Synthesis and crystal structure of RMgSi2 compounds (R=La, Ce, Pr, Nd), a particular example of linear intergrowth

Article abstract of DOI:10.1016/j.jssc.2008.11.034

A new series of rare earth compounds with stoichiometry RMgSi₂ (R=La, Ce, Pr, Nd) is reported. The single crystal X-ray diffraction showed that CeMgSi₂, which melts congruently at 1200 °C, crystallizes in a new tetragonal structure t

Full text of DOI:10.1016/j.jssc.2008.11.034

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Journal of Solid State Chemistry 182 (2009) 716–724
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Journal of Solid State Chemistry
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Synthesis and crystal structure of RMgSi2 compounds (R ¼ La, Ce, Pr, Nd),
a particular example of linear intergrowth
Ã
F. Wrubl a, M. Pani a, , P. Manfrinetti a,b, P. Rogl c
a
`
Dipartimento di Chimica e Chimica Industriale, Universita di Genova, Via Dodecaneso 31, 16146 Genova, Italy
b LAMIA Laboratory, CNR-INFM, Corso Perrone 24, 16152 Genova, Italy
c Institut fu¨r Physikalische Chemie, Universita¨t Wien, Wa¨hringer Strasse 42, A-1090 Wien, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
A new series of rare earth compounds with stoichiometry RMgSi2 (R ¼ La, Ce, Pr, Nd) is reported. The
single crystal X-ray diffraction showed that CeMgSi2, which melts congruently at 1200 1C, crystallizes in
Received 16 September 2008
Received in revised form
17 November 2008
˚
˚
a new tetragonal structure type (I41/amd, tI32, a ¼ 4.2652(4) A, c ¼ 36.830(4) A, Z ¼ 8; wR2 ¼ 0.042
(19 parameters, 393F20), R1 ¼ 0.018 (297F044
sF0). The crystal structure of CeMgSi2 can be formally built
Accepted 23 November 2008
Available online 13 December 2008
up by alternating along the z direction four CeMg2Si2-type CeMg2Si2 slabs with four AlB2-type CeSi2
slabs, one after the other. The structural model obtained from a CeMgSi2 single crystal has been
confirmed for the La, Pr and Nd homologous compounds by means of Rietveld refinement. The trend of
the unit-cell parameters, plotted versus the R3+ ionic radius, shows a linear behaviour, which strongly
suggests a trivalent state for the Ce atoms. An analysis of the features of this new structure is reported,
in comparison with the other known CeMg2Si2/AlB2-type linear intergrowth compounds.
& 2008 Elsevier Inc. All rights reserved.
Keywords:
Rare earth magnesium silicides
X-ray diffraction
Intergrowth structures
1. Introduction
in the analysis of the structural relationships between members of
the same series.
Few literature data exist on the R–Mg–Si phases (R ¼ rare
A large number of known structures can be described as a
linear intergrowth of slabs along a crystallographic axis [1]:
the compounds formed as an intergrowth of Laves phases and
CaCu5-type slabs (e.g., CeNi3, Ce2Ni7, PuNi3, Sm5Co19), those from
CaCu5- and CeCo3B2-type slabs (e.g., CeCo4B, Ce3Co11B4), or also of
CeAl2Si2- and AlB2-type slabs (e.g., CeAlSi2 and Ce3Al4Si4 [2]). In
the latter example, the intergrown slabs share hexagonal meshes
of rare earth atoms at their interface (001 plane), and the c-axis of
the AlB2 cell is parallel to the intergrowth direction.
earths) [4–6]. In particular, while the R2MgSi2 compounds occur
for the whole series of rare earths (Mo2FeB2 type, commonly
viewed as an intergrowth of CsCl and AlB2 related slabs) [6], the
1:1:1 phase only occurs with the divalent Eu and Yb metals, and
CeMg2Si2 (its own type) has been reported only for cerium.
As a consequence of our interest on the physical properties of
compounds in the Ce–Mg–Si system [5], we synthesized a new
phase with stoichiometry CeMgSi2.
In this contribution on the RMgSi2 family of compounds we
report about the synthesis and the structural characterization
from single crystal (R ¼ Ce) and powder data (R ¼ La, Ce, Pr, Nd).
All the phases crystallize in a new tetragonal structure type
formed as a linear intergrowth of slabs deriving from the known
When simple basic structure types are intergrown sharing a
square mesh at their interface instead of an hexagonal one (BaAl4,
CeMg2Si2, AlB2, W, aPo, Cu, AuCu3 and CaF2), other new structure
types are formed. In these cases the AlB2 slabs are arranged so that
the c axis is perpendicular to the intergrowth direction, sharing
the {100} planes at the interface. Considering for example the
combination of slabs of BaAl4 and AlB2, the orthorhombic
structure types CeNiSi2, U3Ni4Si4 and Ce3Ni2Si8 can be derived.
These phases can all be described as a linear combination of the
two aforementioned slabs along one crystallographic direction
(usually the longest of the new orthorhombic cell), following the
stoichiometric equation: mRX4+nRM2 ¼ Rm+nM2nX4m, so that for
CeNiSi2 m ¼ n ¼ 1, for U3Ni4Si4 m ¼ 1 and n ¼ 2 and for Ce3Ni2Si8
m ¼ 2 and n ¼ 1 [3]. This formal approach has proved to be useful
CeMg2Si2 (CeMg2Si2-type) and CeSi2 (a-ThSi2-type) structures.
Structural and chemical features are analyzed in comparison with
the other known CeMg2Si2/AlB2-type intergrowth compounds.
The measurements of the physical properties of all the four
homologues have just been undertaken and will be part of a
future report.
2. Experimental
The elements used were commercial products: 99.9 wt.% for
the rare earth elements, 99.98 wt.% for Mg, ‘electronic-grade’ type
(99.9999 wt.% purity) for Si. Samples with nominal composition
Ã
Corresponding author. Fax: +39 010 3628252.
E-mail address: marcella@chimica.unige.it (M. Pani).
0022-4596/$ - see front matter & 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.11.034

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F. Wrubl et al. / Journal of Solid State Chemistry 182 (2009) 716–724
717
‘‘RMgSi2’’ were prepared for La, Ce, Pr, Nd, Sm and Gd by melting
in a high frequency induction furnace (twice, up to 1500 1C)
stoichiometric amounts of the elements, sealed by arc welding
into outgassed tantalum crucibles in a pure Ar atmosphere. First
syntheses were attempted for Ce and Pr, confirming the presence
of the 1:1:2 new phase in the as-cast alloys, in a nearly single
phase form. A second series of samples was prepared for all the
above mentioned R elements. After melting, the crucibles were
sealed under vacuum in silica ampoules and then annealed at
1000 1C for 4 days. The so obtained alloys were brittle and small
grain sized, showed metallic lustre and were stable toward air and
moisture (no changes were visible after a 6 months air exposure).
Metallographic specimens were prepared by standard techni-
ques and examined by both optical (LOM) and electron micro-
scopy (SEM); the 1:1:2 stoichiometry was confirmed by means of
microprobe analyses (EDX) for La, Ce, Pr, Nd. A differential thermal
analysis (DTA), cycled up to 1300 1C (20 1C/min on heating;
10 1C/min on cooling; 75 1C accuracy) was performed on an as-
cast CeMgSi2 specimen, sealed by arc-welding in a Mo crucible:
the results showed that this compound crystallizes congruently
at 1200 1C (showing the presence of just one strong thermal
effect, detected both on heating and on cooling). After the DTA
cycle, X-ray powder diffraction revealed the presence of the 1:1:2
phase only, as in the other as-cast samples.
X-ray diffraction was carried out both on single crystals and
powders. The single crystal analysis was performed for CeMgSi2,
selecting
a suitable crystal from the DTA sample after a
subsequent annealing at 900 1C for 4 days. The intensity data
were measured at room temperature by means of a Bruker–No-
nius MACH3 diffractometer (graphite-monochromated MoK
a
˚
radiation,
l
¼ 0.7107 A, point detector). The structure solution
was accomplished by means of Patterson methods with
SHELXS-97 [7], and the least-squares refinements were carried
on with SHELXL-97 [8]; the data collection conditions and the
most relevant parameters of the refinement procedure are listed
in Table 1. Further details of the crystal structure investigation can
be obtained from the Fachinformationszentrum Karlsruhe, 76344
Eggenstein-Leopoldshafen, Germany (fax: +497247 808 666;
e-mail: crysdata@fiz.karlsruhe.de), on quoting the depository
number CSD 419887.
X-ray powder diffraction data were collected with a Guinier–
Huber image plate CCD system with Ge monochromated CuKa1
(81o2
yo1001, step D2y ¼ 0.0051, 18,401 points). Precise lattice
parameters were calculated by least-squares fits to the indexed
4
y
-values with Ge as internal standard (99.9999+% purity,
˚
aGe ¼ 5.657906 A). For Rietveld refinements the FULLPROF suite
was used [9]. Pseudo-Voigt functions were used to fit the profiles,
with peak asymmetry correction (133 reflections). Either manual
point selection or a polynomial function with six parameters was
used to fit the background.
Table 1
Crystal data, data collection conditions and refinements results for the single
crystal investigation of the CeMgSi2 compound.
Compound
CeMgSi2
Structure type
Crystal system
Space group
CeMgSi2
All the structural images were produced by means of the free
open-source programs DRAWxtl [10], GIMP [11] and the Open-
Office suite [12].
Tetragonal
I41/amd
a
˚
Lattice parameters (A)
a ¼ 4.2652(4), c ¼ 36.830(4)
3
˚
Cell volume (A )
670.0(2)
Z
8
F(000)
784
Formula unit weight (g)
Pearson code
220.596
tI32
m
(MoK
a
) (mmꢀ1
)
14.19
Calculated density (Mg mꢀ3
Crystal shape
)
4.37
Platelet
0.015 ꢁ 0.13 ꢁ 0.15
Max dimensions (mm)
Scan mode
oꢀy
2–33
y
range (1)
Range in h, k, l
Measured reflections
Indep. reflections
Rint
0php6, 0pkp6,ꢀ56plp56
1560
393
0.044
Refl. with F044
s
(F0)
297
Transm. ratio (Tmax/Tmin
Absorp. correction
Refined parameters
Extinction coefficient
R1
wR2 (F20) all data
shift/e.s.d.
)
2.59
Gaussian integration
19
0.0017(1)
0.018
0.042
0.001
Goodness-of-fit
0.891
ꢀ3
)
˚
+0.99, ꢀ1.16
D
rmax
,
D
rmin (e A
Fig. 1. The unit cell parameters of the RMgSi2 compounds (R ¼ La–Nd) plotted vs.
the R3+ ionic radii.
a
From powder data.
Table 2
Table 3
The a and c unit cell parameters, the VU unit cell volume, the c/a ratio and the
volume contraction for the RMgSi2 compounds.
DV%
Fractional atomic coordinates and anisotropic displacement parameters for
CeMgSi2, as obtained from single crystal data.
3
˚
˚
Compound
Lattice constants (A)
VU (A )
c/a
DV (%)
2
2
2
2
˚
˚
˚
˚
Atom
z
U11 (A )
U22 (A )
U33 (A )
Ueq (A )
a
c
Ce
0.79775(1)
0.12498(7)
0.26412(5)
0.32695(5)
0.0069(2)
0.0072(10)
0.0098(11)
0.0068(10)
0.0064(2)
0.0137(11)
0.0117(11)
0.0082(10)
0.0051(2)
0.0095(9)
0.0057(7)
0.0058(7)
0.0061(1)
0.0101(4)
0.0091(3)
0.0069(3)
Mg
Si1
Si2
LaMgSi2
CeMgSi2
PrMgSi2
NdMgSi2
4.3143(4)
4.2652(4)
4.2487(4)
4.2292(5)
36.927(3)
36.830(4)
36.749(4)
36.690(3)
687.3(1)
670.0(2)
663.4(2)
656.2(2)
8.559
8.635
8.650
8.676
14.7
14.6
15.2
15.8
All atoms are in (8e): 0,14, z; U12 ¼ U13 ¼ U23 ¼ 0.

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Fig. 2. The [010] projection of the CeMgSi2 structure: the different slab types are highlighted (a); the CeSi2 (AlB2-type) structure viewed along [001] (b) and [100] (c); the
CeMg2Si2 cell along [010] (d).
3. Results
LOM and SEM showed that the RMgSi2 samples were single
with the volume contractions1 observed for these phases, are
listed in Table 2. The cell parameters plotted vs. the R3+ ionic
radius show a linear trend (Fig. 1); the Ce point is aligned, so
suggesting a trivalent state for the Ce atoms. It can be observed
that, while the unit cell volume VU regularly decreases from La to
Nd (as expected from the lanthanide contraction), the c/a ratio
shows a slight increase.
phase (R ¼ La, Ce), or nearly single phase (R ¼ Pr, Nd), containing
traces of Si-rich R–Si binary compounds. The corresponding
powder patterns showed immediately that structures isotypic
with that of CeMgSi2 were formed by La, Pr and Nd. For Sm and Gd
the samples were multiphasic, containing a mixture of RSi2, Mg2Si
and R2MgSi2; no presence of the RMgSi2 phase was observed. After
non-plausible results obtained with the TREOR90 program [13],
the preliminary single-crystal data on CeMgSi2, i.e. a tetragonal
body-centered cell, with a very elongated c-axis, were used for the
indexing procedure resulting in a complete interpretation of all
powder patterns. The corresponding lattice parameters, together
The results of the single crystal analysis (Table 3) show that the
CeMgSi2 compound crystallizes in a new ordered structure type.
1
Volume contraction: the relative volume change passing from the separated
elements to the intermetallic compound, defined as
(Z iniVi), where Z is the number of formula units in the unit cell, Vi are the atomic
volumes and ni the stoichiometric coefficients; VU is the observed unit cell volume.
DV (%) ¼ 100(ZSiniVi ꢀVU)/
S

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Table 4
Refined positional parameters of the RMgSi2 (R ¼ La, Ce, Pr, Nd) compounds by the Rietveld method.
Compound
LaMgSi2
R
Mg
Si1
Si2
RB (%)
RF (%)
RWP (%)
w2
8.22
5.87
2.63
2.22
2.71
6.11
6.98
z
0.79800(2)
0.0177(2)
0.12512(6)
0.0206(8)
0.26424(6)
0.0216(8)
0.32725(6)
0.0195(7)
2
˚
Uiso (A )
CeMgSi2
PrMgSi2
NdMgSi2
7.67
8.39
9.33
6.45
7.27
8.77
3.24
2.55
3.06
z
0.79763(1)
0.0180(2)
0.12598(6)
0.0288(9)
0.26405(6)
0.0166(8)
0.32661(7)
0.0362(9)
2
˚
Uiso (A )
z
0.79756(2)
0.0203(1)
0.12546(7)
0.0279(9)
0.26371(7)
0.0141(9)
0.32589(8)
0.041(1)
2
˚
Uiso (A )
z
0.79726(2)
0.0229(3)
0.12498(8)
0.032(1)
0.26459(8)
0.022(1)
0.32510(8)
0.041(1)
2
˚
Uiso (A )
All atoms are in (8e): 0,14, z; RWP: overall agreement index; RB: Bragg agreement index.
Fig. 3. The observed, calculated and difference X-ray powder diffraction patterns for the LaMgSi2 compound, together with the background points (the visible background
peak at 2
ffi211 comes from the sample holder foil).
y
It is a new example of linear intergrowth, resulting from the
combination of four AlB2-type CeSi2 slabs and four CeMg2Si2 cells,
alternating along the c axis, as shown in Fig. 2. For the sake of
comparison, in the same figure the structures of AlB2 (in two
projections) and of CeMg2Si2 are shown. Finally, a possible
deviation from the fixed 1:1:2 stoichiometry was checked at the
end of the anisotropic refinement, allowing the site occupation
factors (sof’s) of all atoms to freely vary. Only minor deviations of
the sof’s from unity were obtained (slightly greater than unity for
the Ce, Mg and Si2 atoms, and slightly lower than unity (within
noting that a slight tendency to be defective was shown just by
the Si1 atoms, belonging to the AlB2-type slabs; this feature is
commonly observed in all known binary phases formed by the
rare earths with Si, which always crystallize with defective AlB2
and/or a-ThSi2 structures [4].
Being unavailable any suitable single crystal, for the other
members of the series (La, Pr, Nd) the structural model was
confirmed by means of Rietveld method and the refined atomic
positions are reported in Table 4, along with the isotropic
displacement parameters. The agreement indexes RB, RF, RWP, as
well as the w2 values, are also reported in Table 4; the relatively
4s) for Si1), correspondingly no significant improvement in the
relevant refinement parameters was obtained. Owing to the very
small extent of this deviation, in the final cycles all atomic
positions were considered fully occupied; however, it is worth
higher w
2 values obtained for the Pr and Nd samples are ascribable
to the presence of few extra unindexed peaks. As an example, the
observed, calculated and difference patterns for the LaMgSi2

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Table 5
Interatomic distances for CeMgSi2, as obtained from single crystal data.
˚
d (A)
d/SrCN12
Ce
2 Si1
3.121(1)
3.202(1)
3.260(1)
3.555(2)
3.556(2)
4.114(1)
4.265(1)
0.99
1.02
1.04
1.13
1.13
1.13
1.17
4 Si2
4 Si1
2 Mg
2 Mg
2 Ce
4 Ce
Mg
2 Si2
2 Si2
4 Mg
2 Ce
2 Ce
2.771(2)
2.772(2)
3.016(1)
3.555(2)
3.556(2)
0.95
0.95
0.94
1.13
1.13
Si1
Si2
2.314(3)
2.373(2)
3.121(1)
3.260(1)
0.88
0.90
0.99
1.04
2 Si1
2 Ce
4 Ce
Si2
Si1
2.314(3)
2.771(2)
2.772(2)
3.202(1)
0.88
0.95
0.95
1.02
2 Mg
2 Mg
4 Ce
same space group of CeSi2, I41/amd. In the CeSi2 phase the Ce
atoms (named as 1, 2, 3, 4 in Fig. 4a) are on the 4a Wyckoff
positions, thus lying on the ꢀ4 rotoinversion axis (0,34,z), exactly in
correspondence with the inversion centre at ð0; 34; 81Þ. In the new
structure of CeMgSi2, the Ce atoms are also lying on such a ꢀ4
axis, but now they occupy the 8e site at z ¼ 0.2022 [(0, 34, 0.2022);
standard description (0, 14, 0.7978)], so that—owing to the action
of the inversion center—a pair of Ce atoms, symmetrically
displaced with respect to z ¼ 18, is generated (atoms 1 and 10 in
Fig. 4b). The distance between these two Ce atoms corresponds to
Fig. 4. (a) CeSi2 and (b) CeMgSi2 structures viewed as slab intergrowth along c; the
Ce atoms are highlighted and numbered to show the symmetry relationship
between the two structures (arrows).
˚
the height of all the CeMg2Si2 slabs, and its value of 5.687 A is only
compound are shown in Fig. 3, together with the background
slightly shorter than the c parameter of the CeMg2Si2 phase
points (the visible background peak at 2
yffi211 comes from the
˚
(c ¼ 5.765 A).
sample holder foil). For the other samples, the pd_cifs and the
corresponding plots are available for download as supplementary
material of the present work.
The comparison of the a parameter of CeMgSi2 with those of
CeMg2Si2 and of CeSi2 (4.2652(4), 4.250 and 4.1874(4) A, respec-
tively) shows that in the formation of the CeMgSi2 intergrowth
compound the CeSi2 slabs slightly expand, in order to share
the square mesh at the interface with the CeMg2Si2 slabs, and
not vice-versa.
˚
4. Discussion
4.1. Relationship between CeSi2
(
a-ThSi2 type) and CeMgSi2
4.2. Coordination polyhedra
A close relationship holds between CeSi2
(
a
-ThSi2 type, tI12,
In the new cell each Ce atom (in the 8e position) has
coordination number CN ¼ 20; the distances list is reported in
Table 5. The coordination polyhedron (Fig. 5a and b), following a
modified version of the maximum gap rule [14] (considering the
I41/amd) and the new CeMgSi2 compound (Fig. 4a and b). In the
unit cell of CeSi2, four AlB2-type CeSi2 slabs are intergrown
along the c direction, sharing a square mesh of Ce atoms at the
interface at which each next slab is rotated by p/2 relative to the
d/S
rCN12 maximum gap instead of the d/dmin [15]) is coded2 as
previous; the presence of the 41 screw axis along c is thus clear
and the complete symmetry is described by the I41/amd space
group. The CeMgSi2 structure has the same overall symmetry;
from a symmetry-oriented point of view, the difference is that
each plane of cerium atoms forming a CeSi2 slab in the parent
64,261,246,043,1 and can be described as formed by a half of the
CeMg2Si2-type Ce polyhedron (position 1a, P4/mmm, point
symmetry 4/mmm, polyhedron code ¼ 84,183,144,2 [14]; Fig. 5d)
and by a half of the
a-ThSi2-type Ce polyhedron (position 4a,
I41/amd, point symmetry ꢀ4m2, polyhedron code 121,244,242,4
a
-ThSi2-type structure is now split into two different planes along
the c direction, thus leaving enough space to host a whole cell of
CeMg2Si2 (CeMg2Si2-type, tP5, P4/mmm). The new CeMgSi2 cell is,
therefore, also tetragonal and its symmetry is described by the
2
A
series of numbers of coordinating atoms (vertices). The superscripts
indicate the number of triangles, squares, pentagons that share each vertex [14].

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721
Fig. 5. (a) The Ce coordination polyhedron in CeMgSi2 and (b) the same polyhedron as a solid figure; (c) the Ce coordination polyhedron in CeSi2; (d) the Ce coordination
polyhedron in CeMg2Si2.
[14]; Fig. 5c). This clearly happens because each Ce atom is
shared by two slabs of different type. The upper part of the
polyhedron, coming from the CeSi2 environment, contributes
with 61,2 (Si1) and 24,2 (Ce) vertices, while the lower part, coming
from the CeMg2Si2 structure, contributes with 43,1 (Si2), 44,2 (Ce)
and 46,0 (Mg) atoms. The point symmetry reduction from 4/mmm
to 2mm, observed at the Ce site passing from CeMg2Si2 to
CeMgSi2, causes the Mg atoms to share six triangles and not
four triangles and a square as before. Two Ce–Si1 distances
shorter than those between the atoms belonging to the same slab
(Si1–Si1 in CeMgSi2); in CeMgSi2, this phenomenon is slightly
˚
attenuated because the bonds Si1–Si1 ¼ 2.373 A are already
˚
relatively shorter than in CeSi2 (intraslab Si–Si ¼ 2.411 A; interslab
˚
Si–Si ¼ 2.280; Si1–Si2 ¼ 2.314 A, comparable to twice as the Si
˚
covalent radius rcov ¼ 1.17 A). This short Si1–Si2 bond is part of a
truncated three-dimensional Si network (Fig. 2a), centered in
every AlB2-type slab, that is clearly related to the three-
dimensional network found in CeSi2 [17]. The presence of this
shorter Si1–Si2 bond is, as expected, also reflected in the Si2
coordination polyhedron, CN ¼ 9. It has the shape of a slightly
distorted mono-capped square antiprism, coded as 45,025,024,014,0
(Fig. 6c), different from that observed in CeMg2Si2 (CN ¼ 10,
bicapped square antiprism, polyhedron code 85,024,0). Also in this
case the change of the space group and Wyckoff position of the
atoms (from 2h, P4/mmm, point symmetry 4mm, to 8e, I41/amd,
point symmetry 2mm.) causes a point symmetry reduction,
so that the polyhedron still has the {100} mirrors, but it has lost
the {110} mirrors and the fourfold axis along c. This is once
again related to the coplanarity loss of the Mg atoms. Besides the
point symmetry, the main difference is related to the two Si–Si
˚
˚
(3.121 A) are shorter and four longer (3.260 A) than those observed
˚
in CeSi2 (3.137 and 3.173 A, calculated from [16], mean distance
˚
3.161 A), so that the mean distance is now only slightly longer
˚
(3.214 A) in CeMgSi2. On the other side, the Ce–Si2 distances
˚
(3.202 A) are shorter than the analogous Ce–Si distances in
˚
CeMg2Si2 (3.269 A).
The Mg atoms retain the same CN ¼ 12 and the polyhedron
code 44,143,142,2 as found in CeMg2Si2, but the point symmetry
changed, passing from the parent CeMg2Si2 structure (P4/mmm,
position 2e, point symmetry mmm.) to CeMgSi2 (I41/amd, position
8e, point symmetry 2 mm.). The loss of the mirror plane
perpendicular to c is strictly related to the fact that the four
coordinating Mg atoms are no longer lying on a plane with the
central Mg and only a two-fold axis is present in the [001]
˚
bond distances, one being remarkably shorter (Si2–Si1, 2.314 A)
˚
and the other longer (Si2–Si2, 3.536 A) than the corresponding
˚
˚
direction (Fig. 6a). Only the Mg–Si2 distances (2.771 and 2.772 A)
ones found in CeMg2Si2 (2.571 and 3.194 A, respectively). The two
˚
are longer than those observed in the parent structure (2.658 A),
suggesting that the Si2 atoms are pulled by the bond with the Si1
atoms in the AlB2-type slabs.
Si2 atoms inside the CeMg2Si2-type slabs are thus symmetrically
moving apart, along c, from the pseudo-square of the four Mg
atoms, each one pulled by the covalent bond with one of the
two symmetrical Si1 atoms in the surrounding AlB2-type slabs.
Since the CeMg2Si2 slabs undergo in the same time a very small
This bond is also affecting both the Si1 and Si2 coordination
polyhedra. The Si1 atoms (CN ¼ 9; Fig. 6b) have the same
polyhedron code 65,034,0 and the same 2mm. point symmetry as
˚
contraction along c as already mentioned (from 5.765 to 5.687 A),
Si in the
a
-ThSi2-type CeSi2. In both structures the bonds between
this implies that the Si2–Ce distance is also contracted, from
˚
Si atoms belonging to different slabs (Si1–Si2 in CeMgSi2) are
3.269 to 3.202 A.

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Fig. 6. The coordination polyhedra of (a) Mg, (b) Si1 and (c) Si2, in CeMgSi2, as bond connections and as solid figures.
3.4. Comparison with other CeMg2Si2/AlB2 linear
intergrowth structures
NdNi2Ga2 slabs3 and two AlB2-type NdGa2 slabs, alternating along
the NdNiGa2 b direction ([18]; Table 6; Fig. 7b; the same structure
is adopted by all the RNiGa2 compounds (R ¼ La–Gd) [19]). No
other compound belonging to this system is known with different
The RMgSi2 compounds are part of the general series
Rm+nM2mX2(m+n) (formally formed by mRM2X2 and nRX2, with
m ¼ 1, n ¼ 1; Fig. 7a). In the R–M–X ternary systems (R ¼ rare
earth, M ¼ transition element, Li or Mg, X ¼ p-element) the only
other known member of this series is represented by NdNiGa2,
oC16, with again m ¼ n ¼ 1, formed by two CeMg2Si2-type
3
The Ga atoms replace the Mg atoms and the Ni the Si ones in the CeMg2Si2-
type slabs, and this change does not affect the overall 1:1:2 stoichiometry of the
intergrowth phase.

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F. Wrubl et al. / Journal of Solid State Chemistry 182 (2009) 716–724
723
Fig. 7. Comparison of CeMgSi2 with the other known CeMg2Si2/AlB2-type intergrowth structures; the different slab types are highlighted.
Table 6
number of fragments with composition APd2X2 (CeMg2Si2-type)
and n is the number of fragments with composition APdX (SrPtSb-
type, ternary variant of the AlB2-type). For each compound m and
n are given in Table 6. As an example, for Ca4Pd5P5 m ¼ 1 and
n ¼ 3; it is formed by the linear intergrowth of two slabs of
CaPd2P2 and six of CaPdP (Fig. 7d). In a ternary A–Pd–X diagram,
all the compounds belonging to the series reported by Mewis lie
on the straight line of the pseudo-binary system formed by the
1:2:2 and the 1:1:1 ternary compounds. The intermediate inter-
growth compounds, found between the 1:2:2 and the 1:1:1
phases, can be regarded as a step-by-step ‘‘dilution’’ of the
1:2:2 phase with A, since the ratio Pd:X ¼ 1 remains constant
along the line and the A content increases stepwise from 20 up to
33 atomic %.
The series of the known CeMg2Si2/AlB2 intergrowth compounds: formula, m and n
parameters, Pearson code.
Formula
m (CeMg2Si2-type) n (AlB2-type) Pearson symbol Ref.
CeMgSi2
NdNiGa2
Eu3Pd4As4
Ca4Pd5P5
Ca5Pd6P6
1 (CeMg2Si2)
1 (NdNi2Ga2)
1 (EuPd2As2)
1 (CaPd2P2)
1 (CaPd2P2)
0
1 (CeSi2)
1 (NdGa2)
2 (EuPdAs)
3 (CaPdP)
4 (CaPdP)
4 (CeSi2)
0
tI32
oC16
oP11
oC56
oP34
tI12
This work
[13]
[19]
[19]
[19]
CeSi2
(
a
-ThSi2)
[13,16]
[13]
CeMg2Si2
1
tP5
m:n ratio. It can be underlined that, in these two cases of
intergrowth structures, the two slab types are present within the
unit cell in different overall numbers (4 and 2) but have the same
relative weight (m ¼ n ¼ 1). Few other CeMg2Si2/AlB2 intergrowth
compounds are known. They are shown in Fig. 7c–e. Johrend and
Mewis [20] reported about two alkaline-earth palladium phos-
phides and one europium palladium arsenide, crystallizing with
their own structure types. All these three structures are members
of the general series Am+nPd2m+nX2m+n (A ¼ alkaline-earth or
europium, X ¼ arsenic or phosphorous), in which m is again the
From a detailed geometric analysis, the five structures can be
divided into three groups: (i) pure tetragonal structures (CeMgSi2
as well as CeSi2), (ii) structures with orthorhombic distortions
(NdNiGa2 and Eu3Pd4As4) and (iii) true orthorhombic structures
(Ca4Pd5P5 and Ca5Pd6P6). The AlB2 slabs always have their c
direction (connecting the triangular bases of the trigonal prisms)
perpendicular to the intergrowth direction. If, within one unit cell
of the derived structure, the c and a directions of the intergrown
AlB2 slabs are regularly interchanged, being either perpendicular
or antiparallel, the two parameters are forced to find a unique

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F. Wrubl et al. / Journal of Solid State Chemistry 182 (2009) 716–724
compromise value, and a tetragonal cell is thus adopted. If instead
they always lie on the same direction, these become distinguish-
able and, as a result, an orthorhombic distortion is obtained.
On the other hand, the intergrowth of consecutive AlB2 slabs is
for these phases also accompanied by the formation of directional
covalent bonds, that can be either hetero-atomic (in Ca5Pd6P6
˚
˚
dPd1ꢀP1 ¼ 2.386 A, in Ca4Pd5P5 dPd5ꢀP4 ¼ 2.389 A) or homo-atomic
˚
(in Eu3Pd4As4 dAs1ꢀAs1 ¼ 2.505 A). All the above-mentioned inter-
4.5. Inter/intra slab interactions
growth compounds are thus formed with m ¼ 1 and n ¼ 1, 2, 3
or 4 (Table 6).
For comparison, it can be useful to consider how the
coordination polyhedra change from the parent structures to the
intergrown slabs in the new compounds, and also for other kinds
of linear intergrowth series.
In the Ce–Mg–Si system, the intergrowth of consecutive AlB2-
type CeSi2 slabs would lead to the formation of Si–Si bonds, like it
happens in the
a-ThSi2 structure, where a three-dimensional
network of silicon atoms is formed. The existence of other
members of the Rm+nM2mX2m+n series, with m:n41 can thus not
be excluded a priori. This chance is currently under investigation
and will be reported in due time.
In the case of the abovementioned CeAlSi2, CeAl2Si2/AlB2
intergrowth compound (m:n ¼ 1, with an hexagonal mesh at the
interface) only the Ce atom is coordinated by atoms belonging to
slabs of both types [2]. The other atoms are coordinated only by
vertices belonging to the same slab: the interaction between slabs
of different type can thus be supposed to be not very different to
that between slabs of the same type. This means that other
intergrowth structures with contiguous CeAl2Si2-type slabs
should be considered as equally stable, and in fact the Ce3Al4Si6
structure type (m ¼ 2, n ¼ 1, m:n ¼ 2) is also observed [2].
On the other hand, in CeMgSi2 and in all the other known
CeMg2Si2/AlB2 intergrowth structures, strong bonding interac-
tions between slabs of different type can be noticed, as resulting
from the covalent bonds between atoms belonging to slabs of
different type. These bonds are absent in the interaction between
slabs of the same type in the bulk CeMg2Si2 phase. As for CeMgSi2,
this phenomenon is clearly reflected on the coordination
polyhedra of the NdNiGa2 structure [14]; the Nd atoms are
surrounded, as expected, by atoms of both the slabs; also the Ni
and the Ga3 atoms are involved in inter-slab bonds, because a
Acknowledgment
The authors thank Prof. F. Merlo for helpful reading and
discussion.
References
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[2] H. Flandorfer, P. Rogl, J. Solid State Chem. 127 (1996) 308.
[3] Y. Yarmolyuk, L. Aksel’rud, Y. Grin’, V. Fundamenskii, E. Gladyshevskii, Sov.
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[4] CRYSTMET—The Metals Database, V.3.7.0, Tooth Information System Inc.,
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[11] /http://www.gimp.orgS.
˚
strong Ni–Ga bond is formed (dNiꢀGa3 ¼ 2.248 A, shorter than the
˚
2.41 A sum of the covalent radii; Fig. 7b). This is indeed the
analogue of the Si1–Si2 bond in the CeMgSi2 structure. Analogous
strong interactions are found in Eu3Pd4As4 between the Pd2 and
˚
As2 atoms (dPd2ꢀAs2 ¼ 2.458 A [20], whereas the sum of the
˚
covalent radii is 2.48 A), in Ca4Pd5P5 between the couples Pd2–P1
˚
˚
and Pd3–P2 (dPd2ꢀP1 ¼ 2.323 A, dPd3ꢀP2 ¼ 2.342 A, whereas the
˚
sum of the covalent radii is 2.34 A) and in Ca5Pd6P6 between the
[12] /http://www.openoffice.orgS.
˚
˚
[13] P. Werner, L. Eriksson, A. Santoro, J. Appl. Crystallogr. 18 (1985) 367.
[14] J.L. Daams, P. Villars, J. van Vucht, Atlas of Crystal Structure Types for
Intermetallic Phases, ASM, 1991.
[15] G. Bruzzone, M.L. Fornasini, F. Merlo, J. Less-Common Met. 22 (1970)
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[16] A. Morozkin, V. Stupnikov, V. Nikiforov, N. Imaoka, I. Morimoto, J. Alloys
Compd. 415 (2006) 12.
[17] W. Zachariasen, Acta Crystallogr. 2 (1949) 94.
pairs Pd2–P4 and Pd6–P5 (dPd2ꢀP4 ¼ 2.310 A, dPd6ꢀP5 ¼ 2.365 A),
as shown in Fig. 7b–d). If two CeMg2Si2-type slabs were
connected, only one short Si–Si bond per unit cell out of two
possible would form, and the structure would be thus less
favoured. As a direct consequence, are only formed intergrowth
structures where the CeMg2Si2-type slabs are strictly intergrown
between AlB2-type slabs, meaning that this kind of sequence is
more stable than that/those with contiguous CeMg2Si2-type slabs.
To the best of our knowledge, structures of this type have not yet
been found.
´
[18] E. Parthe, L. Gelato, B. Chabot, M. Penzo, K. Cenzual, E. Gladyshevskii, Gmelin
Handbook of Inorganic and Organometallic Chemistry, vol. 1, Springer, Berlin,
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[19] Y. Grin’, Y. Yarmolyuk, Dopov. Akad. Nauk Ukr. RSR Ser. A 44 (1982) 69.
[20] D. Johrendt, A. Mewis, Z. Anorg. Allg. Chem. 621 (1995) 57.