Synthesis and Crystal Structure of Hg2V8O20 – the First Ternary Mercury Vanadate with Mixed-valent Vanadium (IV/V)

Abstract

Black needle-shaped single crystals of Hg2V8O20 were obtained by heating amorphous precipitates resulting from mixing aqueous solutions of HgNO3OH and NH4VO3 (Hg :V ratios between 1 : 2.0 and 1 : 2.5) in evacuated silica ampoules at 450 °C for two weeks. Their crystal structure was determined from single crystal diffractometer data [C2/m, Z = 2, a = 22.375(1), b = 3.6312(2), c = 9.6113(4) Å, β = 91.708(5)°, R[F2 ≥ 2σ (F2)] = 0.0681, 807 F2 values, 61 variable parameters]. Two of the four independent vanadium atoms are surrounded by six close oxygen atoms whereas the other two have five oxygen neighbors. These coordination polyhedra share edges and corners resulting in puckered layers with an overall composition [V4O10]− extending parallel to (100). The layers are linked by Hg2 2+ dumbbells (d(Hg-Hg) = 2.510(2) Å) with a short Hg-O distance of 2.13(2) Å(∠(O9−Hg-Hg) = 173.8(5)°). Although Hg2V8O20 crystallizes in a new structure type, it resembles the structures of other layered vanadium oxides with open frameworks and can be classified as being of the Q-UD-type. The near-neighbor environments of the four vanadium positions of this reduced vanadate with an average oxidation number of +4.75 of the vanadium atoms are similar with respect to V-V bonding and it is suggested that the vanadium atoms have delocalized V-V bonding with intermediate valence.