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 synthesis ed by the reaction of gaseous hydrogen with RNiIn compounds at 298 K and hy drogen pressures 1-100 bar and characterised by X-ray diffraction and therm al desorption studies. The hexagonal symmetry of the initial ZrNiAl-type st ructure is not changed on hydrogenation. Hydrogen insertion causes a pronou nced 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 si tes for the accommodation of hydrogen atoms in the lattices of dihydrides R NiInH1.7-2.0 were proposed. A reversible formation of equiatomic RNiIn tern aries accompanies a complete hydrogen desorption from the dihydrides and ta kes place at temperatures near 800 K. Hydrogen evolution proceeds through t wo 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 NdNiIn H0.85, which were structurally characterised as isotropically expanded ZrNi Al-type compounds. The melting points were determined for the LaNiIn (1057 K) and CeNiIn (1083 K) intermetallics. The NdNiIn compound exhibits high th ermodynamic stability and does not disproportionate in hydrogen at P-H2=1 b ar up to 1023 K. RNiIn compounds formed with Y or the heavier rare earth me tals (R=Sm, Gd, Tb, Dy, Ho, Er and Tm) do not form hydrides at hydrogenatio n pressures up to 100 bar, both at room temperature or on heating in hydrog en gas up to 1143 K. (C) 1999 Elsevier Science S.A. All rights reserved.