Zr4Al3D2.68 and Zr3Al2D2.26: New Zr-containing intermetallic hydrides with ordered hydrogen sublattice

Article abstract of DOI:10.1016/S0925-8388(02)01217-3

Two hydrogenated intermetallics with the highest Al/Zr ratio among the hydrogen-absorbing Zr-Al compounds, Zr₄Al₃ and Zr₃Al₂, have been studied by synchrotron X-ray, powder neutron diffraction and thermal desorption spectroscopy. Initial intermetallic compounds are quite different with respect to Al-Al interactions and contain plain Kagome Al nets (Zr₄Al₃) or Al-Al pairs (Zr₃Al₂). In hexagonal Zr₄Al₃D_2.68 (space group (s.g.) P6₃22; a = 11.0017(4); c = 11.1694(5) ?) a 2a×2a×2c superstructure is formed as a result of deuterium ordering in half of the available Zr₄ tetrahedra. These tetrahedra share common corners and edges and form layers separated by 6363 Al-nets. In tetragonal Zr₃Al₂D_2.26 (s.g. P4₂/mnm; a = 7.5970(3); c = 7.2613(3) ?) in addition to the completely filled Zr₄ tetrahedra hydrogen partially occupies Zr₃ triangular sites. Thermal stability of the studied deuterides and Zr-D bonding characteristics can be related to the size of the occupied Zr₄ tetrahedra. Higher thermal stability of Zr₃Al₂D_2.26 agrees well with the existence of large Zr₄ sites and contrasts to the behavior of Zr₄Al₃D_2.68 containing 'contracted' Zr₄ tetrahedra and having weaker Zr-D bonds.

Full text of DOI:10.1016/S0925-8388(02)01217-3

Journal of Alloys and Compounds 356–357 (2003) 91–95
L
www.elsevier.com/locate/jallcom
Zr Al D
and Zr Al D : new Zr-containing intermetallic hydrides
4
3
2.68
3
2
2.26
with ordered hydrogen sublattice
a
b ,
a
b
*
A.B. Riabov , V.A. Yartys , R .V . Denys , B.C. Hauback
a
Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, 5 Naukova Street, 79601 Lviv, Ukraine
b
Institute for Energy Technology, P.O. Box 40, Kjeller N-2027, Norway
Received 11 July 2002; accepted 26 October 2002
Abstract
Two hydrogenated intermetallics with the highest Al/Zr ratio among the hydrogen-absorbing Zr–Al compounds, Zr Al and Zr Al ,
4
3
3
2
have been studied by synchrotron X-ray, powder neutron diffraction and thermal desorption spectroscopy. Initial intermetallic compounds
are quite different with respect to Al–Al interactions and contain plain Kagome Al nets (Zr Al ) or Al–Al pairs (Zr Al ). In hexagonal
4
3
3
2
˚
(space group (s.g.) P6 22; a511.0017(4); c511.1694(5) A) a 2a32a32c superstructure is formed as a result of deuterium
3
Zr Al D
4
3
2.68
ordering in half of the available Zr4 tetrahedra. These tetrahedra share common corners and edges and form layers separated by 6363
˚
(s.g. P4 /mnm; a57.5970(3); c57.2613(3) A) in addition to the completely filled Zr tetrahedra
2 4
Al-nets. In tetragonal Zr Al D
3
2
2.26
hydrogen partially occupies Zr3 triangular sites. Thermal stability of the studied deuterides and Zr–D bonding characteristics can be
related to the size of the occupied Zr tetrahedra. Higher thermal stability of Zr Al D
agrees well with the existence of large Zr sites
4
3
2
2.26
4
and contrasts to the behavior of Zr Al D containing ‘contracted’ Zr4 tetrahedra and having weaker Zr–D bonds.
4
3
2.68

2002 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Gas–solid reactions; Neutron diffraction; X-ray synchrotron diffraction; Crystal structure and symmetry
1
. Introduction
2. Experimental
Hydrogen storage capacities of binary Zr–Al intermetal-
The alloys, Zr Al and Zr Al , were prepared by argon
4
3
3
2
lic compounds strongly depend on the relative Zr content
in the alloys. The highest values, 0.9–1.0 at.H/M, are
reached in Zr AlH [1] and Zr Al H [2] (62.5–66.7
arc melting of the stoichiometric mixtures of high purity
constituent elements, Zr and Al. They were annealed in the
evacuated quartz tubes at 900 8C for 7 days (Zr
at 1100 8C for 48 h (Zr Al ), and quenched into the ice
water after the annealing. X-ray diffraction study of the
prepared alloys (Siemens D 5000 diffractometer; Cu Ka
radiation; Bregg Brentano geometry; PSD) showed that
intermetallic compounds Zr Al and Zr Al were formed
as the main constituents of the 4:3 and 3:2 alloys,
respectively. The Zr Al alloy also contained minor addi-
tions of two secondary phases, Zr Al and ZrAl. The
alloy contained two admixtural intermetallics,
Al ) and
2
2.7
5
3
8.0
4
3
at.% Zr). Following a decrease of Zr content in the
intermetallics to 57.1–60.0 at.% Zr, the hydrogenation
capacities drop significantly, to 0.5 at.H/M in Zr Al H
2.5
3
2
3
2
1
[
1] and Zr Al H [3].
4 3 3.7
Despite the detailed data concerning the hydrogenation
4
3
3
2
properties of a few Zr–Al compounds are available, the
crystal structure data are published only for one deuteride,
oxygen-stabilised Zr Al O0.5D2.7 [4].
4
3
5
3
3
2
This work is aimed on studying by means of powder
neutron and X-ray diffraction the crystal structures of two
Zr
Zr
Zr
3
4
4
Al
Al
Al
2
3
3
and Zr
Al
. The refined unit cell parameters of
5
3
˚
new compounds, Zr Al D
and Zr Al D2.26.
(hexagonal; a55.424(1); c55.387(2) A), Zr
3
Al
2
4
3
2.68
3
2
˚
(
tetragonal; a57.628(1); c56.986 (2) A), ZrAl (ortho-
˚
rhombic; a53.434(3); b510.96(1); c54.305(5) A) and
Zr Al (hexagonal; a58.181(3); c55.682(5) A) agree
˚
5
3
*
Corresponding author. Tel.: 147-63-806-453; fax: 147-64-810-920.
well with the reference data in Ref. [5].
E-mail address: volodymyr.yartys@ife.no (V.A. Yartys).
The prepared alloys were deuterated by charging with
0
925-8388/02/$ – see front matter
 2002 Elsevier B.V. All rights reserved.
doi:10.1016/S0925-8388(02)01217-3

9
2
A.B. Riabov et al. / Journal of Alloys and Compounds 356–357 (2003) 91–95
deuterium (purity 99.8%) at pressures of 1–5 bar and at
room temperature, after preliminary activation in secon-
dary vacuum at 400 8C.
Structures of deuterides were refined by Rietveld-type
analysis (General Structure Analysis System software
(
GSAS) [6]) of powder neutron diffraction data collected
at PUS diffractometer on JEEP II reactor (Kjeller) (l5
˚
1
1
.5555 A; focusing Ge(511) monochromator; 2Q510–
308; D2Q50.058; 2400 data points). Samples were put
into the sealed under argon cylindric vanadium sample
cans with inner diameter 5 mm. Nuclear scattering lengths
were taken from the GSAS library (b 57.16, b 53.45,
Zr
Al
b 56.67 fm). Since the deuterides Zr Al D
and
Zr Al D were present in both studied materials, their
D
3
2
2.21
4
3
2.7
structures were refined step by step, in several iterations.
Two other constituents of the deuterated materials,
Zr Al O D and ZrAl, were introduced into the refine-
Fig. 1. The crystal structure of Zr Al . Altering layers of Zr tetrahedra
and plain 6363 Kagome Al-nets are shown.
4
3
4
5
3
x
2.7
ments based on the available reference data [4,5].
Synchrotron XRD data were collected at ESRF, SNBL
using monochromatic X-rays (l50.49868 A) obtained
chemical surrounding and size considerations are Zr sites
6j.
4
˚
from Si(111). The samples were kept in rotating 0.3 mm
sealed glass capillaries.
Similar to Zr Al , the tetragonal crystal structure of
4
3
Zr Al is characterised by a separation of Zr and Al atoms
3
2
Model of hard spheres has been used in the calculations
of the radii of interstitial sites in the structures of inter-
in the structure. Zr atoms form centred cubes ZrZr , which
stack into columns aligned along [001]. Via sharing the
edges, the columns form a spatial network. Aluminium
8
metallic compounds with the values of the radii, r 5
Zr
˚
˚
1
.602 A and r 51.432 A, taken from Ref. [7]. Crystal
atoms centring the Zr trigonal prisms form Al–Al pairs
Al
6
structure analysis was performed with use of ATOMS, Shape
Software.
located inside the channels of the framework of Zr-col-
umns.
Thermal stability of hydrides was studied by monitoring
pressure changes during their heating in secondary vacuum
with a rate 2 8C/min.
A large number of interstices in the structure of the
tetragonal Zr Al compound includes tetrahedra Zr4,
3
2
Zr Al (four types) and Zr Al and, also, two types of
3
2
2
octahedral sites, Zr Al and Zr Al (Table 2). Similar to
4
2
2
4
Zr Al , preferable for H insertion are Zr sites (8i ).
4
3
4
1
3
. Results and discussion
3
.2. Crystal structure of Zr Al D
4 3 2.68
3
.1. Crystal chemical analysis of intermetallic matrices
as potential H storage materials
The synchrotron X-ray diffraction data from Zr Al D
4 3 2.68
showed that hydrogen absorption leads to the expansion of
the unit cell of the compound by 6%, with no changes in
the original symmetry, Da/a51.23%, Dc/c53.46%.
Analysis of the collected PND data indicated that
Hexagonal crystal structure of Zr Al (own type of
4
3
structure) is closely related to the CaCu -type. It can be
5
obtained from CaCu5 via substitutions 1Ca→2Zr;
2
Cu(2c)→2Zr; 3Cu(3g)→3Al, changing the stoichiometry
deuterium atoms fill tetrahedral Zr sites. It also showed an
4
from RT to R T . An important feature of the CaCu type
appearance of extra rather strong diffraction peaks, which
5
4
3
5
structure, presence of the 6363 Kagome nets, is retained in
the Zr Al structure (Al nets). In turn, Zr atoms form 6
Table 1
4
3
3
a
Interstitial sites in the crystal structure of Zr Al intermetallic compound
4
3
nets built from the regular Zr6 hexagons. The linear Zr
chains centre these hexagons leading to a formation of the
Zr4 tetrahedra combined into the clusters of 6. The
tetrahedra in a cluster are connected by sides and have one
common ‘axis’ edge. As a result, the structure of Zr Al
˚
r (A) Neighbours
Site
Surrounding
x
y
z
6j
12n
Zr1 Zr2
0.272
0.365
0.208 2x
0.139 2x
0
0
0
0.40
236j; 2312n
6j; 12n; 2312o
2312n; 6m; 4h
2312o; 236m
3312o; 4h
2
2
Zr1Zr2
Al
0.188 0.40
0.293 0.37
2
1
2o
Zr1Zr2Al2
4
3
6
4
m
h
Zr1 Al
1/2
0.28
2
2
can be described as an alternation along [001] of the layers
of the clustered Zr4 tetrahedra and Kagome nets of Al
atoms (Fig. 1).
Zr2Al3
1/3
2/3 0.356 0.32
˚
Space group P6/mmm (No. 191); a55.4273; c55.3927 A; 2 Zr1 in
2
e:a 0, 0, z, z51/4; 2 Zr2 in 2c: 1/3, 2/3, 0; 3 Al in 3g: 1/2, 0, 1/2.
In Tables 1 and 2 the sequence of interstitial sites follows a decrease
There are five types of tetrahedral interstices in the
Zr Al structure, including Zr , Zr Al, Zr Al (two) and
4
3
4
3
2
2
of Zr content (increase of Al content) and, for the sites with an equal
surrounding, a decrease in their radii.
ZrAl (Table 1). The most favourable for H insertion from
3

A.B. Riabov et al. / Journal of Alloys and Compounds 356–357 (2003) 91–95
93
Table 2
Interstitial sites in the crystal structure of Zr Al intermetallic compound
3
2
Site
Surrounding
x
y
z
rINT
Neighbours
Tetrahedral interstices
8
8
6k1
6k2
6k3
i1
j1
Zr1 Zr2Zr3
0.146
0.112
0.220
0.250
0.157
0.394
0
0.533
0.112
0.557
0.422
0.707
0.394
0
0
0.49
0.45
0.41
0.40
0.38
0.32
0.29
2316k ; 238i
8j ; 4e; 2316k
1 1
8i ; 8j ; 16k ; 16k
1 1 3
2
1
2
Zr2 Zr3Al
0.478
0.105
0.253
0.261
0.235
0.368
2
1
1
1
Zr1Zr2Zr3Al
Zr1Zr2Zr3Al
Zr1Zr2Zr3Al
Zr2Zr3Al2
Zr2 Al
2
2
16k ; 8j ; 16k ; 8i
1
2
3
16k ; 16k ; 16k ; 8i
2
2
3
1
8
4
j2
e
2316k ; 4e; 2a
2
238j ; 238j
2
2
2
1
Octahedral interstices
8
2
i2
a
Zr1 Zr2Zr3Al
0.315
0
0.060
0
0
0
0.61
0.53
238i ; 2316k ; 2a; 8i ; 2316k
2
2
1
2
2
3
Zr3 Al
438i ; 438j
2
4
2
2
˚
Space group P4 /mnm (No. 136); a57.6244; c56.9862 A; 4 Zr1 in 4d: 0, 1/2, 1/4; 4 Zr2 in 4f: 0.34685, x, 0; 4 Zr3 in 4g: 0.19807, 2x, 0; 8 Al in 8j:
2
0
.1222, x, 0.2091.
were indexed on the basis of the formation of a 2a32a3
c superstructure. Since these peaks are not present in the
2
XRD data, they originate from deuterium ordering (Fig. 2).
From group–subgroup relations and observed extinc-
tions, the space group of symmetry of the formed super-
structure was defined as P6 22 (see Table 3). In total, the
3
supercell contains 48 Zr4 sites. Deuterium ordering
proceeds via occupation of half of the available Zr4
tetrahedra (24). Hydrogen ordering within the Zr4 sites
proceeds in a way that neighbouring side-connected inter-
stices are never simultaneously occupied. The doubling of
the hexagonal unit cell parameters takes place due to the
alternation of the occupancy of the Zr4 sites in the
hexagonal clusters of 63Zr (Fig. 3a, b). Furthermore, the
4
doubling of the c-axis is caused by the alternation of
occupancy/vacancy of the equivalent interstices aligned in
the columns of these clusters.
Fig. 2. PND pattern of Zr Al D at 293 K showing observed (1),
2.68
calculated (2) and difference (line at bottom) plots. The positions of the
peaks of the constituent phases are marked (from bottom to top):
4
3
Zr Al D , Zr Al D and ZrAl. The Bragg indexes are given for the
2.26
strongest observed superstructure peaks only (marked by arrows).
H sublattice is built from the planar D nets separated
along [001] by Al-formed Kagome nets. Thus, in
4
3
2.68
3
2
Table 3
Atomic coordinates, occupancies and temperature factors for Zr Al D
and Zr Al D
from the refinements of the PND data
2.26
4
3
2.68
3
2
Zr Al D
Zr Al D
3 2 2.26
4
3
2.68
Space group
Unit cell
parameters, A
P6 22 (No. 182)
P4 /mnm (No. 136)
2
3
˚
˚
a511.0017(4) A;
a57.5970(3) A;
˚
˚
˚
c511.1694(5) A
c57.2613(3) A
2
2
˚
˚
Atomic
UISO 3100 A
1.1(3)
0.19(9)
0.7(4)
1.0(2)
1.5(7)
UISO 3100 A
coordinates,
occupancies
4 Zr1 in 4e: 0, 0, 0.1148(9);
12 Zr2 in 12i: 0.4966(7), 0.001(1), 0.1256(3);
4 Zr1 in 4d: 0, 1/2, 1/4
4 Zr2 in 4f: 0.3465(3), 0, 0;
4 Zr3 in 4g: 0.1969(3), 2x, 0;
8 Al in 8j: 0.1226(4), x, 0.2103(5);
8.00(2) D1 in 8i: 0.1379(3), 0.5329(3), 0;
0.33(7)
0.32(7)
0.30(9)
0.60(9)
1.02(6)
and temperature 4 Zr3 in 4f: 1/3, 2/3, 0.019(4);
factors
12 Zr4 in 12i: 0.179(1), 0.339(3), 0.007(3);
6
6
1
6
4
1
Al1 in 6h: 0.257(5), 2x, 1/4
Al2 in 6h: 0.758(5), 2x, 1/4
2 Al3 in 12i: 0.007(7), 0.249(6), 0.259(2);
.00(2) D1 in 6g: 0.144(2), 0, 0;
.85(10) D2 in 6g: 0.360(2), 0, 0;
3.0(9)
1.1(3)
1.5(1)
1.6(5)
1.02(5) D2 in 16k: 0.235(3), 0.389(3), 0.274(4); 0.2(7)
0.60(10) D3 in 12i: 0.366(2), 0.511(2), 0.001(3); 2.7(3)
Rpr
5.86
7.86
2.662
4.28
5.41
1.192
R
wpr
2
x

9
4
A.B. Riabov et al. / Journal of Alloys and Compounds 356–357 (2003) 91–95
Fig. 3. Altering layers of the occupied Zr4 interstices in the crystal structure of Zr Al D viewed along [001] at z50 (a) and z51/2 (b). The borders of
2.68
4
3
the original subcell and the superstructure cell are shown.
Zr Al D hydrogen diffusion is not possible along [001]
addition to this site, H atoms enter 16k site inside the
2
4
3
3
˚
Zr Zr Zr Al tetrahedron (r50.43 A). However, since D2
1 2 3
and proceeds via the two-dimensional diffusion paths
located in the planes, which are perpendicular to the
c-axis. The ordering of deuterium atoms within hydrogen
sublattice eliminates short D . . . D separations, so the
atoms are shifted from the ideal centre of the tetrahedral
interstice away from Al atoms, it makes them coordinated
by the Zr Zr Zr triangle only. Such a shift increases
1
2
3
˚
˚
shortest distance between the occupied Zr sites, 2.402 A
Al–D2 distance to 2.082 A and indicates nonbonding
interaction between D2 and neighbouring Al atom. The
occupied tetrahedral interstices form a spatial framework
by sharing common vertices and/or edges. In hydrogen
sublattice D1 atoms are separated from neighbouring D
4
˚
(
D1 . . . D2), exceeds 2 A. The shortest D–D distance
between the neighbouring D layers is very long, 5.75 A.
˚
The occupancies of the three filled Zr sites are slightly
4
different, from 80.9 to 100%. Thus, the overall stoi-
chiometry, Zr Al D2.68, is smaller than the ideal value of
˚
atoms by at least 2.407 A (D1 . . . D2). D2 site is a
4
3
Zr Al D3.0. Since the metal sublattice is slightly deformed,
split-position with the distance to the similar D2 site of
1.65 A. Because of rather small, 6%, occupancy of this
4
3
˚
the occupied Zr sites are non-equivalent in size. Radii of
4
the occupied interstices in the structure fall within the
range 0.42 . . . 0.47 A, which is lower than the size limit
site, neighbouring D2 sites are presumably never simul-
taneously filled and such short D2 . . . D2 separations are
avoided. Deuterium atoms are situated inside the columns
˚
for the occupied interstices in the chemically related
˚
Zr FeD , Zr FeD and Zr FeAl D , 0.48 . . . 0.53 A
formed from the ZrZr cubes and form a 8-vertex coordi-
2
5.0
3
6.7
6
2
9.8
8
[
8,9]. Smaller size of the occupied interstices in
nation polyhedron around Zr1 (Fig. 4). The radius of the
˚
Zr Al D is caused by short Zr1 . . . Zr1 separations in
the structure (2.740 A), which even in the saturated
hydride are 15% shorter than the doubled radius of Zr
atom (3.204 A). Radii of the empty interstices in this
structure vary from 0.39 to 0.44 A.
completely occupied Zr tetrahedron, 0.51 A, fits well with
4
3
2.68
4
˚
the earlier observed range of the radii for the significantly
occupied interstices in the structures of Zr-based hydrides
[8].
˚
˚
Since D atoms avoid filling interstices having Al in their
˚
surrounding, the shortest D . . . Al separation exceeds 3 A.
3.4. Thermal stability of deuterides
Thermal stability of the studied deuterides and Zr–D
bonding characteristics can be related to the size of the
occupied Zr4 tetrahedra. Higher thermal stability of
3
.3. Crystal structure of Zr Al D
3 2 2.26
The hydrogenation of Zr Al intermetallic compound
Zr Al D
(decomposition with the main desorption
3
2
3
2
2.26
preserves its tetragonal symmetry and results in an aniso-
event at 600 8C) agrees well with the existence of large Zr4
sites and contrasts to the behaviour of Zr Al D con-
tropic expansion of the unit cell (Da/a 520.4%; Dc/c 5
0
0
4
3
2.68
3
.9%; DV/V 53.2%). The crystal structure data for this
taining ‘contracted’ Zr4 tetrahedra and having weaker
Zr–D bonds (desorption starts at much lower temperature
of 400 8C).
0
compound are given in Table 3. In Zr Al D hydrogen
atoms completely occupy tetrahedral interstices 8i Zr . In
3
2
2.26
1
4

A.B. Riabov et al. / Journal of Alloys and Compounds 356–357 (2003) 91–95
95
(
0.45 at.D/at.M) to exceed that of Zr Al D (0.38
2.68
4
3
at.D/at.M) and can be related to a more dense packing of
the metal atoms in Zr Al , with shorter distances between
4
3
Zr atoms compared to Zr Al .
3
2
Further studies of Al-containing intermetallics as H
storage materials are important to find the ways of
involving Al into the hydrogenation process thus approach-
ing high H weight content (10 wt.%) of AlH3.
Acknowledgements
The skilful assistance from the project team at the
Swiss–Norwegian Beam Line, ESRF, is gratefully ack-
nowledged.
References
[1] K.H.J. Buschow, P.C. Bouten, A.R. Miedema, Rep. Progr. Phys. 45
Fig. 4. The crystal structure of Zr Al D
2.26
viewed along [001].
(1982) 937.
3
2
Deuterium atoms form 8-vertexes polyhedra around Zr1 atoms and are
located inside the columns built from the Zr1Zr8 cubes. Al atoms form
pairs and are coordinated by the side-connected trigonal prisms Zr6.
[2] N.J. Clarck, E. Wu, J. Less-Common Metals 142 (1988) 148.
[3] K.N. Semenenko, E.E. Fokina, V.N. Fokin, S.L. Troitskaya, V .V .
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[
5] P. Villars, L.D. Calvert, Pearson’s Handbook of Crystallographic
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Materials Park, OH, 1991.
Conclusions
H storage capacity of the chemically related Zr–Al
intermetallics is influenced by the ratio of active (Zr) and
non-active (Al) to the hydrogenation constituents. After
reaching maximum possible occupation of the Zr-formed
Zr4 tetrahedra, hydrogen atoms enter less attractive and
[
6] A.C. Larson, R.B. von Dreele, General Structure Analysis System
(
GSAS), LANSCE, MS-H 805 (1994).
[7] W.B. Pierson, The Crystal Chemistry and Physics of Metals and
Alloys, Interscience, New York, 1972.
[
8] V.A. Yartys, H. Fjellv a˚ g, I.R. Harris, B.C. Hauback, A.B. Riabov,
M.H. Sørby, I.Yu. Zavaliy, J. Alloys Comp. 293–295 (1999) 74.
smaller in size Zr triangular sites revealing the nonbond-
3
[9] V.A. Yartys, Crystal chemistry of novel intermetallic hydrides, 8th
European Conference on Solid State Chemistry, Oslo, Norway, July,
2001, I-6.
ing interactions with the neighbouring Al-atoms. Filling of
Zr sites allows the hydrogenation capacity of Zr Al D
2.26
3
3
2