A.V. Kolomiets et al. / Journal of Alloys and Compounds 402 (2005) 95–97
97
within the unit cell. They occupy the same 4c crystallographic
positions in PrNiH4.3 and in PrNi but the y coordinates are
changed: the atomic positions of Pr and Ni in PrNiH4.3 are
Pr (0, 0.1317(4), 1/4) and Ni (0, 0.4051(7), 1/4), and in
PrNi–Pr (0, 0.13822(4), 1/4) and Ni (0, 0.4276(1), 1/4) [4].
The pronounced changes of the lattice parameters naturally
lead to variations of the interatomic distances. The largest
increase, reaching 23.6%, is found for the Ni–Ni spacing.
The Pr–Pr and Pr–Ni distances increase by ≈8.1 and 12.8%,
respectively. These data are suggestive of one of the possible
hydrogen sites lying in the Pr2Ni4 octahedra with the cen-
ter (E) in the crystallographic position 4a (1/2, 1/2, 0). In
such case, the distances between nearest neighbours would
is higher for the high-pressure hydrogen absorption at room
temperature, leading to 4 hydrogen atoms per formula unit,
PrNi acts as effective hydrogen getter until it absorbs about
3 hydrogen atoms per formula unit.
The relative variation of the lattice parameter a, ꢀa/a for
the CeNi hydride is about four times larger than for the PrNi
hydride (see Table 1), which may be related to a of Ce valence
decrease upon hydrogenation.
Acknowledgements
This work is a part of the research plan MSM 0021620834
financed by the Ministry of Education of the Czech
Republic.
˚
˚
˚
be: E–E, 2.411(1) A; E–Pr, 1.969(4) A and E–Ni, 2.545(4) A.
On the other hand, the series of large empty trigonal prisms
with centers at y = 1/4 and 3/4 can also play an important role
in hydrogenation process.
Taking into account that one unit cell of the CrB-type
structure contains four formula units, one can easily see
that the hydrogen volume concentration in PrNiH4.3 and
CeNiH4.0 is larger than about 10% higher than in LaNi5,
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the standard hydrogen storage material. The specific vol-
3
˚
ume in PrNiH4.3 equals 12.9 A /H atom, in CeNiH4.0 equals
3
˚
14.1 A /H atom. If taking structure data for LaNi5H6.9
˚
˚
(CaCu5 structure type, a = 5.399 A and c = 4.290 A) from
3
˚
Refs. [8–10], we obtain 15.7 A /H atom forLaNi5H6.9
.
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3. Conclusions
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PrNi and CeNi hydrides were obtained for the first time,
using two different synthesis techniques. The sample quality