Systematic trends in structural and thermodynamic properties of ternary oxides in the systems Ln-Pd-O (Ln = lanthanide element)

Abstract

Isothermal sections of the phase diagrams for the systems Ln-Pd-O (Ln = lanthanide element) at 1223 K indicate the presence of two inter-oxide compounds Ln₄PdO₇ and Ln₂Pd₂O₅ for Ln = La, Pr, Nd, Sm, three compounds Ln₄PdO₇, Ln₂PdO₄ and Ln₂Pd₂O₅ for Ln = Eu, Gd and only one compound of Ln₂Pd₂O₅ for Ln = Tb to Ho. The lattice parameters of the compounds Ln₄PdO₇, Ln₂PdO₄ and Ln₂Pd₂O₅ show systematic non-linear variation with atomic number. The unit cell volumes decrease with increasing atomic number. The standard Gibbs energies, enthalpies and entropies of formation of the ternary oxides from their component binary oxides (Ln₂O₃ and PdO) have been measured recently using an advanced version of the solid-state electrochemical cell. The Gibbs energies and enthalpies of formation become less negative with increasing atomic number of Ln. For all the three compounds, the variation in Gibbs energy and enthalpy of formation with atomic number is markedly non-linear. The decrease in stability with atomic number is most pronounced for Ln₂Pd₂O₅, followed by Ln₄PdO₇ and Ln₂PdO₄. This is probably related to the repulsion between Pd²⁺ ions on the opposite phases of O₈ cubes in Ln₂Pd₂O₅, and the presence of Ln-filled O₈ cubes that share three faces with each other in Ln₄PdO₇. The values for entropy of formation of all the ternary oxides from their component binary oxides are relatively small. Although the entropies of formation show some scatter, the average value for Ln = La, Pr, Nd is more negative than the average value for the other lanthanide elements. From this difference, an average value for the structure transformation entropy of Ln₂O₃ from C-type to A-type is estimated as 0.87 J.mol⁻¹.K⁻¹. The standard Gibbs energies of formation of these ternary oxides from elements at 1223 K are presented as a function of lanthanide atomic number. By invoking the Neumann-Kopp rule for heat capacity, thermodynamic properties of the inter-oxide compounds at 298.15 K are estimated.