New layered germanide halides RE2GeX2 (RE = Y, Gd; X = Br, I)

Article abstract of DOI:10.1021/ic062425d

The title compounds were synthesized from RE, REX₃, and Ge under an Ar atmosphere at 1200-1370 K. Y₂GeI₂ and Gd ₂GeI₂ crystallize in space group R3m with lattice constants a = 4.2135(3) and 4.2527(1) A and c = 31.480(2) and 31.657(1) A, respectively. Gd₂GeBr₂ crystallizes in two modifications, the 1T-type (space group P3m1; a = 4.1668(2) A, c = 9.8173(6) A) and the 3R-type (space group R3m; a = 4.1442(9) A, c = 29.487(7) A). The structural motifs of RE₂GeX₂ compounds are Ge-centered slightly distorted RE₆ octahedra connected via their common edges and extending in the a and b directions. The resulting close-packed double layers are separated by halogen atoms. The electrical resistivity measurements revealed semiconductor behavior for Y ₂GeI₂ and Gd₂GeI₂ and a metal-semiconductor transition for 1T-Gd₂GeBr₂. Magnetic susceptibility and heat capacity measurements show long-range magnetic ordering for Gd₂GeI₂ and 1T-Gd₂GeBr₂ at ～15 and ～13 K, respectively.

Full text of DOI:10.1021/ic062425d

Inorg. Chem. 2007, 46, 3231−3235
New Layered Germanide Halides RE2GeX2 (RE
) Y, Gd; X ) Br, I)
Mar’yana Lukachuk, Reinhard K. Kremer, Hansju1rgen Mattausch, and Arndt Simon*
Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
Received December 18, 2006
The title compounds were synthesized from RE, REX3, and Ge under an Ar atmosphere at 1200
and Gd2GeI2 crystallize in space group R3hm with lattice constants a ) 4.2135(3) and 4.2527(1) Å and c )
−
1370 K. Y2GeI2
31.480(2)
and 31.657(1) Å, respectively. Gd2GeBr2 crystallizes in two modifications, the 1T-type (space group P3m1; a
4.1668(2) Å, c 9.8173(6) Å) and the 3R-type (space group R3m; a 4.1442(9) Å, c 29.487(7) Å). The
h
)
)
h
)
)
structural motifs of RE2GeX2 compounds are Ge-centered slightly distorted RE6 octahedra connected via their
common edges and extending in the a and b directions. The resulting close-packed double layers are separated
by halogen atoms. The electrical resistivity measurements revealed semiconductor behavior for Y2GeI2 and Gd2GeI2
and a metal−semiconductor transition for 1T-Gd2GeBr2. Magnetic susceptibility and heat capacity measurements
show long-range magnetic ordering for Gd2GeI2 and 1T-Gd2GeBr2 at
∼
15 and 13 K, respectively.
∼
Introduction
compounds (X ) halogen atom) crystallize in trigonal (1T)
or rhombohedral (3R) types which differ in the stacking
sequence of the slabs. For the 1T type, the translation period
corresponds to one slab in the unit cell, whereas the unit
cell contains three such slabs for the 3R type. Many RE2ZX2
compounds,8,10-12 depending on the synthesis temperature,
were found to crystallize in both 1T and 3R types. Here, the
synthesis, structure, and physical properties of three new rare-
A number of metal-rich rare-earth halides1-4 crystallize
with layered structures resulting from two-dimensional
condensation of RE6 octahedra. These structures relate to
those of the binary compounds ZrCl5,6 and ZrBr.7 Close-
packed bilayers of metal atoms are sandwiched by layers of
halogen atoms, forming slabs which are bonded via van der
Waals interactions. The metal atom framework leaves voids
for different interstitials Z.1-4,8-15 The resulting RE2ZX2
11
earth metal germanide halides closely related to La2GeI2
are reported.
* To whom correspondence should be addressed. E-mail: A.Simon@
fkf.mpg.de.
Experimental Section
(1) Simon, A.; Mattausch, Hj.; Miller, G. J.; Bauhofer, W.; Kremer, R.
K. In Handbook on the Physics and Chemistry of Rare Earths;
Gschneidner, K. A., Jr., Eyring, L., Eds.; North Holland: Amsterdam,
1991; Vol 15, p 191.
(2) Corbett, J. D. J. Chem. Soc., Dalton Trans. 1996, 5, 575.
(3) Meyer, G. Chem. ReV. 1988, 88, 93.
(4) Simon, A.; Mattausch, Hj.; Ryazanov, M.; Kremer, R. K. Z. Anorg.
Allg. Chem. 2006, 632, 919.
(5) Izmailovich, A. S.; Trojanov, S. I.; Tsirelnikov, V. I. Russ. J. Inorg.
Chem. 1974, 19, 1597.
(6) Adolphson, D. G.; Corbett, J. D. Inorg. Chem. 1976, 15, 1820.
(7) Daake, R. L.; Corbett, J. D. Inorg. Chem. 1977, 16, 2029.
(8) Hwu, S.-J.; Ziebarth, R. P.; von Winbush, S.; Ford, J. E.; Corbett, J.
D. Inorg. Chem. 1986, 25, 283.
Preparation. RE metal pieces (RE ) Y, Gd) (Alfa Aesar,
99.99%), REX3 (X ) Br, I), and Ge (99.99%) were used as starting
materials. Y and Gd were hydrogenated, crushed, and then
dehydrogenated under dynamical vacuum (e10-5 mbar) at 1070
K (Mo boat, 24 h). The rare-earth trihalides, REX3, were prepared
from the reaction of the oxides (99.9%) with HX and NH4X and
purified twice by sublimation under high vacuum in Ta containers.
All handling of the educts and products was carried out under
purified Ar either in a glovebox or by using standard Schlenk
technique.
Reactions were performed with appropriate mixtures of the
starting materials in Ta tubes sealed under an Ar atmosphere, which
were subsequently jacketed in silica glass ampules under ca. 10-2
mbar vacuum. The synthesis conditions are compiled in Table 1.
The 3R type of Gd2GeBr2 could only be obtained with a yield of
ca. 10%, which could be the result of a narrow temperature range
of existence.
(9) Schwanitz-Schu¨ller, U.; Simon, A. Z. Naturforsch. 1985, 40b, 710.
(10) Schleid, Th.; Meyer, G. Z. Anorg. Allg. Chem. 1987, 552, 90.
(11) Mattausch, Hj.; Zheng, C.; Ryazanov, M.; Simon, A. Z. Anorg. Allg.
Chem. 2005, 631, 302.
(12) Mattausch, Hj.; Kremer, R. K.; Simon, A. Z. Anorg. Allg. Chem. 1995,
621, 1001.
(13) Cockcroft, J. K.; Kremer, R. K.; Mattausch, Hj.; Raju, N. P.; Simon,
A. J. Alloys Compd. 1992, 183, 241.
(14) Mattausch, Hj.; Eger, R.; Simon, A. Z. Anorg. Allg. Chem. 1991, 579,
145.
Very thin platelike crystals with gold or bronze color were
observed for all products. EDX analyses of the reaction products,
(15) Schleid, Th.; Meyer, G. Z. Kristallogr. 1994, 209, 371.
10.1021/ic062425d CCC: $37.00
Published on Web 03/21/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 8, 2007 3231

Lukachuk et al.
Table 1. Experimental Conditions for the Syntheses of the RE2GeX2
Compounds
reaction
estimated
compound
Y2GeI2
Gd2GeI2
Gd2GeBr2 (1T) 5Gd, 3GdBr3, 3Ge
Gd2GeBr2 (3R) 2Gd, GdBr3, Ge
educts
temp (K) time (days) yield (%)
4Y, 2YI3, 3Ge
4Gd, 2GdI3, 3Ge
1320
1370
1200
1220
8
8
42
12
>90
90
70
∼10
using a TESCAN scanning electron microscope with an Oxford
EDX detector, confirmed the 2:1:2 atomic ratio of the component
elements. All compounds react readily with air and water, resulting
in strongly smelling oxidized or hydrolyzed products.
Structure Refinement. X-ray powder diffraction patterns were
recorded on a STADI P powder diffractometer (Stoe, Darmstadt),
using Ge-monochromatized Mo KR1 radiation (λ ) 0.7093 Å) with
carefully ground samples enclosed in sealed glass capillaries. From
the powder diffraction patterns, Y2GeI2 and Gd2GeI2 and the high-
temperature form of Gd2GeBr2 were found to be isostructural with
the 3R-type of Lu2CCl2,10 whereas the low-temperature form of
Gd2GeBr2 is isostructural with the 1T-type of Gd2CBr2.9 Since the
crystals of RE2GeX2 are extremely soft and stacked as thin layers,
well-shaped specimens suitable for a single-crystal X-ray structure
analysis were difficult to prepare. We therefore performed a Rietveld
profile refinement using the FullProf package.16 In the refinement,
the background was set manually and the reflection profiles were
modeled using the pseudo-Voigt function. Although the samples
were very well ground and mixed with glass powder to avoid
texturing effects, enhanced intensities of the 00l and hhl reflections
were found. A similar result is well-known from early investigations
on ZrCl7 as well as on “t-GdCl” and “t-TbCl”17 (which were later
found out to be hydride chlorides). The effect is only observed
with the ZrCl-type layer stacking and not with that of ZrBr. It is
obviously due to mechanical introduction of layer disorder.
The plots of the final Rietveld refinements are shown in Figure
1, and the refinement parameters for Y2GeI2, Gd2GeI2, and 1T-
Gd2GeBr2 are summarized in Table 2. Because of the small yield
of 3R-Gd2GeBr2, only the lattice parameters (a ) 4.1442(9) Å and
c ) 29.487(7) Å) could be calculated from a least-squares fit of
the powder diffraction data of the selected phase.
The crystallographic information, including the fractional coor-
dinates and selected bond lengths of Y2GeI2, Gd2GeI2, and 1T-
Gd2GeBr2, is listed in Tables 2-4.
Physical Properties. For the measurements of the electrical
conductivity, the polycrystalline samples were pressed into pellets
of 5 mm diameter and ca. 2 mm in thickness. The conventional
four-contact van der Pauw method18 was used. The magnetic
susceptibilities of Gd2GeI2 and Gd2GeBr2 were measured with a
MPMS SQUID magnetometer (Quantum Design) using sample
quantities of ca. 20 mg. Heat capacities were measured in a PPMS
system (Quantum Design) on pressed pellets of ∼20 mg. All
measurements were carried out on pure RE2GeX2 phases selected
under an optical microscope and characterized by X-ray powder
diffraction.
Figure 1. Rietveld refinement plots for Y2GeI2, Gd2GeI2, and 1T-Gd2-
GeBr2; observed intensities indicated with open circles, calculated patterns
with solid lines. The vertical bars indicate the Bragg positions; the difference
between observed and calculated patterns is drawn below.
in a close-packing sequence aBγAb along [001]. Y2GeI2, Gd2-
GeI2, and 3R-Gd2GeBr2 crystallize in the rhombohedral space
group R3hm (Figure 2, right). Their structure consists of the
Results and Discussion
The 1T-Gd2GeBr2 crystallizes in space group P3hm1
(Figure 2, left). The Br-Gd-Ge-Gd-Br slabs are arranged
same types of slabs as those in Gd2GeBr2, but every next
1
layer is shifted by 2/3
/
3
1/3 relative to the previous one. This
(16) Rodriguez-Carvajal, J. FullProf.2k, version 2.70; CEA-CNRS: Gif-
sur-Yvette, France, 2004.
(17) Mattausch, Hj.; Simon, A.; Holzer, N.; Eger, R. Z. Anorg. Allg. Chem.
1980, 466, 7.
results in a tripling of the c lattice parameter and rhombo-
hedral symmetry. The close-packing sequence is aBγAb
cAâCa bCRBc.
(18) van der Pauw, L. Philips Res. Rep. 1958, 13, 1.
3232 Inorganic Chemistry, Vol. 46, No. 8, 2007

Layered Germanide Halides
Table 2. Crystal Structure Data and Rietveld Refinement Parameters
for Y2I2Ge, Gd2I2Ge, and 1T-Gd2Br2Gea
empirical formula
fw
temp (K)
wavelength (Å)
cryst syst
space group
unit cell dimens
a (Å)
Y2GeI2
504.23
293(2)
0.7093
Gd2GeI2
640.91
293(2)
0.7093
1T-Gd2GeBr2
546.91
293(2)
0.7093
trigonal
P3hm1
rhombohedral
R3hm
R3hm
4.2135(3)
31.480(2)
4.2527(1)
31.657(1)
4.1668(2)
9.8173(6)
c (Å)
volume(Å3), Z
484.00(5), 3 495.83(3), 3 147.61(1), 1
5.19
density (calcd) (g/cm3)
6.44
5800
267
17
6.15
5800
238
17
no. of collected points, N 5800
no. of reflns
no of refined params, P
265
17
S
1.55
3.10
2.02
final R indices
Rp
0.0179
0.0242
0.0155
0.1064
0.0545
0.0706
0.0227
0.0607
0.0551
0.0734
0.0363
0.0726
Rwp
Rexp
RB
2
2
a Rp ) ∑|yo - yc|/∑|yo|, Rwp ) [∑w|yo - yc| /∑w|yo| ]1/2, Rexp ) [(N -
P)/∑wyo ]1/2, RB ) ∑|Io - Ic|/∑|Io|, S ) Rwp/Rexp
.
2
Table 3. Atomic Coordinates and Isotropic Displacement Parameters
Figure 2. Projections of the 1T-Gd2GeBr2 (left) and Gd2GeI2 (right)
structures.
(Å2) for Y2GeI2, Gd2GeI2, and 1T-Gd2GeBr2
atom
Wyckoff position
x
y
z
B
The shortest distance between the neighboring Ge atoms
is 4.17 Å, which is too large for forming Ge-Ge bonds.
The Gd-Ge distance of 2.90 Å is only slightly longer than
the sum of the covalent radii of Gd and Ge (2.83 Å).20 As
Ge atoms are not bonded to each other, the charge distribu-
tion for the RE2GeX2 compounds can be approximated in
an ionic formulation as (RE3+)2Ge4-(X-)2 with no valence
electrons in the RE framework.
Y2GeI2 (R3hm)
1
Y
I
Ge
6c
6c
3a
/
/
2/3
1/3
0
0.0499(1)
0.11287(6)
0
1.56(7)
1.85(5)
1.5(1)
3
2
3
0
Gd2GeI2 (R3hm)
1
Gd
I
Ge
6c
6c
3a
/
/
2/3
1/3
0
0.04993(3)
0.11330(3)
0
1.52(2)
1.89(3)
1.55(5)
3
2
3
0
1T-Gd2GeBr2 (P3hm1)
1
Gd
Br
Ge
2d
2d
1a
/
/
2/3
2/3
0
0.1657(3)
0.6579(4)
0
1.79(5)
1.21(8)
1.2(1)
3
Electrical and Magnetic Properties. Figure 3 shows the
results of resistivity measurements for Y2GeI2, Gd2GeI2, and
1T-Gd2GeBr2. The temperature dependence of Y2GeI2 and
Gd2GeI2 indicates semiconducting behavior in the entire
temperature range. The room-temperature resistivity of Y2-
GeI2 amounts to about 4 Ω cm and increases by 8 orders of
magnitude on cooling to low temperatures. The room-
temperature resistivity of Gd2GeI2 is 1 order of magnitude
smaller (0.1 Ω cm) and increases by 4 orders of magnitude
upon lowering of the temperature. 1T-Gd2GeBr2 exhibits a
semiconducting behavior only at temperatures below ∼150
K with an increase of the resistivity by 2 orders of magnitude.
A linear increase of the resistivity above 150 K (see insets
of Figure 3) indicates a transition to metallic characteristics.
1T-Gd2GeBr2 has the smallest room-temperature resistivity
among the title compounds, which amounts to ∼35 mΩ cm.
The F (logarithmic scale) vs 1/T dependences for the title
compounds (Figure 3) between approximately 25 and 100
K are almost linear, indicating a thermally activated conduc-
tion. The analyses with an Arrhenius law, F(T) exp(-EA/
kBT), yield activation energies, EA, of 43, 10, and 3.3 meV
for Y2GeI2, Gd2GeI2, and 1T-Gd2GeBr2, respectively.
The magnetic susceptibilities of Gd2GeI2 and 1T-Gd2GeBr2
(Figure 4) follow a Curie-Weiss law. The experimental
effective magnetic moments obtained from the high-tem-
perature slopes of the 1/ø vs T plots amount to 7.98 and
1
3
0
Table 4. Interatomic Distances (Å) for Y2GeI2, Gd2GeI2, and
1T-Gd2GeBr2
Y2GeI2
Gd2GeI2
1T-Gd2GeBr2
RE-RE
3.972(4),
4.2135(3)
3.139(2)
2.895(2)
4.170(2),
4.2135(3)
4.306(2)
4.2135(3)
4.003(1),
4.2527(1)
3.1706(8)
2.9201(5)
4.177(1),
4.2527(1)
4.3466(8)
4.2527(1)
4.046(3),
4.1668(2)
2.964(3)
2.904(2)
3.924(4),
4.1668(2)
4.131(3)
4.1668(2)
RE-X
RE-Ge
X-X
X-Ge
Ge-Ge
Since Y2GeI2 and Gd2GeI2 are isostructural and have the
same stacking sequence with respect to the layers as that
observed for Gd2GeBr2, we describe the interatomic distances
only for 1T-Gd2GeBr2. The closest Gd-Gd contact is found
between the layers (4.05 Å), and it is much longer than the
average Gd-Gd distance of 3.60 Å in hcp Gd.19 The metal-
metal distance within these layers is somewhat larger (4.17
Å, a lattice parameter). In the isotypic Gd2CBr2 compound,9
the Gd-Gd distances (3.43 and 3.82 Å) are much shorter
due to the smaller atomic radius of the C atom compared
with that of Ge. Despite these differences, the c lattice
parameters in Gd2CBr2 and Gd2GeBr2 are almost equal and
amount to 9.82 Å.
(19) Donohue, J. The Structures of the Elements; Wiley: New York, 1974.
(20) Emsley, J. The Elements; Oxford University Press: Oxford, 1989.
Inorganic Chemistry, Vol. 46, No. 8, 2007 3233

Lukachuk et al.
Figure 4. Magnetic susceptibilities of Gd2GeI2 and 1T-Gd2GeBr2. The
insets show the inverse magnetic susceptibilities.
Figure 3. Temperature dependence of the resistivities for Y2GeI2, Gd2-
GeI2, and 1T-Gd2GeBr2 (from top to bottom).
7.72 µB for Gd2GeI2 and 1T-Gd2GeBr2, respectively. These
values are in good agreement with the expected effective
8
moment of 7.94 µB for Gd3+ with a S7/2 ground state. The
paramagnetic Curie temperatures are negative for both
compounds, indicating predominant antiferromagnetic ex-
change interaction, and they amount to -14 and -11 K for
Gd2GeI2 and 1T-Gd2GeBr2, respectively. Indeed, an antifer-
romagnetic ordering is indicated by a shallow maximum in
the susceptibility with a Ne´el temperature TN ≈ 15 K when
sufficiently small external fields of 0.01 or 0.1 T are applied.
The maximum is suppressed, and steep ferromagnetic-like
increases of the susceptibilities with saturation effects occur
as the magnetic field is increased.
The heat capacity data for Gd2GeI2 and 1T-Gd2GeBr2
(Figure 5a) show λ-type anomalies at ∼15 K for Gd2GeI2
and ∼13 K for 1T-Gd2GeBr2, indicating long-range magnetic
ordering. Comparison with the heat capacity of the nonmag-
netic compound Y2GeI2 reveals that the magnetic contribu-
tions to the specific heat for Gd2GeI2 and 1T-Gd2GeBr2 are
dominant below ∼10 K and extend up to ∼70 K. However,
the majority of the magnetic contributions is concentrated
Figure 5. Heat capacities of Gd2GeI2, 1T-Gd2GeBr2, and Y2GeI2 presented
as Cp/T vs T (a) or Cp vs T (b). The dashed lines in part b indicate a power-
law behavior Cp T 2 or Cp T 3/2
.
below the ordering temperatures. These contributions are
stronger for 1T-Gd2GeBr2 than for Gd2GeI2. Accordingly,
the magnitude of the λ anomaly of 1T-Gd2GeBr2 is somewhat
reduced, and a large fraction of the magnetic entropy is
shifted below the ordering temperature. Similar low-tem-
perature (T < TN) features in the magnetic heat capacity were
observed, e.g., in GdIHx (0.66 e x e 0.86).21 We attribute
(21) Ryazanov, M.; Kremer, R. K.; Simon, A.; Mattausch, Hj. Phys. ReV.
2006, B73, 035114.
3234 Inorganic Chemistry, Vol. 46, No. 8, 2007

Layered Germanide Halides
Figure 6. Sommerfeld plot Y2GeI2 below 4 K.
Figure 7. Temperature dependence of the magnetic entropies, Smag, of
Gd2GeI2 and 1T-Gd2GeBr2.
these features to complex-canted antiferromagnetic order as
a result of the competing interactions in the triangular RE
layers.
The spin entropies Smag(T) of Gd2GeI2 and 1T-Gd2GeBr2,
calculated according to
T Cmag(T ′)
Below the ordering temperature, the magnetic heat capacity
of Gd2GeI2 follows a T 2 behavior (Figure 5b), which can
be attributed to the magnon heat capacity of a two-
dimensional antiferromagnetic system. In contrast, the mag-
netic contribution to the heat capacity of 1T-Gd2GeBr2
decreases with a power-law behavior Cmag T 3/2 common
for spin waves in a three-dimensional ferromagnet.
The heat capacity data of Y2GeI2 between 2 and 4 K are
presented as a Cp/T vs T 2 plot in Figure 6. In this temperature
range, the lattice and electronic contributions can be de-
scribed by the sum of a Debye âT 3 term and a linear
Sommerfeld γT term according to
S(T) )
dT ′
(2)
∫
0
T ′
are plotted in Figure 7. To obtain the magnetic contributions
C
mag for the Gd compounds, Cp of Y2GeI2 was subtracted as
a reference for the phonon contributions. The different atomic
masses of Gd and Y and the different stacking sequences in
the crystal structures of Gd2GeI2 and 1T-Gd2GeBr2 will
certainly cause some changes in the phonon spectrum and a
slight difference of the lattice heat capacities. However, as
can be seen in Figure 5b, the heat capacity of Y2GeI2 at high
temperatures approaches rather well the Cp values of Gd2GeI2
and 1T-Gd2GeBr2 despite the Y-Gd mass difference. To
compensate for the mass differences of Y and Gd, an
empirical factor of 1.09 was applied to adjust the heat
capacities of Y2GeI2 and Gd2GeX2 at high temperature, where
no magnetic contributions are present. The entropy increases
steeply and levels off at temperatures ∼15 K, where the
magnetic orderings occur. About 75% of the magnetic
entropy is removed below TN, indicating that short-range
magnetic ordering effects with sizable contributions to the
heat capacities above TN are not observed. The saturation
value of the entropy is very close to 2R ln 8, as expected for
Cp ) γT + âT 3
(1)
with the Sommerfeld coefficient γ ) 3.0(6) × 10-3 J/mol
K2 and â ) 2.36(8) × 10-3 J/mol K4. The coefficient â
corresponds to a Debye temperature of ΘD(0) ) 160 K. This
value lies well in the range of similar compounds, e.g., YIH0.8
ΘD(0) ) 220 K.22 A linear contribution to the heat capacity
is unexpected in view of the semiconducting behavior of
Y2GeI2. It may be due to disorder or some doping-induced
charge carriers. For example, in Si single crystals, similar
values were observed, which were ascribed to a doping level
of 1020 atoms/cm3.23,24
7
a system with two Gd3+ (S ) /2) atoms per formula unit.
Acknowledgment. We thank E. Bru¨cher and G. Siegle
for the magnetization and resistivity measurements.
Supporting Information Available: Three CIF files. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(22) Ryazanov, M.; Kremer, R. K.; Simon, A.; Mattausch, Hj., unpublished
results.
(23) Gibin, A.; Devyatykh, G. G.; Gusev, A. V.; Kremer, R. K.; Cardona,
M.; Pohl, H.-J. Solid State Commun. 2005, 133, 569.
(24) Keesom, P. H.; Seidel, M. Phys. ReV. 1959, 113, 33.
IC062425D
Inorganic Chemistry, Vol. 46, No. 8, 2007 3235