Crystallographic and Mössbauer study of (Sc, Y, Ln) (Fe, Al) 2 intermetallic compounds

Abstract

Crystal structures have been studied in a series of alloys in the RFe 2-RAl 2 system in which R includes Sc, Y, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu. X-ray studies were made by the Debye-Scherrer method for arcmelted and annealed alloys. Earlier work had shown that where the composition is close to RFe 2 or RAl 2, the compounds are MgCu 2-type (cubic) Laves phases, but that in a composition region intermediate between RFe 2 and RAl 2 the compounds are MgZn 2-type (hexagonal) Laves phases. The present work defines the composition limits of the MgCu 2- and MgZn 2-type structures and reports the existence of a new MgCu 2-type compound based on ScFe 2. In general, the range of stability of the hexagonal compounds is shifted to higher aluminum concentration as the identity of the rare earth changes from Sm to Lu, although anomalous behavior is exhibited by Sc. When the regions of phase stability are depicted in a plot of volume per formula weight vs. composition, the phase boundaries between cubic and hexagonal phases are curved, but have similar shapes at both the iron-rich and aluminum-rich end of the diagram. The tendency to order into the Mg 2Cu 3 Si-type structure in the hexagonal phase has been studied by X-ray diffraction; various models of this ordering are treated. Both the cubic and the hexagonal phases have been studied by the Mössbauer effect with 57Fe. The hexagonal phases exhibit magnetic order at low temperatures but not at room temperature. At room temperature the cubic phases of Ho 33Al x Fe 66− x are magnetic for x = 10 and nonmagnetic for x = 20 or greater.