Synthesis and crystal structure of TbNiGeD1.8

Article abstract of DOI:10.1016/j.jallcom.2004.03.073

A hydride of TbNiGe has been synthesized at <160°C and studied by neutron and X-ray diffraction. TbNiGeD_1.8 (space group P6 ₃/mmc, a=4.10477(13)A?, c=8.0208(3)A?) takes the ZrBeSi-type structure with D surrounded by Tb₃Ni in tetrahedral coordination. Each D-filled Tb₃Ni tetrahedron shares three edges (via Tb-Tb bonds) and one corner (via Ni) with equivalent Tb₃Ni tetrahedra. During deuteration, a displacive transition of the metal lattice takes place, which increases the symmetry from orthorhombic (TiNiSi-type structure) to hexagonal. TbNiGeD_1.8 has the geometry of an AA stacking of close-packed Tb layers.

Full text of DOI:10.1016/j.jallcom.2004.03.073

Journal of Alloys and Compounds 379 (2004) 72–76
Synthesis and crystal structure of TbNiGeD1.8
∗
H.W. Brinks
Institute for Energy Technology, P.O. Box 40, Kjeller 20027, Norway
Received 19 February 2004; accepted 1 March 2004
Abstract
◦
A hydride of TbNiGe has been synthesized at <160 C and studied by neutron and X-ray diffraction. TbNiGeD1.8 (space group P6
3
/mmc,
Ni in tetrahedral coordination. Each D-filled
Ni tetrahedron shares three edges (via Tb–Tb bonds) and one corner (via Ni) with equivalent Tb Ni tetrahedra. During deuteration, a
a = 4.10477(13) Å, c = 8.0208(3) Å) takes the ZrBeSi-type structure with D surrounded by Tb
3
Tb
3
3
displacive transition of the metal lattice takes place, which increases the symmetry from orthorhombic (TiNiSi-type structure) to hexagonal.
TbNiGeD1.8 has the geometry of an AA stacking of close-packed Tb layers.
©
2004 Elsevier B.V. All rights reserved.
Keywords: Powder neutron diffraction; Synchrotron radiation; Crystal structure; Hydrides
1
. Introduction
Both for TbNiGe and TbNiGeD1.8 there are a few weak
unidentified reflections.
◦
Intermetallic TbNiGe takes the orthorhombic TiNiSi-type
After activation in vacuum at 600 C, deuteration was
◦
structure (space group Pnma) [1], which is the most com-
mon structure type among the RTX (R = rare earth,
T = transition metal and X = P-block element) inter-
metallics [2]. Recently, the isostructural LaNiSn, TbNiSi
and CeNi0.82Cu0.18Sn were found to transform into the
ZrBeSi-type structure (ordered AlB2-type structure) upon
deuteration [3–5]. In the present study a TbNiGe-based
deuteride is for the first time presented. The structure is de-
termined by powder neutron and X-ray diffraction data, and
the results are compared with LaNiSnD2 and TbNiSiD1.78.
performed below 150 C and at pD > 3 bar. The absorp-
2
tion is a slow process and at least 24 h is necessary for
saturation. The deuterium content of the samples was de-
termined by a calibrated Sieverts apparatus, which uses a
MKS120AA pressure sensor at pressures less than 30 bar
(accuracy: 0.08% of reading) and MKS870 pressure sensor
at 30–100 bar (1% of reading). The total volume used for
these experiments was 129.36 ml. The temperature for this
volume was (except for the sample holder, its 2 ml connec-
tion line and the MKS120AA pressure sensor) controlled to
◦
4
0.00 ± 0.02 C. TbNiGe (2.530 g) was used for the volu-
metric measurements. The first minutes after exposing the
sample to high pressure have been removed from the plot
due to lack of stable temperatures.
2
. Experimental
TbNiGe was prepared from Tb (purity 99.8%), Ni
PND data of TbNiGeD1.8 were collected at the PUS
instrument at the JEEP II reactor, Kjeller, Norway. The
experimental set-up consisted of focusing Ge(5 1 1)
(99.9%) and Ge (99.999%) by arc melting in an argon at-
mosphere. The ingot was re-melted several times and heat
◦
◦
treated at 600 C for several weeks to increase the homo-
monochromator with take-off angle approximately 90 , λ =
3
geneity. Powder X-ray diffraction (PXD) (Siemens D5000,
Cu K␣1, primary monochromator, position sensitive de-
tector, Si internal standard) confirmed the formation of
TbNiGe with orthorhombic TiNiSi-type crystal structure.
1
.5554 Å and two banks of seven position-sensitive He de-
◦
tectors, each covering 20 in 2θ. Intensities were collected
◦
◦
◦
from 2θ = 10 to 130 in steps of ꢀ(2θ) = 0.05 at 22 C.
The program Fullprof version 2.2c [6] was used for the
PND Rietveld refinements. Scattering lengths were taken
from the program library. Pseudo-Voigt profile functions
were used, and background points were selected manually.
∗
Tel.: +47-64-80-64-99; fax: +47-64-81-09-20.
E-mail address: hwbrinks@ife.no (H.W. Brinks).
0
925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.03.073

H.W. Brinks / Journal of Alloys and Compounds 379 (2004) 72–76
73
3
. Results and discussion
furthermore heating of a sample in the Sieverts-type appa-
ratus directly after absorption at 100 C and 85.7 bar; Fig. 2.
◦
3
.1. Synthesis of TbNiGe deuteride
The sample starts to desorb slowly directly after releasing
the pressure (to less than 1 mbar), and desorbs gradually up
to around 230 C when heated quickly. No signs of any or-
◦
Initial attempts to deuteride the intermetallic were car-
ried out at 5 bar in a closed hydrogenation set-up at different
dered phases could be seen. The final pressure was 3.02 bar.
The remaining hydrogen is removed by heat treatment in
vacuum at 300 C.
temperatures. TbNiGe was found to absorb hydrogen below
◦
◦
1
60 C, but at a slow rate. Metal hydride with a composition
of approximately TbNiGeD did desorb hydrogen in one step
◦
against 3.6 bar D2 in the temperature range 190–205 C at
3.2. Crystal structure
a heating rate of 1.5 K/min. Similarly, by cooling, the inter-
◦
metallic starts to reabsorb at the same pressure at 160 C.
Both PND and PXD data for the deuterated sample could
be refined based on the TiNiSi model (space group Pnma)
of the alloy, but with a strongly anisotropic expansion of
the unit cell (a: +15.5%, b: −3.0%, c: −2.4%). The in-
sertion of hydrogen results in larger distances between the
bc layers, which are distorted closed-packed layers of Tb.
The relation between the refined unit-cell dimensions is
Saturation to TbNiGeD1.8 (determined by Rietveld refine-
ments of PND data) was achieved by slow cooling at 5 bar
to room temperature.
Additional absorption experiments were then carried
with an accurate Sieverts-type apparatus at 24 and 86 bar in
the temperature range 100–145 C, cf. Fig. 1. The sample
was heated to at least 300 C and then heat treated in vac-
uum for desorption between the absorption experiments. At
45 C, even after nearly 100 h at 24 bar, the metal hydride
is not completely saturated. Decreasing of the temperature
to 120 C with the same initial pressure improves the ki-
netics, whereas decreasing the temperature from 125 via
20 to 100 C at the same pressure worsen the kinetics.
◦
◦
in accordance with a simpler hexagonal description with
√
two formula units per unit cell (ah = bo = co/ 3, ch =
ao). There are two hexagonal TiNiSi-related structure types
that may be candidates: LiGaGe- (space group P63mc) and
ZrBeSi-type structure (space group P63/mmc). TiNiSi-type
structure (ordered CeCu2-type structure) can be described
as a deformed LiGaGe-type structure (ordered CaIn2-type
structure), which again is a deformed ZrBeSi-type structure
(ordered AlB2-type structure) [4].
◦
1
◦
◦
1
Hence, the best balance between large driving force and
◦
◦
good dynamics seems to be at around 125 C. At 120 C,
two different initial pressures were used, and an increased
pressure improves, as expected, the kinetics.
Refinements in all three models give very similar fits and
result in structures which are virtually identical. Hence,
the crystal structure with the highest symmetry, namely
ZrBeSi-type structure, was chosen. The fit of the PND
Low temperature is necessary in order to reach high hy-
drogen concentration as evidenced from the PND results and
Fig. 1. The amount of hydrogen absorbed in TbNiGe as a function of time for several temperatures and initial pressures. The hydrogen content is in
deuterium atoms per formula unit.

7
4
H.W. Brinks / Journal of Alloys and Compounds 379 (2004) 72–76
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Desorption of TbNiGeD1.27
Pressure increase from 0 to 3.02 bar
100
150
200
250
300
t/ ºC
◦
Fig. 2. Amount of hydrogen released by heating TbNiGeD1.27 in a closed Sieverts-type apparatus from initially 100 C and a pressure below 1 mbar.
The final pressure was 3.02 bar.
Fig. 3. Rietveld refinement of PND data for TbNiGeD1.8 at PUS. Upper lines show the fit of the observed data (circles). Positions of Bragg reflections
are shown with bars in the middle. The difference between observed and calculated intensity are shown with the bottom line.

H.W. Brinks / Journal of Alloys and Compounds 379 (2004) 72–76
75
Table 1
◦
The refined structural parameters for TbNiGeD1.8 at 22 C
a
c
4.10477(13)
8.0208(3)
0.0491(3)
92(1)
0.16(5)
0.15(5)
0.09(6)
1.05(7)
7.31
Zd
nD (%)
BTb (Å )
Bni (Å )
BGe (Å )
BD (Å )
2
2
2
2
Rwp (%)
2
x
4.81
The space group is P63/mmc, and Tb occupies 2a (0, 0, 0), Ni 2c (1/3,
/3, 1/4), Ge 2d (1/3, 2/3, 3/4) and D 4f (1/3, 2/3, z) sites.
2
Fig. 4. The crystal structure of TbNiGeD1.8. Tetrahedra of D(Tb3Ni) share
Tb–Tb edges and Ni corners with each other.
refinements are shown in Fig. 3 and the resulting structural
parameter in Table 1. The crystal structure is shown in
Fig. 4. D takes a 4f site surrounded by Tb3Ni. The D(Tb3Ni)
tetrahedra are connected via edges (Tb–Tb) and corners
respectively, several possible sites were considered: Tb Ni,
3
(Ni) to other equal tetrahedra. The shortest D–D distance is
Tb Ge, Tb NiGe and Tb NiGe. Their sizes are 0.516, 0.502,
3
3
2
across the edge sharing: 2.497(1) Å.
0.489/0.821 (along Ni–Ge and equatorial, respectively) and
The distances between D and the metal atoms (Tb–D
0.343 Å. The latter interstice is too small and all but Tb Ni
3
2
.402(1) Å, Ni–D 1.611(2) Å) are reasonable compared
to the binary hydride structures [7,8]. Furthermore, Tb–D
distances in TbNiAlD1.1 [9], Tb3Ni Al2D [10] and
contain Ge. The Tb Ni position furthermore complies with
the D–D distance rule. Hence, an occupation of this po-
sition is expected from the crystal chemistry of metal hy-
3
6
6.5
TbNiSiD1.78 [4] are 2.422, 2.205 and 2.360 Å, respectively.
drides. When the Tb Ni position is filled, its size increases
3
The Ni–D distances in the same compounds are 1.602,
to 0.611 Å.
1
.707 and 1.622 Å.
There are several established empirical rules in the crys-
As shown in Fig. 5, the structure becomes considerably
more regular upon hydrogenation. The Tb–Tb distances
become equal, the angles regular in the close-packed layers
of Tb, and Ni is placed midway between the Tb layers. This
increases the symmetry from orthorhombic to hexagonal
symmetry. LaNiSn and TbNiSi, which undergo the same
structural transition upon hydrogenation, are chemically
tal chemistry of intermetallic metal hydrides: a minimum
of P elements in the surrounding of hydrogen, the inter-
stices in the metal lattice have to have a radii of at least
0
.4 Å and the D–D distance has to be at least 2.0 Å. Based
on the radii 1.782, 1.246 and 1.369 Å for Tb, Ni and Ge,
D
2
(a)
D
2
(b)
Fig. 5. Upon hydrogenation the structure becomes less deformed, both (a) in the Tb-layers (bc-layers in TiNiSi-type structure) and (b) the positions of Ni.

7
6
H.W. Brinks / Journal of Alloys and Compounds 379 (2004) 72–76
very related and have R3T sites of similar size: 0.63, 0.46
and 0.52 Å.
[4] H.W. Brinks, V.A. Yartys, B.C. Hauback, J. Alloys Compd. 322
2001) 160.
(
[5] B. Chevalier, M. Pasturel, J.L. Bobet, J. Etourneau, Solid State
Commun. 129 (2004) 179.
[
[
6] J. Rodr ´ı guez-Carvajal, Phys. B 192 (1993) 55.
7] Q. Huang, T.J. Udovic, J.J. Rush, J. Schefer, I.S. Anderson, J. Alloys
Compd. 231 (1995) 95.
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