Crystal Structure snd Hydrogen Sorption Properties of the YNi_5–XGa_X Alloys

Antun Drašner and Želimir Blažina*

Laboratory for Solid State Chemistry, Department of Materials Chemistry, Ruđer Bošković Institute, PO Box 1016, 10001 Zagreb, Croatia
* Corresponding author: E-mail: blazina@irb.hr
Phone: +385 1 4561 111; Fax: +385 1 4680 098

Abstract
Alloys of the general composition YNi_5–xGa_x were prepared and studied by X-ray powder diffraction. It was found that in the YNi_5–xGa_x system the single phase region exists up to the composition YNi₂Ga₃. The hexagonal structure of the prototype compound YNi₅ (CaCu₅ type, space group P6/mmm) is preserved in all alloys for x < 2. Within the composition region YNi₃Ga₂ – YNi₂Ga₃ the existing single phase alloys crystallize also in the hexagonal symmetry with the same space group P6/mmm, but with a larger unit cell of the YCo₃Ga₂ type. All single phase alloys have been exposed to hydrogen gas, whereby binary YNi₅, and ternary alloys with the YCo₃Ga₂ type of structure do not show any significant hydrogen absorption up to 5 MPa of hydrogen. All ternary alloys having the CaCu₅ structure react readily with hydrogen. The pressure composition desorption isotherms for the corresponding alloy-hydrogen systems were determined and it was found that the equilibrium pressure and the hydrogen capacity decrease with the increased gallium content. The change of entropy, the change of enthalpy and the change of the Gibbs free energy of formation for the alloy-hydrogen systems have also been extracted from the pressure composition desorption isotherms. The relevant thermodynamic parameters confirmed that the stability of the hydride increases with the increased amount of gallium.

Keywords: Yttrium-nickel-gallium alloys; crystal structure; hydrogen sorption