Trends in the stability of ternary oxides: systems M-Pb-O (M = Ca, Sr, Ba)

Abstract

Recent experimental investigations of phase equilibria and thermodynamic properties of the systems M-Pb-O, where M = Ca, Sr or Ba, indicate a regular increase in thermodynamic stability of ternary oxides, MPbO₃ and M₂PbO₄, with increasing basicity of the oxide of the alkaline-earth metal. Number of stable interoxide compounds at 1100 K in the systems M-Pb-O (M = Mg, Ca, Sr, Ba) increases in unit increments from Mg to Ba. In this paper, experimentally determined standard Gibbs energies of formation of M₂PbO₄ (M = Ca, Sr, Ba) and MPbO₃ (M = Sr, Ba) from their component binary monoxides and oxygen gas are combined with an estimated value for CaPbO₃ to delineate systematic trends in thermodynamic stability of the ternary oxides. The trends are interpreted using concepts of tolerance factor and acid-base interactions. All the ternary oxides in these systems contain lead in the tetravalent state. The small Pb⁴⁺ ions polarize the surrounding oxygen ions and cause the formation of oxyanions which are acidic in character. Hence, the higher oxidation state of lead is stabilized in the presence of basic oxides of alkaline-earth group. A schematic subsolidus temperature-composition phase diagram is presented for the system BaO-PbO-O-₂ to illustrate the change in oxidation states in binary and ternary oxides with temperature.