THERMODYNAMIC STABILITIES OF TERNARY OXIDES IN BI-CU-O SYSTEM BY SOLID-ELECTROLYTE EMF METHOD

There are four high T-c compounds in the Bi-Sr-Ca-Cu-O system, the thermodynamic characterization of which may require precise Gibbs energy data on ternary compounds in different subsystems. Hence, phase equilibrium studies have been carried out on the less known Bi2O3-Cu2O-CuO system and a partial isothermal phase diagram has been determined at 973 K. Based on these equilibrium studies, three galvanic cells based on stabilized zirconia as the electrolyte and one galvanic cell based on single crystal CaF2 as the electrolyte have been studied in the temperature range 650-950 K. From the EMF results, the following least-squares expressions for the standard Gibbs energy of formation of ternary oxides, Delta Gf,OX degrees rom their constituent binary oxides have been derived. Delta G(f,OX)degrees(Bi5CuO8) = -17 390+11.62T+/-200 J mol(-1) (1) Delta G(f,OX)degrees(Bi4CuO7) = -24 790-6.64T+/-470 J mol(-1) (2) Delta G(f,OX)degrees(Bi2CuO4)= -35 040+11.89T+/-570 J mol(-1) (3) Delta G(f,OX)degrees(Bi2Cu2O5) = -55 210+20.15T+/-830 J mol(-1) (4) The expressions for log P-O2 and log a(CuO) over the relevant mixture of phases have also been derived and reported.