On the coexistence of copper-molybdenum bronzes: CuxMoO₃ (0.2 < x < 0.25; typically x = 0.23) and CuyMoO_3-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 glistering 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.