Thermodynamic properties and nonstoichiometry in the intermetallic compound Ni3Ga

Thermodynamic activities of gallium were measured between 1073 and 1273 K i n the nonstoichiometric intermetallic compound Ni3Ga using an emf method ba sed on solid oxygen-conducting electrolytes. The partial molar enthalpies a nd entropies were derived from their temperature dependence. The variation of the lattice parameter with composition was determined by X-ray diffracti on over the entire homogeneity range. The results of the activity measureme nts are interpreted in terms of a statistical-thermodynamic model for nonst oichiometric phases with the cubic L1(2) superstructure considering four ty pes 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 t he 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 param eter alpha' = 0.0009 at 1123 K which means that at the stoichiometric compo sition about 0.36 % of the gallium sublattice sites are occupied by nickel atoms and 0.12 % of the nickel sites by gallium atoms.