The ternary system: Aluminum–iron–praseodymium

Phase equilibria in the ternary system Pr–Fe–Al have been established in an isothermal section at 800 °C from room temperature x-ray powder diffraction analysis of about 50 alloys, which were melted, annealed at 800 °C, and quenched. Phase equilibria are characterized by the formation of rather extended homogeneous regions, i.e., by a random substitution of Fe/Al in Pr(Al_1−xFe_x)₂, 0 ≤ x ≤ 0.15, in Pr₂(Fe_1−xAl_x)₁₇, 0 ≤ x ≤ 0.65, as well as by the formation of at least four ternary compounds. Whereas the existence of PrFe₄Al₈ with the CeMn₄Al₈-type structure has been confirmed, there were no indications for a compound “PrFe₆Al₆” earlier claimed to crystallize with the ThMn₁₂-type structure. Pr₆(Fe_1−xAlx)₁₄, 0.16 ≤ x ≤ 0.36 with a homogeneous region parallel to the Fe–Al binary, was found to be isotypic with the La₆Co₁₁Ga₃-type of structure. Pr-rich alloys are liquid at 800 °C, and all the alloys Pr₂(Fe_1−xAlx)₁₇ with aluminum concentrations less than 5 at.% Al (x ∼ 0.07) enter a two-phase equilibrium with the Pr-rich liquid. At temperatures below 800 °C, alloys with compositions close to 30 at.% Pr and 5 at.% Al show a further ternary phase on solidification, whose crystal structure is related to the La₆Co₁₁Ga₃-type. PrFe₂Al₈ is a new representative of the CeFe₂Al₈-type structure. The crystal structure of the ternary compound richest in Al, PrFe₂Al₁₀, has not been solved yet.