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 (A2sample) has been investigated by means of X-ray powder diffraction and transmission electron microscopy. GdNi3Sn2 crystallizes at all temperatures, in the hexagonal HoNi2.6Ga2.4-type. GdNi3Ga2 presents two varieties: the high-temperature form (M-sample) adopts the hexagonal CaCu5-type, whereas the HoNi2.6Ga2.4-type is observed for the A2sample. 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 (TC=18.0(5) K), GdNi3Sn2 orders antiferromagnetically (TN=19.5(5) K), and GdNi3Al2 (M- or A1-sample) presents a ferromagnetic behavior (TC=20.0(5) K). Moreover, the hydrogen absorption properties of the Al-based intermetallic are dependent on its synthesis conditions