A3B Intermetallics with L1 2-, Do3-, Do19-Structures

Within the scope of this research project it has been demonstrated successfully that model calculations can be applied to experimental thermochemical data of intermetallic A3 B compounds with an ordered cubic crystal structure (of the L12 -type). In this way it was possible to derive type and concentrations of the defects in the crystal lattice that are responsible for the deviation from the exact (3:1)-composition in the compounds Ni3 Al, Ni3 Ga, Pt 3 Ga, and Pt 3 In. Furthermore, it was also possible to estimate the energies necessary to form such defects. Theoretical model equations were derived for intermetallic A3 B compounds with an ordered cubic crystal structure of the L12 -type. These made possible to obtain certain informations about the point defects present in the crystal lattice (in the present case vacancies and atoms on "wrong" lattice sites) by applying a curve fitting procedure to the concentration dependence of thermochemical properties: type of the predominant defects, their energies of formation and their concentrations. Whereas thermochemical data (aluminum activities) and defect formation energies for Ni3 Al were available in the literature such data were basically missing for the compounds Ni3 Ga, Pt 3 Ga, and Pt 3 In. They had to be determined experimentally. For this purpose an electrochemical method was adapted which uses an oxygen conducting electrolyte, and the gallium and indium activities were determined from the so-called electromotoric force. These thermochemical data were evaluated in terms of the model equations, and it was found that basically only anti-structure atoms, i.e. atoms on "wrong" lattice sites are responsible for the deviation from the ideal (3:1) composition since the energies necessary for the formation of vacancies are considerably higher. Using an improved theoretical approach, it was also possible to calculate the defect formation energies for the compounds Ni3 Al and Ni3 Ga from first principles. The resulting values are in good agreement with those estimated from the experimental thermochemical data. For a further characterization, the specific heat of the compound Ni3 Ga as well as its diffusional properties were determined (using a radioactive tracer method). By measuring the specific electric resistivity it is possible to obtain information about the type and the rate of the reactions that are responsible for establishing order in the crystal lattice.