Defects in lattices of (Fe 1–x Co x ) 17 RE 2 compounds (RE = Gd and Tb)

Constitutional studies on certain Fe-RE, Co-RE, and Fe-Co-RE (RE = Gd and Tb) alloys showed that the intermetallic compounds having the stoichiometry (Fe_1–xCo_x)₁₇RE₂ exhibited deviation from their ideal stoichiometric compositions. The deviation varied as the value of x changed from 0 to 1. To establish the cause of this deviation, theoretical densities for different defect structures were compared with the experimental densities of these compounds. The experimental data were in good agreement with the theoretical densities calculated on the basis of the assumption that there are vacant TM (TM =Fe, Co, or Fe + Co) sites in the lattices of these compounds. These vacant TM sites shift the composition of the respective compounds towards a higher RE content. The number of vacant TM sites was a function of x, e.g. the number of vacant TM sites varied from 1.8% in Fe₁₇Gd₂ to 6.9% in (Fe_0.15Co_0.85)₁₇Gd₂ in the TM-Gd compounds, whereas in the TM-Tb compounds the variation was from 1.8% for Fe₁₇Tb₂ to 5.8% for (Fe_0.23Co_0.77)₁₇Tb₂ and Co₁₇Tb₂.