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

Abstract

Metallic Ru has been found to coexist separately with SrO, RuO₂ and the interoxide phases, Sr₂RuO₄, Sr₃Ru₂O₇, Sr₄Ru₃O₁₀ and SrRuO₃, present along the pseudo-binary system SrO–RuO₂ at 1300 K. The standard Gibbs energies of formation (Δ_f(ox)G°) of the four strontium ruthenates have been measured in the temperature range from 925 to 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 ternary compounds from their component binary oxides SrO and RuO₂ can be represented by linear equations as a function of temperature: Sr₂RuO₄: Δ_f(ox)G°=−117 440−4.38 T (±135) J mol⁻¹ Sr₃Ru₂O₇: Δ_f(ox)G°=−231 120−9.56 T (±225) J mol⁻¹ Sr₄Ru₃O₁₀: Δ_f(ox)G°=−343 330−14.78 T (±300) J mol⁻¹ SrRuO₃: Δ_f(ox)G°=−109 900−4.68 T (±80) J mol⁻¹ A three-dimensional oxygen potential diagram for the system Sr–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 four strontium ruthenates from elements and their standard entropies at 298.15 K are evaluated.