Magnetic properties and crystal structure of HoNiAl and UNiAl hydrides

The effect of hydrogenation on the magnetic properties and crystal structur e of 4f and 5f intermetallics was studied on HoNiAlH2.0, UNiAlH2.3 and UNiA lD2.1. The incorporation of hydrogen into HoNiAl leads to a decrease of the magnetic ordering temperature from 13 to 6 K, while it increases from 19 K to values in the range 90-100 K in the UNiAl-H system. The crystal lattice s of all compounds are anisotropically expanded. Moreover, the crystal symm etry is lowered to orthorhombic in HoNiAlH2.0, but remains hexagonal for UN iAlH(D)(x). Three different deuterium positions were found for both compoun ds. (C) 2000 American Institute of Physics. [S0021-8979(00)94208-7].