A3B Intermetallics: Defect chemistry and nonstoichiometry

Abstract

The defect chemistry of different ordered intermetallic compounds with the A3B stoichiometry was investigated. Three groups were distinguished according to their crystal structure: L12 compounds (Ni3Al, Ni3Ga, Pt3Ga, Pt3In), D019 compounds (Ti3Al), and D03 compounds (Fe3Al, Ni3Sb). Statistical-thermodynamic models were derived based on a Wagner-Schottky approach, and the calculated activity curves (thermodynamic activity vs. composition) were compared with experimental activity data. In this way, we attempted to obtain at least estimated values for the energies of formation of the different types of point defects present in the corresponding compound, both as configurational defects (which are responsible for nonstoichiometry) and as thermal defects. In the majority of cases, thermodynamic activities had to be determined experimentally in the present study, using either an emf method with a solid electrolyte (Ni3Ga, Pt3Ga, Pt3In, Fe3Al) or a Knudsen cell-mass spectrometric method (Ni3Sb).