Effect of aluminum substitution and rare-earth content on the structure of R2(Fe1-xAlx)17 (R=Tb, Dy, Ho, Er) phases

The phase equilibria in the iron rich parts of the R-Fe-Al (R=Tb, Dy, Ho, Er) phase diagrams at 500 degreesC have been studied by X-ray powder diffraction analysis. As the aluminum contents in the series R-2(Fe1-xAlx)(17) are increased the hexagonal Th2Ni17 type structure (or a substitution derivative of that structure) transforms - usually via a hexagonal TbCu7 type derivative structure - into the rhombohedral Th2Zn17 type structure (or a substitution derivative of;hac structure). The hexagonal structures dominate at or below the molar ratio R/(Fe,Al)=2/17 (10.5 and 9.5 at.% R), while the rhombohedral structure dominates. at higher R contents (11.5 at.%). The solubility of aluminum in the hexagonal structure increases as the atomic size of R decreases (from x=0-3.42 in Tb2Fe17-xAlx to x=0-4.75 in Er2Fe17-xAlx), while that in the rhombohedral structure decreases (from x=7.70-9.41 in Tb2Fe17-xAlx to x=0-4.75 in Er2Fe17-xAlx). The cell parameters suggest that the nature of the atomic substitutions in the various crystal structures changes as a function of both structure type and atomic size of R. (C) 2001 Elsevier Science B.V. All rights reserved.