Hydrogenation induced valence change and metal atom site exchange at room temperature in the C14-type sub-structure of CeMn1.8Al0.2H4.4

CeMn1.8Al0.2 absorbs more than 4.4 hydrogen atoms near ambient conditions, thereby undergoing a volume expansion (DeltaV/Vsimilar to43%) that is the largest known among metal hydrides. While the Mn/Al distribution in the C14-type alloy [P6(3)/mmc, a=5.37393(6), c=8.76321(12) Angstrom] is partially ordered (preference of Al for site 2a). it tends to become disordered in the hydride (equal Mn/Al occupancies for sites 2a and 6h). Only very slow hydrogenation while cooling the sample to - 70 degreesC is capable of maintaining a partial Mn/Al order. Deuterium in CeMn1.8Al0.2D4.36(7) [a=5.9788(6), c=9.7600(13) Angstrom] occupies exclusively tetrahedral Ce-2(Mn,Al)(2) type interstices. Magnetic susceptibility data suggest that the volume expansion during hydrogenation is enhanced by a valence change of cerium (Ce-IV-->Ce-III). (C) 2003 Elsevier B.V. All rights reserved.