The Ag₂O-V₂O₅-HF_(aq) system and crystal structure of α-Ag₃VO₄

Reactions between the three components Ag₂O, V₂O ₅, and HF_(aq) were investigated under hydrothermal conditions, and the recovered phases were, in increasing Ag:V content, Ag ₂V₄O₁₁, β-AgV03, Ag₄V ₂O₆F₂, Ag₄V₂O ₇, and α-Ag₃-VO₄. A higher ratio of Ag₂O to V₂O₅, as compared to a stoichiometric ratio, was required to synthesize Ag₄V₂O₆F ₂, Ag₄V₂O₇, and α-Ag ₃VO₄. Owing to their solubility differences, the crystallization regions are not centered around the respective 2:1 and 3:1 Ag₂O/V₂O₅ tie-lines but rather are centered along the 4:1 and 8:1 Ag₂O/V₂O₅ tie-lines. Reactions with a 4:1 Ag/V ratio either resulted in Ag₄V ₂O₆F₂ at 150 °C or Ag₄V ₂O₇ at 200°C. Products were recovered in between 80% and 100% yield based on V₂O₅. Red transparent crystals of α-Ag₃VO₄ crystallize in the monoclinic space group C2/c, with cell parameters a = 10.1885(16) Å, b = 4.9751(8) A, c = 10.2014(17) Å, β = 115.754(3)°.