Thermodynamic properties and nonstoichiometry in the intermetallic compound Ni3Ga

Thermodynamic activities of gallium were measured between 1073 and 1273 K in the nonstoichiometric intermetallic compound Ni3Ga using an emf method based on solid oxygen-conducting electrolytes. The partial molar enthalpies and entropies were derived from their temperature dependence. The variation of the lattice parameter with composition was determined by X-ray diffraction over the entire homogeneity range. The results of the activity measurements are interpreted in terms of a statistical-thermodynamic model for nonstoichiometric phases with the cubic L1(2) superstructure considering four types of point defects, i.e. antistructure atoms and vacancies on the nickel and gallium sublattices. The energies of formation of the point defects at the stoichiometric composition are used as adjustable parameters yielding the following set of values: E-f (Ni-Ga) = 0.60 eV, E-f (Ga-Ni) = 0.60 eV, E-f(V) (Ni) = 1.5 eV, E-f(V) (Ga) = 2.0 eV. This results in a disorder parameter alpha' = 0.0009 at 1123 K which means that at the stoichiometric composition about 0.36 % of the gallium sublattice sites are occupied by nickel atoms and 0.12 % of the nickel sites by gallium atoms.