Construction of Chemical Potential Diagrams for Metal‐Metal‐Nonmetal Systems: Applications to the Decomposition of Double Oxides

A chemical potential diagram for A‐B‐O (A, B = metallic elements) systems has been constructed to make visually clear the thermodynamic stability of double oxides; the stability area of metallic elements, their binary oxides, and double oxides can be presented as convex polygons in a log ( a A / a B ) vs log P (O 2 ) plot, a and P (O 2 ) being the activity of metallic elements and the oxygen partial pressure, respectively; the axis variable, log ( a A / a B ), is used to treat two metallic elements equivalently. The Co‐Ti‐O and La‐Co‐O systems have been chosen as examples to demonstrate how the present diagram can provide complicated phase relations in a compact and visualized manner. An attempt has also been made to utilize this diagram to present graphically products of kinetic decomposition of double oxides placed in an oxygen potential gradient.