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

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 (Δf(ox)G deg. ) 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 (Δf(ox)G deg. ) of the ternary compounds from their component binary oxides SrO and RuO2 can be represented by linear equations as a function of temperature: - Sr2RuO4: Δf(ox)G deg. =-117 440-4.38 T (±135) J mol-1; - Sr3Ru2O7: Δf(ox)G deg. =-231 120-9.56 T (±225) J mol-1; - Sr4Ru3O10: Δf(ox)G deg. =-343 330-14.78 T (±300) J mol-1; - SrRuO3: Δf(ox)G deg. =-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