PHASE-RELATIONS, CHEMICAL-POTENTIALS AND THERMODYNAMIC PROPERTIES OF INTEROXIDE COMPOUNDS IN THE SYSTEM BA-PB-O

Phase relations in the system Ba-Pb-O have been established by equilib rating 18 samples in the ternary in either pure oxygen, inert gas or e vacuated capsules at 1100 K. Phase identification of quenched samples was performed using optical and electron microscopy, X-ray diffraction and energy dispersive analysis of X-rays. Three interoxide compounds, Ba2PbO4, Ba4Pb4O10 and BaPbO3 were found to be stable: tie-lines conn ect each of these compounds with PbO and oxygen gas. Lead is present i n tetravalent state in all the three interoxide compounds: All alloys and intermetallics (Ba2Pb2Ba5Pb3 and BaPb) are in equilibrium with BaO . The oxygen chemical potential corresponding to the three-phase field s, BaO + PbO + Ba2PbO4, PbO + Ba2PbO4 + Ba4Pb3O10 and PbO + Ba4Pb3O10 + BaPbO3 were measured using solid state cells incorporating (Y2O3)ZrO 2 as the solid electrolyte in the temperature range 750-1080 K. A spec ial cell design, with a buffer electrode in addition to the normal ref erence and measuring electrodes, was used to avoid polarization of the three-phase electrode. The standard Gibbs free energies of formation of the three ternary oxides were derived from the emf of three solid o xide galvanic cells. The results were cross-checked by measuring the c hemical potential of BaO in the three-phase field PbO + BaPbO3 + O-2 ( 26.2 Pa) and Gibbs energy of formation of BaPbO3 using a cell based on BaF2 as the electrolyte. The following equations summarize the result s: 2BaO + PbO + 1/20(2) --> Ba2PbO4; Delta G(degrees) = -202640 + 91.6 7 T (+/- 400) J mol(-1); 4BaO + 3PbO + 3/20(2) --> Ba4Pb3O10; Delta G( degrees) = -556835 + 283.12 T (+/- 900) J mol(-1); BaO + PbO + 1/20(2) --> BaPbO3; Delta G(degrees) = -165970 + 87.45 T (+/- 200) J mol(-1). The results obtained in this study differ significantly from data rep orted in the literature. The earlier investigators incorrectly assumed that BaO exists at unit activity under pure oxygen in the temperature range 700-875 K. (C) 1998 Elsevier Science S.A. All rights reserved.