Phase relations in the system Sr–Ir–O and thermodynamic measurements on SrIrO₃, Sr₂IrO₄ and Sr₄IrO₆ using solid-state cells with buffer electrodes

Abstract

The standard Gibbs energies of formation of SrIrO₃, Sr₂IrO₄ and Sr₄IrO₆ have been determined in the temperature range from 975 to 1400 K using solid-state cells with (Y₂O₃) ZrO₂ as the electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. For the design of appropriate working electrodes, phase relations in the ternary system Sr–Ir–O were investigated at 1350 K. The only stable oxide detected along the binary Ir–O was IrO₂. Three ternary oxides, SrIrO₃, Sr2IrO₄ and Sr₄IrO₆, compositions of which fall on the join SrO–IrO₂, were found to be stable. Each of the oxides coexisted with pure metal Ir. Therefore, three working electrodes were prepared consisting of mixtures of Ir+SrO+Sr₄IrO₆, Ir+Sr₄IrO₆+Sr₂IrO₄, and Ir+Sr₂IrO₄+SrIrO₃. These mixtures unambiguously define unique oxygen chemical potentials under isothermal and isobaric conditions. Used for the measurements was a novel apparatus, in which a buffer electrode was introduced between reference and working electrodes to absorb the electrochemical flux of oxygen through the solid electrolyte. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. The standard Gibbs energies of formation of the compounds, obtained from the emf of the cells, can be represented by the following equations: SrIrO₃: Δ_{f (ox)}G°/J mol^-1=-58 860-1.23T (±650) Sr₂IrO₄: Δ_{f (ox)}G°/J mol^-1=-82 315-2.86T (±960) Sr₄IrO₆: Δ_{f (ox)}G°/J mol^-1=-84 690-6.78T (±1520) where Δ_{f (ox)}G° represents the standard Gibbs energy of formation of the ternary compound from its component binary oxides SrO and IrO2. Based on the thermodynamic information, chemical potential diagrams for the system Sr–Ir–O were developed.