Phase relations in the system Sr-Ir-O and thermodynamic measurements on SrIrO3,Sr2IrO4 and Sr4lrO6 using solid-state cells with buffer electrodes

The standard Gibbs energies of formation of SrIrO3, Sr4IrO6 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: Delta(f(ox))G degrees/J mol(-1) = - 58 860 - 1.23T (+/-650) Sr2IrO4: Delta(f(ox))G degrees/J mol(-1) = - 82 315 - 2.86T (+/-960) Sr4IrO6: Delta(f(ox))G degrees/J mol(-1) = - 84 690 - 6.78T(+/-1520) where Delta(f(ox))G degrees 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. (C) 1999 Elsevier Science S.A. All rights reserved.