REAu4Al8Si: the end member of a new homologous series of
intermetallics featuring thick AuAl2 layers†
Susan E. Latturner and Mercouri G. Kanatzidis*
Department of Chemistry, Michigan State University, East Lansing, Michigan.
E-mail: kanatzid@cem.msu.edu; Fax: +1 517-353-1793; Tel: +1 517-355-9715
Received (in West Lafayette, IN, USA) 10th June 2003, Accepted 18th July 2003
First published as an Advance Article on the web 7th August 2003
DOI: 10.1039/b306641j
REAu4Al8Si (RE = Ce-Gd, and Yb) is the end member of a
new series of intermetallic structures grown from aluminium
flux; this new structure contains thick AuAl2 slabs and the
ytterbium analog exhibits mixed-valent behavior.
2340
When silicon and transition metals are included to improve the
properties of commercial aluminium alloys, they often form as
yet unexplored multinary intermetallics within the aluminium
matrix.1 Exploratory synthesis, particularly using aluminium
flux, has made great strides in mapping out the wide range of
possible phases produced in the alloying process. Addition of
silicon to the synthesis often results in more complex structures,
along with improved physical properties such as hardness,
chemical stability, and high melting point.2 New quaternary
structures such as Sm2Ni(NixSi12x)Al4Si6,3 RE8Ru12Al4
5
49Si9(AlxSi122x), and RE4Fe2+xAl72xSi8 have been obtained
from the combination of rare earth metals, transition metals, and
silicon in aluminium flux. Expanding this investigation to the
third row transition metals has resulted in a number of new
compounds such as the Th2(AuxSi12x)[AuAl2]nSi2 series of
interrelated structures.6 In this work, we describe the end
member of a new homologous series of structures — RE[AuAl2]nAl2(AuxSi12x)2 (n = 0–3; x ≈ 0.5) — formed by the
combination of an early rare earth metal, gold, and silicon in
aluminium flux.7
REAu4Al8Si grows from aluminium flux as large silver
rectangular plates, sometimes coated with a reddish–purple
film. The crystals are stable to air and moisture, and fairly stable
to acids and bases. Other reaction products include REAuAl2Si
and REAu2Al4Si and recrystallized gold and silicon. As will be
described, REAu4Al8Si with RE = Ce–Sm form in a tetragonal
P4/mmm structure with one mixed site; analogs with RE = Sm
and Gd form in a closely related polymorph — an ordered I4/
mmm supercell.
The disordered REAu4Al8Si structure is shown in Fig. 1a.
This structure adopts the tetragonal space group P4/mmm with
the early rare earth elements from cerium to europium; a
lanthanum-containing analog has not been isolated.8 The rare
earth ions are arranged in a square grid with a RE–RE distance
of around 4.25 Å (the a cell parameter). The local coordination
environment of the rare earth is similar to that of barium in the
BaAl4 structure type.9 These rare earth containing layers are
separated from each other by large slabs of gold and aluminium
atoms in an antifluorite type arrangement. The overall configuration can therefore be considered an intergrowth of AuAl2 and
BaAl4-type structures. Parthe et al. investigated a number of
RE/Ni/Ga ternary compounds that were also described as
intergrowths of BaAl4 and the antifluorite (CaF2) structure. In
these compounds, however, the BaAl4 blocks were the
dominant feature. Their synthesis was carried out via arc
melting with high amounts of RE and a high RE : Ni ratio,
promoting the formation of BaAl4 layers.10 The RE[AuAl2]nAl2(AuxSi12x)2 series explored in our work contains thick
† Electronic Supplementary Information (ESI) available: crystallographic
data for all analogues (tables of atomic positions, thermal parameters, bond
lengths) in the form of cif files. See http://www.rsc.org/suppdata/cc/b3/
b306641j/.
CHEM. COMMUN., 2003, 2340–2341
layers of antifluorite AuAl2 and is not obtainable via arc
melting.
In the BaAl4 type layer, the 2h wyckoff site forms a cube
around the rare earth ion, at a distance of 3.2–3.3 Å. Analysis of
the X-ray data indicate that this site is occupied by a mixture of
gold and a lighter atom, either Al or Si; for all the analogs, the
ratio is close to 1 : 1. The bond lengths from this site to
neighboring atoms include a 2.465(2) Å bond to its symmetry
generated equivalent, and a 2.573(1) Å bond to four aluminium
atoms. A mixture of gold and aluminium would be expected to
have longer bonds; it is therefore apparent that the site is
occupied by a mixture of Au and Si. This is also supported by
electronegativity arguments. Au and Si, as the most electronegative atoms in the structure, would be expected to be in close
vicinity to the electropositive RE ion. The assignment of Au and
Si to this site in a 1 : 1 ratio results in the stoichiometry
REAu4Al8Si, in agreement with elemental analysis.
The other predominant motif in the REAu4Al8Si structure is
the AuAl2 slab, highlighted in Fig. 1. Gold and aluminium
atoms are arranged in an antifluorite structure, as is the case in
bulk AuAl2 (cubic, Pm-3m, a = 5.99 Å).9 The Au–Al bond
lengths in the middle of this layer (Au(1)–Al(1), at 2.600(1) Å
in the Pr analog, for instance) are almost identical to those in
bulk AuAl2 (2.597 Å); toward the edges of the layer they begin
to vary from this value (to 2.607(1) Å and 2.573(1) Å, also in the
Pr analog). This trend was also apparent in the Th2Au5Al8Si2
structure, which also possesses a thick AuAl2 slab.6 The
antifluorite layer is approximately 12 Å thick in the REAu4Al8Si and Th2Au5Al8Si2 structures; examples with thicker
AuAl2 moieties have not been found as of yet, indicating that
these two compounds are the end members of their respective
series.
In the majority of these compounds gold and silicon appear to
randomly occupy the capping sites of the AuAl2 layer.
However, an ordered superstructure was also observed for the
Fig. 1 A. Structure of REAu4Al8Si (disordered, space group P4/mmm),
viewed down the a-axis. Bond distances shown are for the Pr analog. B.
Structure of REAu4Al8Si (ordered supercell, space group I4/mmm) viewed
down the 110 direction.
This journal is © The Royal Society of Chemistry 2003
Sm and Gd analogs. Ordering of Au and Si in a strictly
alternating fashion produces a cell with a* = A2a and c* = 2c
and I4/mmm symmetry.11 This structure is shown in Fig. 1b.
The Au–Si bond length is 2.42 Å; again, the Au–Al distances
approach those in bulk AuAl2 toward the center of the
antifluorite slab. The synthetic conditions that promote the
formation of the ordered superstructure are not clear at this
time.
The cell parameters of REAu4Al8Si vary regularly with the
rare earth element with the exception of the Eu analog, as shown
in Fig. 2. The c-cell parameter actually increases as the rare
earth radius decreases, but this is compensated by the shrinking
of the a-cell edge, so the overall unit cell volume still decreases
as expected. Magnetic susceptibility measurements (Fig. 3a)
confirm that Eu is present as the divalent ion in EuAu4Al8Si.
The high temperature data can be fit to the Curie–Weiss law,
indicating a magnetic moment of 8.11 mB, in good agreement
with the theoretical value of 7.94 mB for Eu2+ ions. An
antiferromagnetic transition is also evident at 10 K. The unit cell
parameters of the cerium analog also deviate slightly from the
linear trend observed by the other rare earths, but there was no
clear evidence of Ce3+/Ce4+ mixed valence in the magnetic
susceptibility data of CeAu4Al8Si.
Fig. 2 Plot of unit cell parameters vs. rare earth for REAu4Al8Si compounds.
The Gd analog has been normalized to the subcell to allow for
comparison.
Fig. 3 Magnetic susceptibility data for EuAu4Al8Si and YbAu4Al8Si. Filled
circles represent cm data; open triangles represent 1/cm data.
An Yb analog with this structure was also synthesized. Since
its cell parameters do not follow the trend established by the
early rare earth data, it is likely either divalent or mixed valent.
Magnetic susceptibility data for YbAu4Al8Si, Fig. 3b, supports
a mixed valence description. At high temperatures the molar
susceptibility is nearly temperature independent with a low
magnitude of 2 3 1023 emu mol21. This is consistent with Pauli
paramagnetic behavior, as opposed to the higher moment and
Curie–Weiss behavior resulting from localized spins. The
ytterbium atoms are therefore likely to be in a predominantly
divalent state at these temperatures. As the temperature
decreases, the contraction of the lattice may favor the formation
of Yb3+ by a promotion of an f14 electron to a conduction band.
The presence of the paramagnetic Yb3+ ion is evidenced by the
increase in the susceptibility and its temperature dependence
below 100 K. Formation of a Yb2+/Yb3+ fluctuating valence
state at low temperatures is of interest as it may result in unique
thermal and magnetic properties.12‡
Notes and references
‡ Experimental section: in a nitrogen-filled dry box, rare earth metal,
gold, aluminium, and silicon were combined in an alumina crucible in a 1
: 2 : 10 : 5 ratio. The crucible was sealed in a silica tube under vacuum and
then heated to 1000 °C in 12 h. It was held at this temperature for 15 h, then
cooled to 860 °C in 20 h. After heating at 860 °C for 48 h, the reaction was
cooled to rt over 3 d. The REAu4Al8Si (RE = Ce, Pr, Nd, Sm, Eu, and Gd)
was isolated from the excess aluminium by soaking the crucible in 5 M
NaOH overnight. Other by-products of the reaction include recrystallized
Au and Si, and other members of the RE[AuAl2]nAl2(AuxSi12x)2 series.
Single crystal X-ray diffraction data were collected on a Bruker SMART
CCD diffractometer with Mo Ka radiation. The structure was solved with
the SHELXTL suite of programs. Microprobe elemental analysis was
carried out with a JEOL JSM-35C SEM with energy dispersive spectroscopy (EDS) capability. Magnetic susceptibility data were collected at a field
of 1000 G on a Quantum Design MPMS-5 SQUID magnetometer. Work
funded by the Department of Energy.
1 S. Suresh, A. Mortensen and A. Needleman, Fundamentals of Metal
Matrix Composites, Butterworth-Heinemann, Boston, MA, 1993; S.
Ochiai, Mechanical Properties of Metallic Composites, Marcel Dekker,
New York, 1994.
2 G. H. Meier, in High Temperature Ordered Intermetallic Alloys II, Eds.
N. S. Stoloff, C. Koch, C. T. Liu and O. Isumi, Materials Research
Society Symposium Proceedings 81, Materials Research Society,
Pittsburg, PA, 1987.
3 X. Z. Chen, S. Sportouch, B. Sieve, P. Brazis, C. R. Kannewurf, J. A.
Cowen, R. Patschke and M. G. Kanatzidis, Chem. Mater., 1998, 10,
3202–3211.
4 B. Sieve, X. Z. Chen, R. Henning, P. Brazis, C. R. Kannewurf, J. A.
Cowen, A. J. Schultz and M. G. Kanatzidis, J. Am. Chem. Soc., 2001,
123, 7040–7047.
5 B. Sieve, S. Sportouch, X. Z. Chen, J. A. Cowen, P. Brazis, C. R.
Kannewurf, V. Papaefthymiou and M. G. Kanatzidis, Chem. Mater.,
2001, 13, 273–283.
6 S. E. Latturner, D. Bilc, S. D. Mahanti and M. G. Kanatzidis, Chem.
Mater., 2002, 14, 1695–1705.
7 S. E. Latturner and M. G. Kanatzidis, in preparation.
8 PrAu4Al8Si: tetragonal space group P4/mmm, a = 4.2604(5) Å, c =
14.224(2) Å; V = 258.17(6) Å3. Z = 1, r = 7.542 g cm23. Total
reflections: 4056, unique reflections: 443; index ranges 27 5 h 5 7, 27
5 k 5 7, 224 5 l 5 23. R1/wR2 for I > 2s(I): 3.15/7.48%; for all data:
3.68/7.69%.
9 P. Villars, Pearson’s Handbook, Desk Edition, Crystallographic Data
for Intermetallic Phases, ASM International: Materials Park, OH, 1997,
Vol. 1.
10 (a) Y. P. Yarmolyuk, Y. N. Grin, I. V. Rozhdestvenskaya, O. A. Usov,
A. M. Kuz’min, V. A. Bruskov and E. I. Gladyshevskii, Sov. Phys.
Crystallogr., 1982, 27, 599; (b) Y. N. Grin, Y. P. Yarmolyuk, I. V.
Rozhdestvenskaya and E. I. Gladyshevskii, Sov. Phys. Crystallogr.,
1982, 27, 418.
11 SmAu4Al8Si: tetragonal space group I4/mmm, a = 5.9683(5) Å, c =
28.594(3) Å; V = 1018.55(17) Å3. Z = 4, r = 7.710 g cm23. Total
reflections: 5084, unique reflections: 427; index ranges 27 5 h 5 7, 27
5 k 5 7, 237 5 l 5 38. R1/wR2 for I > 2s(I): 3.26/7.66%; for all data:
3.75/7.78%.
12 (a) C. M. Varma, Rev. Mod. Phys., 1976, 48, 219; (b) C. Schank, G.
Cordier, R. Kniep and F. Steglich, et al., J. Alloys Compd., 1994,
207/208, 333.
CHEM. COMMUN., 2003, 2340–2341
2341