Crystal structure of the La5Al4 compound

Article abstract of DOI:10.1016/j.jallcom.2003.08.024

The crystal structure of the La₅Al₄ compound, probably stabilized by some metallic admixtures, has been studied by X-ray powder diffraction. It was shown that La₅Al ₄ crystallizes in a new structure type (hexagonal symmetry, a = 9.1628(7) ?, c = 11.2309(7) ?).

Full text of DOI:10.1016/j.jallcom.2003.08.024

Journal of Alloys and Compounds 367 (2004) 132–136
Crystal structure of the “La Al ” compound
5
4
∗
N.M. Belyavina , V.Ya. Markiv, V.V. Zavodyanny
Department of Physics, Taras Shevchenko University, 6 Glushkov Ave., 03022 Kyiv, Ukraine
Abstract
The crystal structure of the “La
diffraction. It was shown that “La Al
2003 Elsevier B.V. All rights reserved.
5
Al
4
” compound, probably stabilized by some metallic admixtures, has been studied by X-ray powder
5
4
” crystallizes in a new structure type (hexagonal symmetry, a = 9.1628(7) Å, c = 11.2309(7) Å).
©
Keywords: Lanthanum–aluminum system; Crystal structure; Intermetallic compound; X-ray powder diffraction
1
. Introduction
the ingot. After arc melting the samples were wrapped in
molybdenum foil, sealed in an evacuated quartz tube and
annealed at 400 C for 1200 h, with subsequent quenching
◦
The La–Al phase diagram first presented in [1] exhib-
ited the existence of four intermetallic compounds: La3Al2,
LaAl, LaAl2 and LaAl4. Further investigations, carried out
using high-purity metals by Buschow [2], resulted in a vari-
ant of the La–Al phase diagram with six intermetallic com-
pounds: La3Al, LaAl, LaAl2, LaAl2+x, LaAl3 and La3Al11
in cold water.
The X-ray powder diffraction data were collected with a
DRON-3 automatic diffractometer (Cu K␣ radiation). The
phase composition of the as-cast and annealed alloys and the
crystal structure of the compounds were determined from
the X-ray diffraction patterns. These patterns were obtained
in a discrete mode using the following scanning parameters:
(the existence of the La3Al2 compound was not confirmed).
According to Buschow [2], and Gschneidner and Calder-
wood [3], two of them form in the La-rich region (50 at.% La
and more) and crystallize in the Ni3Sn (La3Al, a = 7.228 Å,
c = 5.517 Å) and CeAl (LaAl, a = 9.531 Å, b = 7.734 Å,
c = 5.809 Å) structure types, respectively.
◦
step scan 0.05 , counting time per step (3–5 s). The peak
positions and integral intensities of the observed reflections
were determined using full-profile analyses. After removal
of the Cu K␣2 component, the diffraction profiles were fitted
using Lorentz functions.
The existence of the La3Al2 compound in the La-rich
region of the La–Al phase diagram was confirmed by Chaban
and Kuz’ma [4] during their study of an isothermal section
of the La–Al–B system. A single crystal investigation of
La3Al2 [4] displayed hexagonal symmetry with a = 9.26 Å,
c = 11.20 Å.
For the phase composition of the alloys and the crystal
structure determinations we used our own software program
packages with special databases for the X-ray diffraction
data and for the structure types of intermetallic and inorganic
compounds (more than 10,000 information units). The Ito
method [5] was used to determine the symmetry and unit
cells of the compounds. Unit cell refinements were carried
out by a least-square method. By using the original software
complex [6], tests of the structure models and refinements
of the structural parameters were carried out.
2
. Experimental details
Alloys of the binary La–Al system (35–50 at.% Al) were
prepared by arc melting of lanthanum (97.67 wt.% purity)
and electrolytic aluminum (99.99 wt.%) under purified ar-
gon atmosphere. The ingots were remelted several times in
order to ensure perfect homogeneity. The chemical compo-
sition of the alloys was checked via the weight losses of
3
. Results and discussion
Since the previous investigations carried out using
high-purity metals (La 99.9 wt.% and Al 99.99 wt.%) [2] led
to the conclusion that the La Al compound does not exist,
∗
Corresponding author.
3
2
E-mail address: belmar@univ.kiev.ua (N.M. Belyavina).
first of all the raw lanthanum was tested with a “Philips”
0
925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.08.024

N.M. Belyavina et al. / Journal of Alloys and Compounds 367 (2004) 132–136
133
Table 1
Crystallographic data for the “La5Al4” compound
Atom
Site
x
y
z
La(1)
La(2)
La(3)
La(4)
Al(1)
Al(2)
Me(1)
Me(2)
6I
3g
3f
4h
6i
2e
2d
3f
0.3891(6)
0.7399(9)
0.237(1)
0.333
0.714(3)
0
0
0
0
0.336(1)
0.5
0
0.1572(9)
0.206(2)
0.732(4)
0.5
0.667
0
0
0.667
0
0.333
0.572(3)
0
¯
Space group
Independent reflections
Total B factor (Å )
Reliability factor
Me = 0.95 Al + 0.05 La
P62m (#189)
150
2
1.56(3)
RW = 0.042
X-ray spectrometer. The microprobe analysis showed the
presence (wt.%) of Pr (0.30), Nd (0.10), Gd (1.12) and Al
metry. It should be noted that the symmetry and the
refined values of the lattice constants of the “La Al4”
5
(
0.16) as main impurities (the content of other impurities
compound (a = 9.1628(7) Å, c = 11.2309(7) Å) are in
a satisfactory agreement with those obtained earlier for
the La3Al2 compound by single crystal investigation [4].
Because of the absence of systematic extinctions, the pos-
sible space groups are: P6/mmm, P622, P62m, P6m2 and
P6mm.
was less than 0.05 wt.%). The percentage of lanthanum was
equal to 97.67 and the total percentage of lanthanum and
other RE metals was about 99.3.
Five alloys of the La–Al system with compositions near
La3Al2 were checked by X-ray powder and microprobe
analysis. The diffraction patterns display the single-phase
state at ∼44 at.% Al, rather than at 40 at.% Al. It should
also be noted that the lattice constants of the binary phases,
coexisting with the “La3Al2” compound (LaAl, for exam-
ple) differ from those reported in [2,3]. Moreover, the lattice
¯
¯
Comparing the X-ray powder diffraction pattern for the
“La Al4” compound, as well as its crystallographic charac-
5
teristics, with those presented in the structure type database
did not reveal any probable structural analogues. There-
fore, the crystal structure determination was carried out
by the trial-and-error method. The calculations of several
possible structure models led to an atomic arrangement
◦
constants of LaAl annealed at 750 C (a = 9.465(7) Å,
b = 7.754(5) Å, c = 5.801(8) Å) and LaAl annealed at
◦
¯
4
00 C (a = 9.388(9) Å, b = 7.745(9) Å, c = 5.773(6) Å),
(space group P6m2) that gave a good agreement between
in turn, differ from each other. Certainly, the large dif-
ference in the lattice constants observed for the La-rich
compounds cannot be caused only by the observed amount
of impurities in the raw lanthanum. Due to these facts, the
observed and calculated intensities (Table 1, Fig. 1). It
should be noted that the statistical occupations of the 2d
and 3f positions (Me = 0.95 Al + 0.05 La) were used in
this model to provide the observed composition (44 at.%
studied compound will hereinafter be denoted as “La Al4”.
Al) of “La Al4”. The rest of the lanthanum and aluminum
5
5
The indexing of the diffraction pattern of the alloy
atoms were assumed to completely fill their positions in the
structure.
with 44 at.% Al (“La Al4”) indicated a hexagonal sym-
5
Fig. 1. Part of the observed X-ray diffraction powder pattern (points) and calculated pattern (solid line) for “La5Al4” (Cu K␣ radiation).

1
34
N.M. Belyavina et al. / Journal of Alloys and Compounds 367 (2004) 132–136
Table 2
Part of the X-ray diffraction pattern for “La5Al4”, Cu K␣ radiation
dcal (Å)
dobs (Å)
Ical
Iobs
h k l
dcal (Å)
dobs (Å)
Ical
Iobs
h k l
4
4
4
3
3
3
3
3
3
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
.584
.581
.242
.968
.744
.741
.550
.386
.240
.999
.899
.898
.808
.723
.647
.646
.645
.575
.394
.393
.341
.292
.291
.246
.244
.201
4.583
85
157
0 1 2
1 1 0
1 1 1
0 2 0
0 0 3
0 2 1
1 1 2
0 1 3
0 2 2
1 2 0
1 1 3
1 2 1
0 0 4
0 2 3
0 1 4
1 2 2
0 3 0
0 3 1
1 1 4
0 3 2
1 2 3
0 2 4
2 2 0
0 0 5
2 2 1
1 3 0
2.161
2.160
2.160
2.121
2.050
2.049
2.017
1.984
1.955
1.954
1.954
1.925
1.897
1.872
1.871
1.822
1.820
1.798
1.979
1.775
1.753
1.733
1.732
1.732
1.732
2.159
0
7
16
0 1 5
0 3 3
1 3 1
2 2 2
1 2 4
1 3 2
1 1 5
0 4 0
0 2 5
2 2 3
0 4 1
0 3 4
1 3 3
0 06
0 4 2
0 1 6
2 3 0
1 2 5
2 3 1
2 2 4
0 4 3
1 1 6
1 3 4
2 3 2
1 4 0
71
119
4
4.244
3.961
3.743
118
4
33
25
21
13
30
2
9
3
57
0
4
2.121
2.050
22
41
4
30
74
12
37
151
660
339
4
162
66
69
297
3
71
13
53
6
5
0
3.550
3.377
3.236
2.999
2.898
73
12
34
150
1000
–
1
10
61
1.984
1.954
1.930
1.897
1.870
5
34
111
2.808
2.721
2.649
4
163
432
30
63
50
3
1.819
1.797
8
4
–
1
78
57
41
23
0
15
48
15
25
99
2.392
1.775
–
1.732
30
1
103
2.340
2.290
43
10
2.242
2.201
20
46
27
48
The results of the refinement of the structure model for
The projections of these crystal structures on the xy-plane
the “La Al4” compound (reliability factor RW = 0.042),
and the typical polyhedrons of the aluminum atoms are
5
as well as the diffraction data, are summarized in Tables 1
and 2, and are shown in Fig. 1.
shown in Fig. 2. The main fragments of the “La Al4”
5
structure are: atomic grids (z = 0) similar to those in the
Fig. 2. Projection of the “La5Al4” structure on the xy-plane and coordination polyhedrons of the (a) Al(1), (b) Al(2), (c) Me(1) and (d) Me(2) atoms.

N.M. Belyavina et al. / Journal of Alloys and Compounds 367 (2004) 132–136
135
Fig. 3. Atomic grids in the “La5Al4” structure at (a) z = 0, (c) z ≈ 0.3, (d) z ≈ 0.5 and (b) in Y5Ga3 (Mn5Si3-type structure) at z = 0.25.
Mn Si3-type structure (Fig. 3a and b), crimped graphite-like
are comparable with the sum of the La + Al and Al + Al
5
grids (z ≈ 0.3, 0.7) (Fig. 3c) and other slabs (z ≈ 0.5)
atomic radii, respectively.
(Fig. 3d).
A comparison of the crystallographic characteristics of the
A set of interatomic distances is given in Table 3. The
“La Al4” compound with those contained in the database
5
shortest bond lengths (d
= 3.1 Å and dAl−Al = 2.7 Å)
of structure types of intermetallic and inorganic compounds
La−Al
Table 3
Interatomic distances in the crystal structure of “La5Al4”
Atom–atom
d (Å)
Atom–atom
d (Å)
Atom–atom
d (Å)
La(1)–1Al(1)
La(1)–1Me(1)
La(1)–1Me(1)
La(1)–1La(4)
La(1)–1La(4)
La(1)–2Al(1)
La(1)–2La(2)
La(1)–1Al(2)
La(1)–1La(1)
La(1)–1La(2)
La(1)–1La(3)
La(1)–1Me(2)
La(2)–2Al(1)
La(2)–1Me(1)
La(2)–1Me(1)
La(2)–2Al(2)
La(2)–4La(1)
La(2)–2La(1)
La(2)–2La(2)
La(3)–1Me(2)
La(3)–4Al(1)
La(3)–2Me(2)
3.317(2)
3.376(3)
3.379(3)
3.471(2)
3.474(1)
3.516(2)
3.644(3)
3.645(3)
3.684(2)
3.706(1)
4.022(2)
4.129(3)
3.310(3)
3.435(2)
3.441(1)
3.531(2)
3.644(3)
3.706(2)
4.126(3)
3.070(2)
3.353(2)
3.403(1)
La(3)–2Al(2)
La(3)–2La(3)
La(3)–2La(4)
La(3)–2La(4)
La(3)–2La(1)
La(4)–1Me(2)
La(4)–1Me(2)
La(4)–1Me(2)
La(4)–1Al(1)
La(4)–1Al(1)
La(4)–1Al(1)
La(4)–1La(1)
La(4)–1La(1)
La(4)–1La(1)
La(4)–1La(4)
La(4)–1Me(1)
La(4)–1La(3)
La(4)–1La(3)
La(4)–1La(3)
Al(1)–1Me(2)
Al(1)–1Al(2)
Al(1)–1La(2)
3.711(2)
3.761(2)
3.985(2)
3.990(2)
4.022(2)
3.244(2)
3.245(2)
3.251(2)
3.334(2)
3.339(2)
3.342(2)
3.471(2)
3.474(2)
3.479(2)
3.526(2)
3.852(2)
3.985(2)
3.990(2)
3.993(2)
2.654(2)
2.711(2)
3.310(2)
Al(1)–1La(1)
Al(1)–1La(4)
Al(1)–1La(4)
Al(1)–2La(3)
Al(1)–2La(1)
Al(2)–3Al(1)
Al(2)–3La(2)
Al(2)–3La(1)
Al(2)–3La(3)
Me(1)–2La(1)
Me(1)–2La(1)
Me(1)–2La(1)
Me(1)–1La(2)
Me(1)–1La(2)
Me(1)–1La(2)
Me(1)–2La(4)
Me(2)–2Al(1)
Me(2)–1La(3)
Me(2)–2La(4)
Me(2)–2La(4)
Me(2)–2La(3)
Me(2)–2La(1)
3.317(2)
3.334(2)
3.339(2)
3.353(2)
3.516(2)
2.711(2)
3.531(2)
3.645(2)
3.711(2)
3.376(2)
3.379(2)
3.384(2)
3.435(2)
3.441(2)
3.444(2)
3.852(2)
2.654(2)
3.070(2)
3.244(2)
3.245(2)
3.403(2)
4.129(2)

1
36
N.M. Belyavina et al. / Journal of Alloys and Compounds 367 (2004) 132–136
(more than 10,000 U) permits us to conclude that it crystal-
References
lizes in a new structure type.
[1] M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York,
Toronto, London, 1958.
4
. Conclusions
[
[
2] K.H.J. Buschow, Philips Res. Rep. 20 (1965) 337.
3] K.A. Gschneidner, F.M. Calderwood, Bull. Alloy Phase Diagr. 9 (1988)
686.
The obtained data did not give conclusive evidence for the
[
4] N.F. Chaban, Yu.B. Kuz’ma, Dokl. Akad. Nauk Ukr. SSR, Ser. A 11
existence of an intermetallic compound containing ∼44 at.%
Al in the La-rich part of the La–Al system. Even a small
amount of metallic impurities (such as Pr, Nd, Gd) leads to
the formation of the “La Al4” compound (earlier La3Al2).
This compound was also revealed in ternary systems. An
(1971) 1048.
[
5] V.P. Seryh, Collected Programs, Kharkov Physico-Technical Institute,
Kharkov, 1980.
5
[6] V. Markiv, N. Belyavina, in: Proceedings of the Second In-
ternational Scientific Conference of Engineering and Func-
tional Materials (EFM’97), Naukove Tovarystvo Shevchenka,
Taras Shevchenko Scientific Society, L’viv, 14–16 October 1997,
p. 260.
extended solid solution based on the “La Al4” compound
was detected, for example, in the La–Al–Ga system.
5
¯
A structure model for “La Al4” in the P6m2 space group
5
was proposed and refined.