The magnetochemistry and stoichiometry of the copper-oxygen system

An enclosed magnetic balance has been used to study the changes in magnetic susceptibility which take place during the oxidation of copper and during the treatment of cuprous and cupric oxides in oxygen and in vacuo. Cuprous oxide prepared by high tem perature oxidation and cooled in vacuo from 1060 °C has a gram susceptibility of — 0·155x10-6, independent of temperature in the range 14 to 293 °K. Cuprous oxide quenched from 1045 °C in air has a gram susceptibility which varies with temperature from Xg = —0·150 x 10-6 a t 293 °K to Xg = —0·127 x 10-6 at 14 °K. The paramagnetic component of the susceptibility of the latter oxide is attributed to magnetic centres (cation vacancies and positive holes) arising from the oxygen excess, and its absolute value is compared with that expected from different models for the disorder. It is suggested that single vacancies aggregate on cooling. Cuprous oxide prepared by oxidation a t low temperatures (120 to 160 °C) and low oxygen pressures may be rendered much more paramagnetic than the oxide prepared at high temperatures. A survey of the magnetic properties has been made by carrying out such oxidations in situ on the magnetic balance. It is concluded that beyond the stoichiometric point on the oxygen excess side an extensive metastable non-stoichiometry may be sustained at these temperatures. Further oxidation or, alternatively, a treatment in vacuo at a high temperature causes the oxygen excess to be nucleated and antiferromagnetic cupric oxide is formed. Similar studies with copper supported on silica show that the metastable para-­ magnetism is associated more especially with the surface. Attention is given to the fact that cupric oxide may be rendered paramagnetic under certain circum stances, but a distinction is drawn between this effect and the paramagnetism of non-stoichiometric cuprous oxide. The significance of the results for the mechanism of the oxidation of copper is discussed. It is suggested that CuO nucleates internally in Cu2O, and a close analogy then exists between the oxidation of copper to Cu2O and the oxidation of Cu2O to CuO. The surface reaction, nam ely, incorporation of oxygen into cuprous oxide, is the same in bo th cases. A mechanism is put forward to account for the relative ease of cation vacancy production (and hence non-stoichiometry) in the surface layers during this process.