Relationship between the synthesis conditions and the structural, electrical, magnetic, and hydrogen absorption properties of the ternary compounds GdNi3X2 with X = Al, Ga, or Sn

The crystal structure of the ternary compounds GdNi3X2 (X = Al, Ga, or Sn) obtained by melting (M-sample) and subsequent annealing at 1273 K (A1-sample) or 1073 K (A2-sample) has been investigated by means of X-rag powder diffraction and transmission electron microscopy. GdNi3Sn2 crystallizes at all temperatures, in the hexagonal HoNi2.6Ga2.4-type GdNi3Ga2 presents tno varieties: the high-temperature form (M-sample) adopts the her;agonal CaCu5-type, whereas the HoNi2.6Ga2.4-type is observed for the A2-sample. The structural properties of GdNi3Al2 (M- and A1-samples) are more complicated; the electron diffraction patterns suggest the occurrence of a new superstructure of the hexagonal CaCu5-type. The study of the electrical and magnetic properties of these compounds shows that GdNi3Ga2 is ferromagnetic (T-C = 18.0(5)K), GdNi3Sn2 orders antiferromagnetically (T-N = 19.5(5)K), and GdNi3Al2 (M- or Al-sample) presents a ferromagnetic behavior (T-C = 20.0(5) K). Moreover, the hydrogen absorption properties of the Al-based intermetallic are dependent on its synthesis conditions. (C) 2000 Academic Press.