Heavy fermion itinerant antiferromagnetic UNiAl is one of the very few U-co
ntaining compounds which absorbs H-2/D-2 without disproportionation. The pr
esent neutron diffraction studies on UNiAlDy (y = 2.2) are directed towards
resolving controversies with regard to the occupancy of Ni atoms and the a
ssociated interstitial sites for (H/D) atoms, as well as the nature of magn
etic ordering in the higher hydride phase with y greater than or equal to 2
. The fit to the neutron diffraction data is found to improve considerably
if the Ni atoms originally lying in the U-atoms' plane in UNiAl get shifted
to the Ni-Al atoms' plane in the deuteride. This is in agreement with an e
arlier neutron diffraction report on a deuteride sample of similar composit
ion [T. Yamamoto , J. Alloys Compd. 269, 162 (1998)] and our x-ray structur
al studies on UNiAlH2.3 [P. Raj , Phys. Rev. B 63, 94414 (2001)], but diffe
rs from those of Bordallo , [H. N. Bordallo , Physica B 276-278, 706 (2000)
] and of Kolomiets [A. V. Kolomiets , J. Appl. Phys. 87, 6815 (2000)]. Our
values of the structural parameters including the D-site occupancies are br
oadly in agreement with the results of Yamamoto The magnetization studies o
n UNiAlD2.2 show a single antiferromagnetic transition with Neel temperatur
e, T-N = 95 K. (C) 2001 American Institute of Physics.