No abstract

Thermodynamic activities of gallium were measured between about 1000 and 1300 K in the nonstoichiometric intermetallic compound Pt3Ga using an emf-method based on an oxygen conducting solid electrolyte. The variation of the lattice parameter with composition was determined by powder X-ray diffraction. The results of the activity measurements are interpreted in terms of a statisticalthermodynamic model for L12-phases considering four types of point defects, i. e. anti-structure atoms and vacancies on the platinum and gallium substructures. The energies of formation of the point defects at the stoichiometric composition are estimated from a curve fitting procedure yielding Ef(PtGa) = Ef(GaPt) = 1.25 eV, assuming that Ef(VPt) = Ef(VGa) = 2.0 eV. This results in a disorder parameter α’ = 3 · 10−6 at 1173 K which means that at the stoichiometric composition 0.0012% of the gallium substructure sites are occupied by platinum atoms and 0.0004% of the platinum sites by gallium atoms at this temperature.

A statistical-thermodynamic model was derived which allows to describe thermodynamic activities in intermetallic compounds with L1 2 -structure as a function of composition and temperature. The energies of formation of the four types of point defects (anti-structure atoms and vacancies on both sublattices) were used as adjustable parameters. The model was applied to the three compounds Ni 3 Al, Ni 3 Ga, and Pt 3 Ga, and it permitted to estimate for the first time the defect formation energies for Ni 3 Ga and to provide initial estimates for Pt 3 Ga.