Pt 3 Ga: Thermodynamics and Nonstoichiometry

Thermodynamic activities of gallium were measured between about 1000 and 1300 K in the nonstoichiometric intermetallic compound Pt 3 Ga 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 L1 2 -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 E f (Pt Ga ) = E f (Ga Pt ) = 1.25 eV, assuming that E f (V Pt ) = E f (V Ga ) = 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.