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Samples with composition R2Fe14M3 (M = Ga and Si; R = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm) have been prepared by are-melting. Crystal-structure studies have shown that the prepared samples are single phase with the rhombohedral Th2Zn17 and/or the hexagonal Th2Ni17 structure. Substitution of Ga for Fe in R2Fe17 leads to an increase of the unit-cell volume, whereas Si substitution reduces the unit-cell volume. The Curie temperatures of the R2Fe14Ga3 and R2Fe14Si3 compounds are much larger than those of the corresponding R2Fe17 compounds. The Fe moment decreases upon Ga or Si substitution. The compounds R2Fe14Ga3 and R2Fe14Si3 With R = Er and Tm exhibit a spin-reorientation transition at low temperature. (C) 1997 Elsevier Science Ltd.