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

• Published in: Journal of the Electrochemical Society Vol. 150; no. 4; pp. E227 - E232
• Main Authors: JACOB, K. Thomas, KAY THI LWIN, WASEDA, Yoshio
• Format: Journal Article
• Language: English
• Published: Pennington, NJ Electrochemical Society 01.04.2003
• Subjects: Condensed matter: structure, mechanical and thermal properties; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Physics; Solid-solid transitions; Specific phase transitions; Platinoid Compounds; Gibbs free energy; Phase transformations; Electrochemical method; Thermodynamic activity; Ruthenium oxides; Transition elements Compounds; Experimental study; Calcium oxides; Alkaline earth metals Compounds; Phase equilibria; Chemical potential; Thermodynamic properties; Heat of formation
• ISSN: 0013-4651; 1945-7111
• DOI: 10.1149/1.1557082

Metallic Ru has been found to coexist separately with CaO, RuO2, and the interoxide phases, Ca2RuO4, Ca3Ru2O7, and CaRuO3, present along the pseudobinary system CaO-RuO2. The standard Gibbs energies of formation (*Df(ox)GDG) 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 (*Df(ox)GDG) of the compounds can be represented by Ca2RuO4: *Df(ox)GDG/J mol-1 = -38,340-6.611 T (#+120), Ca3Ru2O7: *Df(ox)GDG/J mol-1 = -75,910-11.26 T (#+180), and CaRuO3: *Df(ox)GDG/J mol-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.