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
Riabov
