Pt₃Ga: Thermodynamics and Nonstoichiometry

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.

A3B Intermetallics: Defect chemistry and nonstoichiometry

“…Similar experiments were performed for Pt 3 Ga [39] and Pt 3 In [40], and the defect formation energies were determined by a corresponding statistical-thermodynamic evaluation of the obtained activity values. Table 2 lists the defect formation energies of all L1 2 compounds investigated in our laboratory.…”

A Statistical‐Thermodynamic Modeling of Ordering Phenomena in Binary Intermetallic B2‐ and L12‐Structures

“…In this part, we show some of the principles which were used for the modeling of a defect structure in intermetallics with B2‐, and L1 2 ‐structures39, 70, 71, 76–78, 81–88 applying a Wagner–Schottky Approach 81. The Wagner–Schottky Approximation works under the assumption that (at constant pressure) the internal energy, volume, and vibrational entropy of the crystal are linear functions of the number of atoms or vacancies, respectively, in the different sublattices 39.…”

“…The modeling approach selected here uses the energies of formation of the different point defects as adjusted parameters,32, 70, 71, 76, 77, 81–88 although, of course, some disorder or long range order parameter could be used equally well 39. The advantage of using the energy parameters in the present case lies in the possibility to test the values that are available in the literature.…”

“…This modeling approach was applied to compounds with B2‐structure, e.g., NiAl in ref.,32 and to several intermetallic compounds with L1 2 ‐strucuture as Ni 3 Al,70, 71, 83 Ni 3 Ga,81–85, 93 Pt 3 Ga,83, 86, 87 and Pt 3 In in ref 83, 85, 86…”

“…Therefore, these parameters have been approximated by fitting to experimental thermodynamic activity data 86, 87, 97, 98. Figure 2–6 demonstrate theoretical activity and vacancy concentration curves calculated according to GDCM compared to the experimental data and results obtained according to the above mentioned statistical‐thermodynamic model based on a Wagner–Schottky Approach 70, 71, 81–88. As can be seen from the presented diagrams, in all cases good agreement is found between the experimental and the theoretical results.…”