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

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.