  A2B7 compounds (A = rare earth or Mg, B = transition metal) are widely
studied as active materials for negative electrode in Ni-MH batteries. By
playing on the substitution rate of both A and B elements, it is possible to
prepare various compositions. This strategy will help to improve the
properties, for example get a higher reversible hydrogen capacity and a
well-adapted hydrogen sorption pressure. Indeed, A can be almost all light rare
earths (La to Gd), yttrium and alkaline earth metals (Mg or Ca), whereas B can
contain various late transition metals (Mn to Ni). To understand the effects of
composition on the physicochemical properties of La2Ni7-based compound, various
pseudo-binary systems have been investigated: Gd2 xLaxNi7 (x = 0, 0.6, 1, 1.5),
Sm2-xLaxNi7 (x = 0, 0.5, 1, 1.5), Y2-xLaxNi7 (x = 0, 0.4, 0.5, 0.6, 0.8, 1,
1.5, 1.75) and A0.5La1.1Mg0.4Ni7 (A = Sm, Gd and Y). To determine their
crystallographic properties, X-ray diffraction analysis was performed followed
by Rietveld refinement. Thermodynamic properties regarding reversible hydrogen
sorption were investigated at room temperature. Quaternary compounds present
drastically improved sorption properties in the frame of practical energy
storage applications with reversible capacity equivalent to 400 mAh/g for the
compound La1.1Sm0.5Mg0.4Ni7.
