Thermodynamics and nonstoichiometry in the intermetallic compound Pt3In

Thermodynamic activities of indium were measured between 973 and 1273 K in the nonstoichiometric intermetallic compound Pt3In 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, and the corresponding results point to a considerably wider homogeneity range on the indium-rich side than previously reported. The results of the activity measurements are interpreted in terms of a statistical-thermodynamic model for L12-phases considering four types of point defects, i.e. anti-structure atoms and vacancies on the platinum and indium sublattices. The energies of formation of the point defects at the stoichiometric composition are estimated from a curve fitting procedure yielding Ef(PtIn)=Ef(InPt)=1.15 eV, assuming that EfV(Pt)=EfV(In)=2.0 eV. This results in a disorder parameter α′=5·10−6 at 1173 K which means that at the stoichiometric composition 0.002% of the indium sublattice sites are occupied by platinum atoms and 0.0007% of the platinum sites by indium atoms at this temperature.