On the coexistence of copper–molybdenum bronzes: CuxMoO3 (0.2 < x < 0.25; typically x ∼ 0.23) and CuyMoO3−z (0.1 < y < 0.2; typically y ∼ 0.15) in the Cu–MoO2–O quasi-ternary system

Two copper–molybdenum bronzes: CuyMoO3−z (0.1 < y < 0.2; typically y ∼ 0.15) and CuxMoO3 (0.2 < x < 0.25; typically x ∼ 0.23) were prepared as glistening black polycrystalline materials by the solid state reaction of Cu and MoO3 at 600 °C under argon in Pt crucibles. Powder XRD showed that the material with global composition ‘0.1Cu·MoO3’ comprises ∼Cu0.15MoO3 and MoO3; whilst ‘0.2Cu·MoO3’ comprises ∼Cu0.15MoO3 and ∼Cu0.23MoO3. DTA performed on ‘0.2Cu·MoO3’ reveals a reversible solid state phase transition ∼520 °C under argon. Reacting equimolar amounts of Cu2O and MoO2 at 600 °C in a Cu crucible under argon yields: Cu6Mo5O18, Cu and MoO2. A tentative subsolidus Cu–MoO2–O isothermal (∼25 °C) phase diagram under argon is drawn from these data. Oxidation states of Cu and Mo within this system are discussed.