Preparation of single crystals of LaAl and X-ray structure determination

Article abstract of DOI:10.1016/S0925-8388(98)00636-7

Single crystals of the compound LaAl have been prepared by the reaction of a mixture of lanthanum and aluminum under nitrogen at 950 °C. Previously reported crystal structure data determined by X-ray powder diffraction are now confirmed by an X-ray single crystal structure analysis on LaAl: space group Cmcm, Z = 8, a = 9.455(1) angstroms, b = 7.753(1) angstroms, c = 5.791 (1) angstroms, R1/wR2 = 2.4/5.6, N(F²≥2σ(I²)) = 685, N(var.) = 16. The structure of LaAl contains zig-zag chains formed by two crystallographically independent Al atoms with alternating angles Al-Al-Al of 64° and 180°. The chains are connected via lanthanum atoms.

Full text of DOI:10.1016/S0925-8388(98)00636-7

Journal of Alloys and Compounds 278 (1998) L10–L12
L
Preparation of single crystals of LaAl and X-ray structure determination
Letter
*
Andreas Leineweber, Herbert Jacobs
¨
Fachbereich Chemie der Universitat Dortmund, D-44221 Dortmund, Germany
Received 11 May 1998
Abstract
Single crystals of the compound LaAl have been prepared by the reaction of a mixture of lanthanum and aluminium under nitrogen at
9508C. Previously reported crystal structure data determined by X-ray powder diffraction are now confirmed by an X-ray single crystal
2
2
˚
˚
˚
structure analysis on LaAl: space group Cmcm, Z58, a59.455(1) A, b57.753(1) A, c55.791(1) A, R1/wR252.4/5.6, N(F $2s(I ))5
685, N(var.)516. The structure of LaAl contains zig-zag chains formed by two crystallographically independent Al atoms with alternating
angles Al–Al–Al of 648 and 1808. The chains are connected via lanthanum atoms.
1998 Published by Elsevier Science S.A. All rights
reserved.
Keywords: Compound in the system La/Al; Crystal structure of LaAl; Zintl–Klemm compound LaAl
1. Introduction
boat and heated in a quartz tube under N2 atmosphere up
to 9508C. After 2 days the tube was cooled down to r.t. (1
day) and opened in a glove box filled with Ar [6].
The outer part of the regulus mainly consisted of black
LaN. The inner part was metallic and brittle. By powder
diffraction (CuKa1) LaAl (see below) and La3Al (a5
The phase diagram of the system La/Al was first
investigated by Canneri [1]. Three intermetallic phases
were found: LaAl4 (later on verified as La3Al11 [2]),
LaAl2 and LaAl. Further studies additionally showed the
existence of La3Al and LaAl3 (compilation of known
compounds see e.g. [3]). The structure of LaAl was
Table 1
Technical data concerning the single crystal structure analysis of LaAl
`
determined together with those of CeAl and PrAl by Becle
Crystal dimensions/mm3
0.1530.1530.05
9.455(1)
7.753(1)
5.791(1)
2.948
and Lemaire [4]. Powder patterns of the three isotypic
compounds were indexed, a structure was solved and
refined for data of CeAl. Neutron diffraction was per-
formed on CeAl and PrAl [5]. The magnetic structures
were determined but no structural parameters were refined.
So X-ray single crystal data of a CeAl-type structure are
not published up to now.
In the course of investigations on the system La/Al/N
we obtained single crystals of LaAl sufficient in size for an
X-ray structure determination.
˚
lattice parameters a/A
˚
b/A
˚
c/A
rx /(g cm23
)
Z
8
space group type
1/m(MoKa)/mm
radiation
Cmcm (Nr. 63)
0.0497
MoKa
monochromator
scan type
graphite
v/2q
qmin, qmax
h, k, l
Rint
3.48; 408
0–17, 614, 610
5.7%
absorption correction
number of reflections
Fo asym. unit
c-scan
3251
733
685
2. Experimental
2.1. Synthesis
F2o$2s(F2o)
extinction coefficient
minimal transmission
number of variables
R1/wR2 (685 reflections)
R1/wR2 (733 reflections)
0.0093(5)
28.3%
16
2.4%; 5.6%
2.7%; 5.8%
2.8; 21.9
A pellet from aluminium powder (99.5%, Fa. Alfa
(Karlsruhe)) and lanthanum turnings (3N, Rare Earth Prod.
Ltd.) in a molar ratio of 1:2 was placed into an alumina
3
˚
max./min. residual electron density/A
*
Corresponding author.
0925-8388/98/$19.00
1998 Published by Elsevier Science S.A. All rights reserved.
PII: S0925-8388(98)00636-7

A. Leineweber, H. Jacobs / Journal of Alloys and Compounds 278 (1998) L10–L12
L11
˚
Table 2
5.050(1) A with Cu3Au structure [3]) were identified as its
components. From the inner part silver-coloured single
crystals of LaAl could easily be isolated whilst La3Al only
was microcrystalline.
Positional and isotropic displacement parameters of LaAl
2
˚
Site
Atom
x
y
z
100?Ueq /A
8g
4a
4c
La
Al(1)
Al(2)
0.32168(2)
0
0
0.16106(2)
0
0.3019(2)
1/4
0
1/4
0.994(8)
1.27(2)
1.01(3)
2.2. Structure analysis
The crystal quality was checked by precession photo-
graphs (MoKa). Intensity data were collected on a
MACH3 diffractometer (Enraf–Nonius, Delft) using
MoKa radiation. An orthorhombic unit cell analogous to
that given in [4] was found. Details of data collection are
given in Table 1 and the results of structure refinement
(SHELXL93 [7]) in Table 2 and Table 3.
Table 3
2
˚
Components of the Uij tensors of LaAl in A /100
Atom
U11
U22
U33
U12
U13
U23
0
20.03(4)
0
La
Al(1)
Al(2)
0.87(1)
1.84(7)
1.03(6)
1.05(1)
0.87(5)
0.92(5)
1.06(1)
1.09(5)
1.08(5)
0.035(5)
0
0
0
0
0
3. Discussion
Table 4
Comparison of structure parameters of LaAl and CeAl [4] (for LaAl in
˚
˚
˚
[4] unit cell parameters are given a59.531 A, b57.734 A, c55.809 A)
The positional parameters of LaAl obtained by us are
close to the results from the refinement of powder data of
CeAl given in [4] (Table 4).
Two non-equivalent Al atoms form plane zig-zag chains
running along the direction [001] (Fig. 1), with an angle of
648 (Al(1)–Al(2)–Al(1), Fig. 2). Thus the distances
LaAl
CeAl [4]
La
x50.32168(2)
y50.16106(2)
y50.3019(2)
x50.321
y50.161
y50.290
Al(2)
Fig. 1. Perspective view of the structure of LaAl along the direction [001]; short distances in Al–Al chains are indicated.

L12
A. Leineweber, H. Jacobs / Journal of Alloys and Compounds 278 (1998) L10–L12
in contrast to Al exhibits localized bonding (forming Ga–
Ga dumbbells [10]). For a comparison of LaAl with LaGa
the structural features are compatible with the higher
electronegativity of Ga vs. Al.
La2Al(1)Al(2) is crystallographically closely related to
e.g. the structures of Na2CuAs, K2CuAs and K2CuSb [11].
Acknowledgements
The authors are indebted to Dr. U. Zachwieja for
performing the data collection on LaAl, the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen
Industrie for financial support of this investigation.
˚
Fig. 2. Zig-zag chains of Al in LaAl (distances in A, angle in degree).
˚
˚
d(Al(1)–Al(2))52.75 A are only slightly shorter than
d(Al(1)–Al(1))52.90 A.
Between different chains the shortest distances d(Al(2)–
˚
Al(2)) in the plane (100) are 4.22 A. The chains are
References
˚
connected by lanthanum atoms with d(Al–La)$3.23 A.
The Zintl–Klemm-rule leads for LaAl to ‘La31Al32
’
[1] G. Canneri, Metall. Ital. 24 (1932) 3.
[2] A.H. Gomes de Mequita, K.H.J. Buschow, Acta Crystallogr. 22
(1967) 497.
[3] P. Villars, L.D. Calvert. Pearson’s Handbook of Crystallographic
Data for Intermetallic Phases, ASM International (1991).
with Al32 being isoelectronic to sulfur. Twofold coordina-
tion of Al atoms by itself (like in simple chains or rings) is
expected but the situation appears to be somewhat more
complicated. Only Al(2) has clearly two Al(1) neighbours
(Fig. 2) whilst Al(1) has two additional close contacts to
Al(1) suggesting an additional bonding interaction. In
contrast to LaAl the homologue LaGa [8] (as well as
isoelectronic CaSi [9]) crystallizes in the CrB structure
type with simple zig-zag chains of Ga atoms (bonding
angles of approx. 1108).
`
[4] C. Becle, R. Lemaire, Acta Crystallogr. 23 (1962) 840.
[5] H. Asmat, B. Barbara, D. Ginoux, J. Solid State Chem. 22 (1977)
179.
[6] H. Jacobs, D. Schmidt, Curr. Top. Mater. Sci. 8 (1981) 379.
[7] G.M. Sheldrick. SHELXL93, Programm for Crystal Structure
¨
refinement, University of Gottingen, Germany (1993).
´
[8] E. Parthe, D. Hohnke, W. Jeitschko, O. Schob, Naturwissenschaften
52(7) (1955) 155.
Thus LaAl has a structure slightly more complicated
than that of the homologue LaGa, with a tendency of Ga to
form more stable two-centres-2e-bonds. This property is
also obvious from the structures of the elements, where Ga
[9] E. Hellner, Z. Anorg. Allg. Chem. 261 (1950) 226.
[10] R. Nesper, Angew. Chem. Int. Ed. Engl. 20 (1991) 789.
¨
[11] B. Eisenmann, G. Savelsberg, H. Schafer, Z. Naturforsch. 31b
(1976) 1344.