A new design for the solid-state cell incorporating a buffer electrode for high-temperature thermodynamic measurements is presented. The function of the buffer electrode. placed between the reference and working electrodes, is to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevents polarization of the measuring electrode and ensures accurate data. The application of this novel design and its advantages are demonstrated by measurement of the standard Gibbs energies of formation of Nd6Ir2O13 (low-temperature form) and Nd2Ir2O7 in the temperature range from 975 to 1450 K. Yttria-stabilized zirconia is used as the solid 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 Nd-Ir-O were investigated at 1350 K. The two ternary oxides, Nd6Ir2O13 and Nd2Ir2O7, compositions of which fall on the join Nd2O3-IrO2, were found to coexist with pure metal Ir. Therefore, two working electrodes were prepared consisting of mixtures of Ir + Nd2O3 + Nd6Ir2O13 and Ir + Nd6Ir2O13 + Nd2Ir2O7. These mixtures unambiguously define unique oxygen chemical potentials under isothermal and isobaric conditions. The standard Gibbs energies of formation (Delta G(f)degrees (ox)) Of the compounds from their component binary oxides Nd2O3 and IrO2, obtained from the emf of the cells, can be represented by the equations: Nd6Ir2O13: Delta G(f)degrees ((ox))/J mol (- 1) = - 115 890 + 7.67 T/K (+/- 2640) Nd2Ir2O7: Delta G(f)degrees ((ox))/J mol (-) (1) = - 87 690 + 6.23 T/K (+/- 1460) Based on the thermodynamic information, chemical potential diagrams for the system Nd-Ir-O are developed. (C) 2000 Elsevier Science Ltd. All rights reserved.