounds with Layered Structures (Part III*): Synthesis and Crystal Structure of Ag1-x[W2O2Cl6] and Investigations on the Ag+ Ion Mobility. Journal of Inorganic and General Chemistry / Zeitschrift für anorganische und allgemeine Chemie, 2010, 636 (9-10), pp.1827. ⟨10.1002/zaac.201000080⟩. ⟨hal-00552461⟩