Effect of hydrogenation on crystal structure and magnetic properties
of UT Si „ T Ä Pd,Ni… intermetallics
A. V. Kolomiets,
1,2,
* L. Havela,
1
A. V. Andreev,
3
F. Wastin,
4
J. S
ˇ
ebek,
3
and M. Marys
ˇ
ko
3
1
Department of Electronic Structures, Charles University, Ke Karlovu 5, 121 16 Prague 2, The Czech Republic
2
Department of Physics, National University ‘‘Lvivska Politehnika’’, Bandery 12, 79013 Lviv, Ukraine
3
Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, The Czech Republic
4
European Commission, Joint Research Centre, Institute for Transuranium Elements, Post Box 2340, D-76 125, Karlsruhe, Germany
Received 21 March 2002; published 31 October 2002
The crystal structure and magnetic properties of UPdSi and UNiSi hydrides synthesized at T =923 K and
hydrogen pressure p =130 bars were studied. Hydrogenation results in modification of the lattice symmetry
from the orthorhombic TiNiSi structure type to hexagonal ZrBeSi type and in the increase of magnetic
ordering temperatures in both compounds. UPdSiH
x
orders antiferromagnetically at T
N
=46 K compared to
T
N
=31 K for UPdSi. UNiSiH
x
is a ferromagnet with T
C
=98 K, whereas UNiSi is an antiferromagnet with
T
N
=85 K. The observed changes are attributed to the increased uranium-uranium spacing in the hydrides.
DOI: 10.1103/PhysRevB.66.144423 PACS numbers: 75.30.-m, 61.10.-i, 81.40.Rs
I. INTRODUCTION
Hydrogenation of uranium-based intermetallic com-
pounds, leading to enlarged U-U spacing and modified bond-
ing conditions, is an interesting probe of 5 f -electron sys-
tems. Magnetic properties, which are very sensitive to the
degree of the 5 f localization, are important indicators of
electronic structure variations.
1
In addition, hydride studies
are also relevant for hydrogen storage issues.
2
However, not all U intermetallics absorb hydrogen under
the usual laboratory conditions. Attempts to hydrogenate the
UTX materials ( T =transition metal atom, X = p -electron el-
ement crystallizing in the ZrNiAl structure type were suc-
cessful in a few cases only,
3,4
although rare-earth R iso-
types absorb H
2
willingly.
5
In this work we describe the synthesis and study of two
hydrides based on UPdSi and UNiSi, representatives of the
UTX compounds crystallizing in the orthorhombic TiNiSi
structure type.
UPdSi was thoroughly investigated by several groups by
means of x-ray and neutron diffraction, magnetization, and
transport measurements.
6–11
It was established that UPdSi
crystallizes in the TiNiSi-type of structure space group
Pnma ) having the shortest inter-uranium spacing d
U-U
350 pm. This value is close to the critical region between
localized and itinerant 5 f -electron behavior.
1
UPdSi is an
antiferromagnet, which undergoes two magnetic phase tran-
sitions at T
N
=29 K and T
1
=10 K according to Ref. 6 or
T
N
=33 K and T
1
=27 K according to Refs. 8 and 9. It has
also two metamagnetic transitions at
0
H
c
1,2
=4 T and 7 T
( T =4.2 K), respectively, at which the antiferromagnetic
coupling of the U moments is gradually broken.
9
The other intermetallic compound studied, UNiSi, has
also the TiNiSi-type of unit cell, but with a lower d
U-U
330 pm.
12
It shows more complicated magnetic behavior
and three magnetic phase transitions are reported for this
compound. Magnetization measurements point to antiferro-
magnetic ordering at T
N
=80 K followed by another phase
transition at T
1
=7.5 K.
6
Specific heat studies revealed an
additional transition at T =18 K.
8
The nonzero spontaneous
magnetization observed at lower temperatures is associated
with ferromagnetic ordering or the presence of a ferromag-
netic component. Below T =10 K, the magnetization curves
have a complicated S-shaped character, which indicates that
in the ground state UNiSi is a ferrimagnet or a ferromagnet
with a canted structure.
10
The low value of the effective mo-
ment
eff
=2.1
B
/f.u. given in Ref. 6, as well as the poor
approach to saturation of the ‘‘free-powder’’ magnetization
curve even in the field of 35 T ( M
35T
=0.52
B
/f.u.), points
to strongly delocalized U 5 f states see Ref. 13 and refer-
ences therein. The ferromagnetic component disappears
above T =18 K.
10
The negative value of the paramagnetic
Curie temperature
P
=-13 K estimated from the magnetic
susceptibility measured above 80 K points to antiferromag-
netic interactions dominating in the system.
6
II. EXPERIMENTAL DETAILS
The parent compounds were obtained by arc melting of
stoichiometric amounts of constituents under Ar atmosphere.
The phase composition was checked by means of x-ray dif-
fraction on the Siemens D-500 diffractometer equipped with
Cu used for UPdSi-H and Co used for UNiSi-H anodes.
Prior to exposing the samples to hydrogen, they were
crushed to submillimeter particles and their surface was ac-
tivated by annealing for 2 h at T =473 K in the dynamic
vacuum of p =2 10
-5
mbar. Hydrogenation was per-
formed at T =923 K under hydrogen pressure p
H
2
=130 bars. Several attempts to obtain the hydrides at lower
H
2
pressure and lower temperature were unsuccessful. The
absorption process was monitored by the pressure variations
in closed calibrated volume. It was considered to be com-
pleted after the pressure drop finished. The crystal structures
of the synthesized products were checked by x-ray diffrac-
tion. It was not possible to determine precisely enough the
hydrogen content directly during the synthesis, so the weight
change due to the H
2
uptake was used to estimate the hydride
composition. It appeared that the samples of both compounds
PHYSICAL REVIEW B 66, 144423 2002
0163-1829/2002/6614/1444237/$20.00 ©2002 The American Physical Society 66 144423-1