Hydrides of the RNiIn (R = La, Ce, Nd) intermetallic compounds: crystallographic characterisation and thermal stability

LaNiInH2.0, CeBiInH1.8 and NdNiInH1.7 intermetallic hydrides were synthesised by the reaction of gaseous hydrogen with RNiIn compounds at 298 K and hydrogen pressures 1-100 bar and characterised by X-ray diffraction and thermal desorption studies. The hexagonal symmetry of the initial ZrNiAl-type structure is not changed on hydrogenation. Hydrogen insertion causes a pronounced anisotropic expansion of the unit cells along [001] (Delta c/c=14.9-18.3%) and results in a volume increase of 8.9-9.3%. Possible interstitial sites for the accommodation of hydrogen atoms in the lattices of dihydrides RNiInH1.7-2.0 were proposed. A reversible formation of equiatomic RNiIn ternaries accompanies a complete hydrogen desorption from the dihydrides and takes place at temperatures near 800 K. Hydrogen evolution proceeds through two steps with peaks at 425-540 and 630-710 K and at temperatures 500-600 K leads to the formation of lower hydrides LaNiInH0.9, CeNiInH0.8, and NdNiInH0.85, which were structurally characterised as isotropically expanded ZrNiAl-type compounds. The melting points were determined for the LaNiIn (1057 K) and CeNiIn (1083 K) intermetallics. The NdNiIn compound exhibits high thermodynamic stability and does not disproportionate in hydrogen at P-H2=1 bar up to 1023 K. RNiIn compounds formed with Y or the heavier rare earth metals (R=Sm, Gd, Tb, Dy, Ho, Er and Tm) do not form hydrides at hydrogenation pressures up to 100 bar, both at room temperature or on heating in hydrogen gas up to 1143 K. (C) 1999 Elsevier Science S.A. All rights reserved.