Determination of stoichiometry and site disorder in the complex intermetallics

A combination of characterization methods (SEM, X-ray diffraction, DTA, neutron diffraction and phase diagram analyses) has been used to determine the actual composition of a complex intermetallic compound of nominal composition Pr6Ni2Si3 where neither X-ray nor neutron diffraction alone are capable of differentiating between vacancies or Si substitution on one of the Ni sites due to the scattering factor of two Si substitutions being equivalent to one Ni atom plus a vacancy. Using a combination of quantitative phase analysis of the diffraction data based on Reitveld refinement, metallography and mass balance calculations, the correct stoichiometry was found to necessarily contain excess Si, and is close to the stoichiometry, Pr6Ni1.6Si3.2. Nonetheless this composition still contains around 1% second phase following long term anneal. Due to the close proximity of neighboring phases, this corresponds to an error in stoichiometry of approximately 0.02%. Assuming similar occupancies of Si and Ni on the Ni sites for other members of the homologous series R(n+2)(n+1)Nin(n−1)+2Sin(n+1), compositions of Pr5Ni1.85Si3.1 and Pr15Ni6.7Si10.2 were predicted for n = 3 and n = 4 alloys, respectively, and verified experimentally. The approach present in this paper is applicable to many other systems.