Calcium ruthenates: determination of Gibbs energies of formation using electrochemical cells

Metallic Ru has been found to coexist separately with CaO, RuO₂ and the interoxide phases, Ca₂RuO₄, Ca₃Ru₂O₇, and CaRuO₃, present along the pseudobinary system CaORuO₂. The standard Gibbs energies of formation (Δ_f(ox)G°) of the three calcium ruthenates from their component oxides have been measured in the temperature range 925-1350 K using solid-state cells with yttria-stabilized zirconia as the electrolyte and Ru+RuO₂ as the reference electrode. The standard Gibbs energies of formation (Δ_f(ox)G°) of the compounds can be represented by Ca₂RuO₄; Δ_f(ox)G°/Jmol^-1=-38.340-6.611 T (±120), Ca₃Ru₂O₇: Δ_f(ox)G°/Jmol^-1=-75.910-11.26 T (±180), and CaRuO₃: Δ_f(ox)G°/Jmol^-1=-35.480-3.844 T (±70). The data for Ca₂RuO₄ corresponds to the stoichiometric composition, which has an orthorhombic structure, space group Pbca with short c axis (“S” form). The structural features of the ternary oxides responsible for their mild entropy stabilization are discussed. A three-dimensional oxygen potential diagram for the system Ca-Ru-O is developed as a function of composition and temperature from the results obtained. Using the Neumann-Kopp rule to estimate the heat capacity of the ternary oxides relative to their constituent binary oxides, the standard enthalpies of formation of the three calcium ruthenates from the elements and their standard entropies at 298.15 K are evaluated