Alkali-metal thiogermanates: Sodium channels and variations on the La3CuSiS7 structure type

Article abstract of DOI:10.1021/ic502418s

Five new isotypic quaternary chalcogenides containing rare-earth metal atoms crystallizing in the hexagonal noncentrosymmetric space group P6₃ (No. 173) with the La₃CuSiS₇ structure type have been synthesized by reacting the appropriate anhydrous rare-earth trichloride with sodium thiogermanate, Na₂GeS₃. The reaction between LnCl₃ and Na₂GeS₃ in an evacuated fused-silica ampule produced high yields of good-quality crystals of NaLn₃GeS₇ [Ln = Ce (I), Nd (II), Sm (III), Gd (IV), and Yb (V)], while a similar reaction between EuCl₃ and Na₂GeS₃ yielded a quinary chloride thiogermanate, Na_1.2Eu_3.4Cl₂Ge₃S₉ (VI), incorporating a cyclic trimeric Ge₃S₉ building unit and adopting a structure related to La₃CuSiS₇. The crystal structure of the compounds comprises a complex network of bicapped trigonal-prismatic LnS₈ and GeS₄ tetrahedra, which creates channels along the [001] direction. The Na⁺ cations reside in these channels within trigonally distorted octahedral coordination environments, surrounded by six S atoms. For compounds III-V, the temperature dependence of the magnetic susceptibility indicates that these compounds are paramagnetic with μ_eff. = 1.86, 8.01, and 3.87 μ_B, for III-V, respectively. The experimental μ_eff for IV is close to the theoretical value of 7.94 for free Gd³⁺ ions, while μ_eff values for III and V deviate from their theoretical values of 0.86 and 4.54 μ_B for Sm³⁺ and Yb³⁺ ions, respectively. These compounds are semiconductors with optical band gaps of around 1.3 eV for III and V. Extended Hückel calculations suggest that the valence band comprises primarily S 3p and the bottom of the conduction band is dominated by empty rare-earth 5d orbitals. Compound VI exhibits a sharp optical absorption of around 2.18 eV, which is attributed to the f → d transition of Eu^II. The effective magnetic moment of 7.94 μ_B/Eu is in excellent agreement with the theoretical value of 7.94 μ_B for the free Eu²⁺ ion.

Full text of DOI:10.1021/ic502418s

Article
pubs.acs.org/IC
Alkali-Metal Thiogermanates: Sodium Channels and Variations on
the La3CuSiS7 Structure Type
Amitava Choudhury† and Peter K. Dorhout*,‡
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States
S
* Supporting Information
ABSTRACT: Five new isotypic quaternary chalcogenides containing rare-
earth metal atoms crystallizing in the hexagonal noncentrosymmetric space
group P63 (No. 173) with the La3CuSiS7 structure type have been
synthesized by reacting the appropriate anhydrous rare-earth trichloride
with sodium thiogermanate, Na2GeS3. The reaction between LnCl3 and
Na2GeS3 in an evacuated fused-silica ampule produced high yields of good-
quality crystals of NaLn3GeS7 [Ln = Ce (I), Nd (II), Sm (III), Gd (IV),
and Yb (V)], while a similar reaction between EuCl3 and Na2GeS3 yielded a
quinary chloride thiogermanate, Na1.2Eu3.4Cl2Ge3S9 (VI), incorporating a
cyclic trimeric Ge3S9 building unit and adopting a structure related to
La3CuSiS7. The crystal structure of the compounds comprises a complex
network of bicapped trigonal-prismatic LnS8 and GeS4 tetrahedra, which
creates channels along the [001] direction. The Na+ cations reside in these
channels within trigonally distorted octahedral coordination environments,
surrounded by six S atoms. For compounds III−V, the temperature dependence of the magnetic susceptibility indicates that
these compounds are paramagnetic with μeff. = 1.86, 8.01, and 3.87 μB, for III−V, respectively. The experimental μeff for IV is
close to the theoretical value of 7.94 for free Gd3+ ions, while μeff values for III and V deviate from their theoretical values of 0.86
and 4.54 μB for Sm3+ and Yb3+ ions, respectively. These compounds are semiconductors with optical band gaps of around 1.3 eV
for III and V. Extended Huckel calculations suggest that the valence band comprises primarily S 3p and the bottom of the
̈
conduction band is dominated by empty rare-earth 5d orbitals. Compound VI exhibits a sharp optical absorption of around 2.18
eV, which is attributed to the f → d transition of EuII. The effective magnetic moment of 7.94 μB/Eu is in excellent agreement
with the theoretical value of 7.94 μB for the free Eu2+ ion.
structure type.10 Today, a large number of compounds are
INTRODUCTION
■
known with this structure type with the general formula
A6B2C2Q14 or A3BCQ7, while some have vacancies within the
[A−B] metal network.11−49 In all of these compounds, A is a
trivalent rare-earth metal and Q is chalcogen, S, or Se. The B
sites are localized on the hexagonal axis and occupy all of the
positions from the center of an octahedron (for example, Mg,
Mn, Fe, Co, Ni, and Zn12,15,23,28) to the trigonal side faces of
octahedra (as in Cu and Ag13,17,27,32) having coordination
numbers ranging from 6 to 3, in an environment of Q atoms. A
great variety of atoms having different charges (such as 1+, 2+,
and 3+)18 occupy the B sites, and often they are only partially
occupied. The C sites are localized on the ternary axis and are
found in a tetrahedral coordination environment of Q atoms.
They are occupied by tetravalent Si, Ge, and Sn,18,35,39,41,42
trivalent Al, Ga, In, and Fe,18,24 and divalent Be18 or Fe.23,34
Light alkali-metal cations occupying the B sites were unknown
until the first report by the Schleid group in rare-earth
thiosilicates, NaY3SiS7.33,47 In this paper, we report the
The multicomponent system involving a rare-earth chalcoge-
nide in combination with several other main-group elements
has attracted a great deal of attention not only because they can
form a large number of structure types but also because of their
ability to be flexible in composition over a wide range within
one structure type.1−7 In general, in the solid-state chemistry of
rare earths, the gradually changing size of the rare-earth metal
becomes the only determining factor as to whether an
isostructural member within a particular structure type will
form or not. Such a variation of components within one
structure type has practical implications on the fundamental
properties of the materials, for example, in the case of rare-earth
metals, spectroscopic and magnetic properties resulting from
the f electrons. However, it becomes even more interesting in
terms of tuning the properties when one is able to vary multiple
components of the system while keeping the structure type
intact. One such group of quaternary chalcogenides belongs to
the hexagonal crystal system space group P63, first reported in
1968 as the cell parameters calculated from powder X-ray
diffraction data, and has proven to be a remarkably robust
structure type.8 Single-crystal structure investigations revealed
that these compounds belong to the Ce6Al3.33S149 or La3CuSiS7
Special Issue: To Honor the Memory of Prof. John D. Corbett
Received: October 2, 2014
Published: November 20, 2014
© 2014 American Chemical Society
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Article
Table 1. Crystal Data and Structural Refinement Parameters for Compounds I−VI
compound
chemical formula
fw
I
II
III
IV
V
NaCe3GeS7
740.36
NaNd3GeS7
752.72
NaSm3GeS7
771.05
NaGd3GeS7
791.75
NaYb3GeS7
839.12
cryst syst
space group
T/K
hexagonal
P63 (No. 173)
293(2)
hexagonal
P63 (No. 173)
293(2)
hexagonal
P63 (No. 173)
100(2)
0.71073
9.9366(7)
5.8018(8)
496.10(8)
2
hexagonal
P63 (No. 173)
293(2)
hexagonal
P63 (No. 173)
293(2)
́
wavelength /Å
0.71073
10.2248(3)
5.8035(4)
525.45(4)
2
0.71073
10.0820(4)
5.7897(5)
509.66(5)
2
0.71073
9.8646(4)
5.8007(5)
488.84(5)
2
0.71073
9.5421(6)
5.8833(7)
463.92(7)
2
a/Å
c/Å
volume/Å3
Z
ρcalc /Mg m−3
μ/mm−1
Flack parameterx
GOF (S)
4.679
4.905
5.162
5.379
6.007
16.946
19.354
21.939
24.597
34.713
0.046(8)
1.214
0.04(3)
1.303
0.00(4)
1.097
0.03(2)
1.188
0.02(3)
1.093
a
R1 [I > 2σ(I)]
0.015
0.027
0.032
0.019
0.034
b
wR2(F2) (all data)
δF/e Å−3
0.0395
0.0756
0.0886
0.0546
0.0857
0.890, −0.758
0.944, −2.553
1.265, −2.289
VI
0.809, −2.087
1.357, −1.232
compound
chemical formula
fw
Na1.2Eu3.4Cl2Ge3S9
1121.46
hexagonal
P63 (No. 173)
293(2)
cryst syst
space group
T/K
́
wavelength/Å
0.71073
9.5388(3)
11.7178(9)
923.35(8)
2
a/Å
c/Å
volume/Å3
Z
ρcalc/Mg m−3
μ/mm−1
Flack parameter x
GOF (S)
4.034
17.498
0.00(2)
1.072
a
R1 [I > 2σ(I)]
0.0322
b
wR2(F2) (all data)
δF/e Å−3
0.0733
1.337, −1.035
a
b
R1 = ∑|||Fo| − |Fc||/∑|Fo|. wR2 = {∑[w(Fo2 − Fc2)2]/∑[w(Fo )2]}1/2, w = 1/[σ2(Fo)2 + (aP)2 + bP], where P = [Fo2 + 2Fc2]/3; a = 0.0172 and b
= 0.2723 for I; a = 0.0424 and b = 1.1100 for II; a = 0.0610 and b = 5.4309 for III; a = 0.0282 and b = 1.7229 for IV; a = 0.0583 and b = 1.2194 for
V; a = 0.0467, b = 0.
2
temperature-controlled furnace, and heated at 825 °C for 96 h. The
details of the reactions are given in ref 52. For the reactions with CeCl3
and NdCl3, only a few high-quality crystals of NaCe3GeS7 and
NaNd3GeS7, respectively, were obtained. The major phase for these
two reactions was Ln4(GeS4)3 (Ln = Ce and Nd).52 Similar reactions
with SmCl3, GdCl3, and YbCl3 yielded only the NaLn3GeS7 (Ln = Sm,
Gd, and Yb) phase as the major product, and no Ln4(GeS4)3 could be
detected in our analyses. The reaction between EuCl3 and Na2GeS3
yielded red crystals of Na1.2EuII3.4Cl2Ge3S9 as the major product. The
reduction of Eu from Eu3+ in starting EuCl3 to Eu2+ in VI could be
explained by either oxidation of S2− to S or oxidation of some of Cl− to
Cl2 gas. The presence of elemental S was not evident in the final
product; however, the formation of Cl2 could not be ruled out. In all of
the as-synthesized products, the presence of traces of GeS2 (cold end
of the ampule) and NaCl could be detected.
structures and some properties of the first few members of Na-
containing thiogermanates in the La3CuSiS7 structure type,
NaLn3GeS7 (Ln = Ce, Nd, Sm, Gd, and Yb), as well as a new
quinary compound Na1.2Eu3.4Cl2Ge3S9 related to the La3CuSiS7
structure type, synthesized from anhydrous rare-earth chloride
(LnCl3) and ternary thiogermanate (Na2GeS3).
EXPERIMENTAL SECTION
■
Synthesis. Good-quality single crystals of the family of compounds
NaLn3GeS7 [Ln = Ce (I), Nd (II), Sm (III), Gd (IV), and Yb (V)]
and Na1.2Eu3.4Cl2Ge3S9 (VI) were obtained from the reaction of the
appropriate anhydrous rare-earth chloride (LnCl3) and the ternary
sodium thiogermanate, Na2GeS3. Na2GeS3 was prepared by heating a
stoichiometric mixture of Na2S2, Ge, and S in a sealed fused-silica
ampule at 725 °C for 3 h. Na2S2 was prepared from the stoichiometric
combination of the elements in liquid ammonia as described
previously for several alkali polychalcogenides.50,51 CeCl3, NdCl3,
SmCl3, EuCl3, GdCl3, and YbCl3 (Sigma-Aldrich, 99.99% anhydrous
powder) were used as received. Typically, 0.36 mmol of LnCl3 (Ln =
Ce, Nd) and 0.28 mmol of Na2GeS3 (9:7 LnCl3/Na2GeS3) were mixed
and ground in an agate mortar inside a N2-filled glovebox. The mixture
was then transferred to a graphite crucible, which was then placed
inside a fused-silica tube, evacuated, flame-sealed, placed vertically in a
It is important to comment on the chosen synthesis method. The
initial aim was to substitute trivalent rare earths in the Na2GeS3
structure through a metathesis route:
LnCl3 + 2Na2GeS3 = NaLn(GeS3)2 + 3NaCl
(1)
The structure of Na2GeS3 comprises infinite anionic chains of
corner-shared GeS4 tetrahedra, where the Na+ cations reside in the
interchain spaces. Our hypothesis has been to incorporate rare-earth
ions in place of Na ions and retain the integrity of the corner-shared
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Article
a
Table 2. Selected Interatomic Distances (Å) and Angles (deg) for the Coordination Polyhedra of I−V
Ce (I)
Nd (II)
Sm (III)
Gd (IV)
Yb (V)
Ln−S
Ln1−S1
3.0442(9)
3.0012(15)
2.7976(18)
2.8112(18)
2.8705(19)
2.919(3)
2.9532(16)
2.763(2)
2.788(2)
2.840(2)
2.860(3)
2.924(2)
3.003(2)
3.212(3)
2.9276(12)
2.7337(17)
2.7703(17)
2.8240(18)
2.832(2)
2.812(2)
2.722(4)
2.653(4)
2.780(3)
2.694(4)
2.826(3)
3.031(4)
3.465(4)
Ln1−S2#1
Ln1−S2#2
Ln1−S3#3
Ln1−S2#4
Ln1−S3#5
Ln1−S3#6
Ln1−S2#7
2.8377(10)
2.8385(10)
2.9023(11)
2.9732(15)
2.9998(11)
3.0414(11)
3.1642(15)
2.959(2)
2.9057(19)
3.0081(17)
3.248(2)
3.0169(19)
3.172(3)
Na−S
Ge−S
Na1−S2#1
Na1−S2#2
Na1−S2#7
Na1−S2#8
Na1−S2#9
Na1−S2#10
2.629(3)
2.629(3)
2.646(3)
2.629(3)
2.646(3)
2.646(3)
2.630(6)
2.630(6)
2.647(6)
2.630(6)
2.647(6)
2.647(6)
2.622(10)
2.622(10)
2.670(10)
2.622(10)
2.670(10)
2.670(10)
2.616(5)
2.616(5)
2.610(5)
2.610(5)
2.610(5)
2.616(5)
2.573(8)
2.573(8)
2.536(8)
2.573(8)
2.536(8)
2.536(8)
Ge1−S3
2.2249(11)
2.2249(11)
2.2249(11)
2.1710(19)
2.226(2)
2.226(2)
2.226(2)
2.172(3)
2.227(2)
2.227(2)
2.227(2)
2.177(4)
2.2246(18)
2.2246(18)
2.2246(18)
2.177(3)
2.214(4)
2.214(4)
2.214(4)
2.195(6)
Ge1−S3#3
Ge1−S3#5
Ge1−S1#11
S−Ge−S
S1#11−Ge1−S3
S1#11−Ge1−S3#3
S1#11−Ge1−S3#5
S3−Ge1−S3#3
S3−Ge1−S3#5
S3#3−Ge1−S3#5
112.96(4)
112.96(4)
112.96(4)
105.76(4)
105.76(4)
105.76(4)
113.40(6)
113.40(6)
113.40(6)
105.27(7)
105.27(7)
105.27(7)
114.02(7)
114.02(7)
114.02(7)
104.57(8)
104.57(8)
104.57(8)
114.11(5)
114.11(5)
114.11(5)
104.45(6)
104.45(6)
104.45(6)
115.26(11)
115.26(11)
115.26(11)
103.12(13)
103.12(13)
103.12(13)
a
Symmetry transformations used to generate equivalent atoms: #1, −x + y + 1, −x + 1, z; #2, x − 1, y, z; #3, −y + 1, x − y + 1, z; #4, y, −x + y + 1, z
+ 1/2; #5, −x + y, −x + 1, z; #6, y − 1, −x + y, z + 1/2; #7, y, −x + y + 1, z − 1/2; #8, −y, x − y − 1, z; #9, x − y − 1, x − 1, z − 1/2; #10, −x + 1, −y, z
− /2; #11, x, y, z − 1.
1
GeS4 chains. When we did not get our intended product, we modified
the ratios of rare-earth chloride and Na2GeS3 and found that the
reaction of LnCl3 and Na2GeS3 with a 9:7 ratio gave the best yield of
NaLn3GeS7 in the case of SmCl3. Consequently, we carried out similar
reactions for the other rare-earth metals, and the reactions are listed
below in the form of unbalanced equations:
However, the same reactions with LaCl3 yielded a completely different
product: La3Cl(GeS4)2. We did not succeed in preparing phase pure
NaLn3GeS7 by heating a stoichiometric combination of pure elements
and the binaries Na2S and GeS2 under similar conditions, suggesting
that the metathesis route may be the only way to prepare phase-pure
materials.
X-ray Structure Determination. Intensity data sets for compounds
I−V were collected on a Bruker Smart CCD diffractometer. These
data were integrated with SAINT,53 the program SADABS was used for
the absorption correction for compounds I − IV,54 and the HABITUS
was used for the numerical absorption correction of compounds V and
VI.55 The structures were solved by direct methods using SHELXS-
9756 and difference Fourier syntheses. Full-matrix least-squares
refinement against |F2| was carried out using the SHELXTL-PLUS57
suite of programs. On the basis of systematic absences, the space group
P63 was chosen for all of the compounds, which immediately revealed
that I−V are isostructural to La3CuSiS7.10 The rare-earth metal cation
was easily located on the 6c sites, while subsequent refinement
identified Ge on the 2b sites, three S atoms on the 2b and 6c sites,
respectively, and Na on the 2a site approximately 2.6 Å away from one
of the S atoms. The last cycles of refinement for I−V included
anisotropic thermal parameter refinement for all of the atoms. The
refinement of compound V did not merge to a good R factor (R1 =
20%), indicating that the crystal might be merohedrally twinned. So, a
twin law, TWIN 0 −1 0 −1 0 0 0 0 1, was used,58 which immediately
improved R1 from 20% to 2%, and BASF was refined to 0.49(3). The
crystal was also checked for the simultaneous presence of racemic
twinning using TWIN 0 −1 0 −1 0 0 0 0 1 −4 with three BASF
variables, but the negligible value of the last two BASFs (0.05 and
0.06) indicated the absence of racemic twinning. The structure was
finally refined with one BASF variable, with the −4 term removed
9CeCl3 + 7Na2GeS3
→ NaCe3GeS7 (I; minor) + Ce4(GeS4)3 (major) + GeS2
(2)
+ NaCl
9NdCl3 + 7Na2GeS3
→ NaNd3GeS7 (II; minor) + Nd4(GeS4)3 (major) + GeS2
(3)
+ NaCl
9SmCl3 + 7Na2GeS3
→ NaSm3GeS7 (III; major) + GeS2 + NaCl
(4)
9GdCl3 + 7Na2GeS3
→ NaGd3GeS7 (IV; major) + GeS2 + NaCl
(5)
9YbCl3 + 7Na2GeS3 → NaYb3GeS7 (V; major) + GeS2 + NaCl
(6)
9EuCl3 + 7Na2GeS3
→ Na1.2Eu3.4Cl2Ge3S9 (VI) + GeS2 + NaCl
(7)
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from the TWIN law. For compound VI, the rare-earth atoms (Eu1 and
Eu2) and one Ge and five S atoms were easily located and refined
isotropically. At this point, another atom at 2.915 Å away from one of
the S atoms was assigned as Na and refined. The refinement resulted
into a high thermal parameter for Na (U11 = 0.05336), indicating that
Na may be partially occupied. So, the in the subsequent refinement,
the occupancy of Na was varied and refined at 0.593(2). However,
elemental analysis employing energy-dispersive spectrometry (EDS)
indicated the presence of Cl in the crystal with the following ratio:
Na:Eu:Cl:Ge:S = 1.3:3.5:2:3:9. To account for the presence of Cl
atoms in the structure and to balance the charge of the compound, we
simply assigned two S atoms that were not bonded to a Ge atom to Cl
atoms and refined. Because there is hardly any difference between the
scattering factor of the S and Cl atoms, it was not possible to locate Cl
crystallographically. At this point, to balance the charge and to satisfy
elemental analysis, more Na and less Eu were required, which
obviously indicated that the Eu sites may have mixed occupancy of Eu
and Na. The introduction of Na into both Eu sites by varying the
occupancy and keeping the coordinates fixed clearly indicated that Eu2
has 20% of Na in its site. The mixed-occupancy model of Eu and Na in
the 6c site also brought down the weighted R factor from 20% to 18%.
Finally, anisotropic refinement was carried out for all of the atoms, and
the refinement merged to a very good R factor. On the basis of the
occupancy of Eu2/Na2 (80/20%) and Na1 (60%) in the final round of
refinement, we came to the formula Na1.2Eu3.4Cl2Ge3S9 for compound
VI. Details of the final refinements and cell parameters for I−VI are
given in Table 1. The final atomic coordinates for compounds I−V
and VI are supplied as Supporting Information (SI; Tables S1 and S2),
and the important interatomic distances and angles for I−V and VI are
given in Tables 2 and 3, respectively. Further details of the crystal
reference. The Kubelka−Munk function was applied to obtain band-
gap information.59 Magnetic susceptibilities of III−VI were measured
at 1 T over the temperature range 1.8−300 K, and the isothermal
magnetization at 1.8 K up to a field of 5 T was measured with a
Quantum Design SQUID magnetometer.
RESULTS AND DISCUSSION
Compounds I−V belong to the same structure type, namely,
the La3CuSiS7 family, and are analogous to the Na-containing
■
Figure 1. Perspective view of the crystal structure of I along the c axis
showing the complex networks of rare-earth polyhedra and GeS4
tetrahedra. Na+ ions are located in the one-dimensional channel made
by S atoms.
Table 3. Selected Interatomic Distances (Å) and Angles
a
(deg) for the Coordination Polyhedra of VI
moiety
distance (Å)
moiety
angle (deg)
Eu1−S2#1
Eu1−S2#2
Eu1−S2
3.024(2)
3.024(2)
3.024(2)
3.032(3)
3.037(2)
3.037(2)
3.037(2)
2.898(2)
2.933(3)
3.105(2)
3.184(2)
3.214(2)
3.315(2)
3.460(2)
2.154(2)
2.165(2)
2.243(4)
2.244(2)
S3−Ge1−S2
S3−Ge1−S1#8
S2−Ge1−S1#8
S3−Ge1−S1
S2−Ge1−S1
S1#8−Ge1−S1
114.82(8)
115.40(8)
99.90(8)
Eu1−Cl1
Eu1−S3#3
Eu1−S3#4
Eu1−S3#5
Eu2−Cl2
Eu2−Cl1
Eu2−S2#6
Eu2−S3#2
Eu2−S1
110.32(9)
105.34(8)
110.25(9)
Eu2#2−Cl1−Na2#1
Eu2#2−Cl1−Eu2#1
Eu2#2−Cl1−Eu2
Eu2#1−Cl1−Eu2
Eu2#2−Cl1−Eu1
Eu2#1−Cl1−Eu1
114.06(5)
114.06(5)
114.06(5)
114.06(5)
104.36(6)
104.36(6)
Figure 2. (a) Coordination environment around the rare-earth cation.
(b) Coordination environment around the Na+ cation. The thermal
ellipsoids are given at 90% probability. The numbers in parentheses are
symmetry codes given in Table 2
Eu2−S3
Eu2−S2#2
Ge1−S3
Ge1−S2
Ge1−S1#8
Ge1−S1
thiosilicate structure NaY3SiS7.33,47 Structures I−V contain six
atoms in their asymmetric unit: the crystallographically unique
Ln, Na, and Ge atoms are located on the 6c, 2a, and 2b Wyckoff
positions, respectively, while three crystallographically distinct S
atoms are located on a special position (S1 on the 2b Wyckoff
position) related by 3-fold symmetry and on general positions
(S2 and S3 on the 6c Wyckoff position), respectively. The
structure of NaLn3GeS7 (Ln = Ce, Nd, Sm, Gd, and Yb) is built
up from an intricate connectivity of GeS4 tetrahedra, NaS6
octahedra, and LnS8 bicapped trigonal prisms (Figure 1). The
rare-earth cations (Ln3+) are in a distorted trigonal-prismatic
environment capped by two S atoms, one at roughly 3.0 Å
(S3#6) and the other at a longer distance, roughly 3.2−3.4 Å
(S2#7), as seen in Figure2a. The LnS8 bicapped trigonal prisms
are connected to neighboring LnS8 polyhedra through common
a
Symmetry transformations used to generate equivalent atoms: #1, −y
+ 1, x − y + 1, z; #2, −x + y, −x + 1, z; #3, x − y, x, z − 1/2; #4, y, −x
+ y + 1, z − 1/2; #5, −x + 1, −y + 1, z − 1/2; #6, −x + 1, −y + 1, z +
1/2; #7, −x, −y, z + /2; #8, −x + y + 1, −x + 1, z.
1
structure investigations may be obtained from Fachinformationszen-
trum Karlsruhe, Eggenstein-Leopoldshafen, Germany [fax (49) 7247-
808-666; e-mail crysdata@fiz.karlsruhe.de] on quoting the depository
numbers CSD 421445 (I), 421443 (II), 421442 (III), 421444 (IV),
421441 (V), and 421446 (VI).
Physical Property Measurements. Diffuse-reflectance spectrometry
(DRS) measurements for the compounds were performed on a Varian
Cary 500 UV−vis−near-IR spectrophotometer equipped with a
Praying Mantis accessory. A poly-Teflon standard was used as the
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Figure 4. Coordination environment around the two crystallo-
graphically distinct Eu atoms in VI.
compounds33,47 but are definitely shorter than many Na−S-
containing compounds.60 Moreover, these NaS6 octahedra are
trigonally distorted wherein Na+ ions are slightly displaced
along the shared face, creating three short bonds and three long
bonds, reminiscent of the chains found in the polar hexagonal
ternary chlorides of AMX3 (A = alkali metal, M = transition
metal, and X = halogen).61 The trigonal distortion also explains
the lack of any center of inversion at the Na1 site.
The GeS4 tetrahedra are isolated from each other and share
common edges and corners with the neighboring LnS8
polyhedra. The GeS4 tetrahedra also surround the columns of
NaS6 octahedra, but they do not share Ge−S−Na linkages.
These GeS4 tetrahedra all point in one direction along the 3-
fold axis, again displaying the noncentrosymmetric nature of
the structure. The GeS4 tetrahedra are distorted, with S−Ge−S
angles varying between ∼103 and 114°, while the average Ge−
S distances range between 2.209(2) and 2.214(4) Å in all five
structures (see Table 3). The GeS4 tetrahedra are trigonally
distorted with one short and three long Ge−S bonds, and this
trigonal distortion increases as the size of the rare earth
decreases from I through V, as reflected by the S−Ge−S angles
(Table 3). Out of three crystallographically distinct S atoms,
two (S1 and S3) belong to the GeS4 unit and participate in
forming Ge−S−Ln linkages, while the other S one (S2) only
forms Na−S−Na, Ln−S−Na, or Ln−S−Ln linkages. For this
reason, some authors prefer a different structural formula for
this family of compounds, for example, NaY3S3[SiS4].33
Figure 3. Perspective view of the structure of VI along the c axis
showing the presence of a cyclic trimer of corner-shared GeS4
tetrahedra (in green) and rare-earth atoms. Na+ ions and Cl atoms
are also located in the 63 channel.
face, edge, and corner sharing to create a three-dimensional
network. Such connectivity creates a tunnel of octahedral holes
along the 63 axis and creates tetrahedral holes in the lattice. The
Na+ and Ge4+ cations fill the octahedral holes in the tunnel and
the tetrahedral holes in the lattice, respectively. The average
interatomic distances between the Ln3+ cations and S2− anions
[d(Ce−S)ave = 2.975(1) Å, d(Nd−S)ave = 2.943(2) Å, d(Sm−
S)ave = 2.918(3) Å, d(Gd−S)ave = 2.906(2) Å, and d(Yb−S)ave
= 2.873(3) Å] are within the comparable range of those
reported for Ce3+-, Nd3+-, Sm3+-, Gd3+-,42 and Yb3+-containing
sulfides.60 Moreover, they show the trend in lanthanide
contraction. It is to be noted here that one of the longer
capping Ln−S distances gets increasingly longer with
decreasing size of the lanthanides and becomes quite long for
Yb3+ [3.465(4) Å].
The alkali cation position (Na+) is fully occupied and
surrounded by six S atoms in an octahedral environment
(Figure 2b). The NaS6 octahedra share opposite faces and form
a chain that runs along the 63 axis. Apart from the face sharing
within themselves, NaS6 octahedra share common faces with
neighboring rare-earth polyhedra through Ln−S−Na linkages.
The absence of Ge−S−Na linkages indicates that channel
building S atoms do not belong to the GeS4 tetrahedra. The
average interatomic distances between the Na+ cations and S2−
anions [d(Na−S)ave = 2.637(2), 2.638(3), 2.646(1), 2.613(2),
and 2.554(4) Å, respectively, for I−V] do not show any
particular trend with decreasing size of the lanthanides, and the
distances are comparable to the Na−S distances in related
The S2 atom has shown unique structural and substitutional
chemistry. For example, if S2− is partially substituted by a
mononegative anion like chloride (Cl−), no alkali-metal cation
may be required in the channel to balance the charge. So,
essentially an empty channel structure can be created and it has
already been found in Sm3S2Cl[SiS4] by the Schleid group.47
Table 4. Comparison between the Atomic Coordinates of VI and I
VI
I
atom
Wy
site
occ
x
y
z
atom
Wy
occ
site
x
y
z
1
2
1
2
Eu1
Eu2
Na2
Ge1
Cl1
Cl2
S1
2b
6c
6c
6c
2b
2a
6c
6c
6c
2a
3..
1
1
/
/
1/3*
Ge1
Ce1
2b
6c
1
1
3..
1
/
/
3
1/3*
3
3
3
0.8
0.2
1
0.2522(2)
0.2522(6)
0.3366(2)
0.3366(6)
0.6542(3)
0.6542(3)
0.4926(5)
0.5921(3)
0.6404(4)
0.4296(3)
0.3939(2)
0.6746(9)
0.3951(2)
0.1273(2)
0.3566(2)
0.7513(1)
1
1
0.6631(3)
0.5529(2)
2
1
1
2
3..
3..
1
1
/
/
3
S1
2b
1
3..
/
/
0.9592(2)
3
3
3
1
0
0
1
0.4420(2)
0.6748(2)
0.6497(3)
0
0.3297(2)
0.7503(2)
0.5760(2)
0
S2
1
1
S2
6c
6c
2a
1
1
1
1
0.8421(1)
0.4792(2)
0
0.0875(1)
0.8951(2)
0
0.7339(3)
0.4829(2)
0.4866(8)
S3
1
1
S3
1
Na1
3..
0.6
Na1
3..
*
1
The z coordinate has been fixed to /3; occ = occupancy, Wy = Wyckoff.
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Figure 5. Plots showing the magnetic susceptibility (χ) and inverse susceptibility (χ−1) of compound III as a function of the temperature. The inset
shows the isothermal magnetization at 1.8 K.
Figure 6. Plots showing the magnetic susceptibility (χ) and inverse susceptibility (χ−1) of compound IV as a function of the temperature. The inset
shows the isothermal magnetization at 1.8 K.
One can also think of a hypothetical composition; for example,
S2− can be replaced by a different chalcogen, Se2− or Te2−,
creating a mixed chalcogen phase of this structure type,
NaLn3Q3GeS4 (Q = Se or Te). Such a substitution can
potentially tune the band gap of the material.
general site. The structure of VI is a complex network of cyclic
trimers of corner-shared GeS4 tetrahedra, Eu−S/Cl polyhedra,
and NaS3Cl2 trigonal bipyramids. Eu1 adopts a Cl-capped
trigonally distorted octahedral coordination with six S atoms;
Eu2, on the other hand, forms a very distorted S-capped
trigonal prism with two Cl and four S atoms (Figure 4). The
capping S atom forms a very high Eu2−S interatomic distance
[3.4599(2) Å]. Eu1-centered polyhedra are isolated from their
symmetrically generated Eu1 polyhedra but share edges with
Eu2-centered polyhedra. Eu2-centered polyhedra, on the other
hand, are edge-shared in a plane perpendicular to the c axis and
form a layer, while such layers are again fused to the next layer
along the c axis, through corner and edge sharing between the
Eu2 and Eu1 polyhedra, respectively, to form an infinite three-
dimensional Eu−S/Cl−Eu network. The GeS4 tetrahedra, on
the other hand, are not isolated but connected through the
corner to form a cyclic Ge3S9 trimeric unit. These Ge atoms are
placed in the tetrahedral voids created by the S atoms. Out of
the two Cl atoms, one (Cl1) adopts a trigonally distorted
The structure of VI, on the other hand, is new and related to
the La3CuSiS7 structure type (Figure 3). There are nine atoms
in the asymmetric unit, in which one site has a mixed
occupancy of Eu and Na (Eu2/Na2). The similarity between
the La3CuSiS7 structure type and VI becomes evident once the
coordinates and the atoms are compared between VI and I
(Table 4). Clearly, the cations Eu1 and Eu2/Na2 in VI are
located at the 2a and 6c sites, which were occupied by Ge1 and
Ln1, respectively, in I−V, while the anions Cl1, S2, and S3 in
VI, which are located at the 2b and 6c sites, were occupied by
the anions S1, S2, and S3, respectively, in I−V. Along the 63
axis, the Na+ and Cl− ions alternate in the channel and the c axis
is doubled in VI, compared to I−V, and in addition there is one
more anionic site in VI occupied by S (S1) located at the 6c
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Figure 7. Plots showing the magnetic susceptibility (χ) and inverse susceptibility (χ−1) of compound V as a function of the temperature. The inset
shows the isothermal magnetization at 1.8 K.
Figure 8. Plots showing the magnetic susceptibility (χ) and inverse susceptibility (χ−1) of compound VI as a function of the temperature. The inset
shows the isothermal magnetization at 1.8 K.
Figure 10. Optical absorption spectra (taken on the crushed single
crystals) of VI, transformed from the DRS data.
Figure 9. Optical absorption spectra (taken on the crushed single
crystals) of III and V, transformed from the DRS data.
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Table 5. Exponents ζi and Valence-Shell Ionization Potentials Hii of Slater-Type Orbitals χI Used for EHTB Calculations
symbol
Sm
χi
Hii (eV)
ζ1
coeff 1
ζ2
coeff 2
6s
6p
5d
4f
−5.860000
−2.850000
−6.400000
−8.080000
−16.000000
−9.000000
−20.000000
−13.300000
−5.100000
−3.000000
1.750000
1.390000
2.650000
9.900000
2.160000
1.850000
2.122000
1.827000
0.733000
0.733000
0.562300
1.000000
0.520000
0.429300
1.000000
1.000000
1.000000
1.000000
1.000000
1.000000
1.000000
0.000000
1.200000
4.900000
0.553000
0.000000
0.640000
0.715900
Ge
S
4s
4p
3s
3p
3s
3p
Na
Figure 11. Total and partial (Sm, Ge, and S) DOSs calculated for III. The dashed line indicates the position of the Fermi level.
tetrahedral coordination surrounded by four Eu atoms, while
Cl2 is located in the 63 channel and adopts a five-coordinate
trigonal-bipyramidal geometry. The Eu−Cl distances are in the
range 2.898(2)−3.032(3) Å, which is well within the
comparable range of Eu−Cl distances in Eu2+-containing
compounds.62 The 2+ oxidation state of Eu is also confirmed
by magnetic measurement (vide infra). Na1, which is also
located in the 63 axis has five-coordinate trigonal-bipyramidal
geometry, with the three S atoms in the trigonal plane and two
Cl atoms occupying the axial positions. The Na−S distances are
2.812(2) Å, certainly longer than the NaLn3GeS7 (I−V) series
of compounds but similar to the values reported in NaLnS2.60
The most remarkable features of this structure are the presence
of a Ge3S9 cyclic trimeric unit and the alternating presence of
Na and Cl along the 63 channel. The chalcogenides of the
main-group elements are always interesting because of their
ability to form various building units. These building units in
the chalcogermanate family include the most common
tetrahedral GeQ4,63 edge- and corner-shared tetrahedra
(Ge2Q6 and Ge2Q7),64,65 a supertetrahedron (Ge4Q10),66 an
ethane-like dimer (Ge2Q6),67 and chairlike and five-membered
the presence of Cl in the 63 channel of VI makes it a unique
structure type. The only other example of a quinary sulfide/
chloride containing a rare-eath metal and a main-group element
is LaCa2GeS4Cl3.68
Magnetic Measurements. The plots of the temperature
dependence of the magnetic susceptibility (χ) and inverse
susceptibility (χ−1) of III−V in the range 1.8−300 K are given
in Figures 5−7, respectively. The asymptotic nature of the χ(T)
plots at 1 T for III−V respectively under field-cooled and zero-
field-cooled (FC and ZFC) conditions confirms the para-
magnetic behavior of the compounds, and no magnetic
transitions were observed under these conditions. The absence
of any transition in III−V was also confirmed by the FC and
ZFC χ(T) measurements under low applied field (10 Oe, data
not shown). The plot of χ−1(T) for the Sm compound (III) at
high field (1 T) does not show linear behavior at any
temperature range and could not be fitted to the Curie−Weiss
law (Figure 5). The curved nature of the χ−1(T) plot is typical
of Sm3+ (4f5), known as van Vleck paramagnetism,69 and occurs
as a result of the competing nature of the thermal energy (kBT)
and energy difference between the ground and first excited
states arising from the splitting of the 6H ground term of Sm3+
due to spin−orbit coupling. At higher temperatures, the
thermal energy term exceeds the energy gap and the excited
states may be sufficiently populated. Similar magnetic behavior
of Sm3+ has been observed in many other chalcogenides.70−72
67
ring Ge4Q10 units besides some more complex units.
However, to our knowledge, such a corner-shared cyclic
trimeric unit of GeS4 tetrahedra was not known thus far. On
the other hand, the presence of Na+ or Cu+ along the 63
channel was a signature of the La3CuSiS7 structure type, while
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The room temperature magnetic moment of Sm in III is
calculated to be 1.86 μB, which is greater than the calculated
value according to μeff = g[J(J + 1)]1/2 for free Sm3+ ions (0.85
μB) but close to the room temperature paramagnetic moment
calculated by using the van Vleck formula (1.55 μB).69 Such
high moments are often observed in many Sm3+-containing
compounds.72 The isothermal magnetization (M) measured at
1.8 K for III exhibits a linear increase with increasing applied
field (inset in Figure 5). This behavior is indicative of
antiferromagnetic behavior, and the value of the saturation
moment at 5 T is about 0.025 μB/Sm, which is significantly less
than that expected for a fully ordered Sm3+ moment (gJ = 5 μB/
Sm). The χ−1(T) plot of IV at 1 T, on the other hand, is linear
in the entire temperature range (1.8−300 K), and fitting of the
data in 100−300 K range according to the Curie−Weiss law χ =
C/(T − θ) yields the value of the effective moment μeff = 8.01
μB and the paramagnetic Curie temperature (θp) = −8.27 K
(Figure 4). The effective paramagnetic moment of IV is in good
agreement with the theoretical magnetic moment of 7.94 μB
expected for Gd3+ with a 4f7 configuration, and the negative
sign and small magnitude of θp indicated a very weak
antiferromagnetic coupling between the Gd3+ ions. The
isothermal magnetization (M) of IV measured at 1.8 K exhibits
a steady increase in magnetization with increasing field; the
saturation moment at 5 T is about 5.94 μB/Gd (see inset of
Figure 6). The saturation moment of ∼6 μB/Gd for IV at 5 T
and 1.8 K is slightly less than that expected for a fully ordered
Gd3+ moment of 7 μB/Gd. The χ−1(T) plot of V at 1 T is linear
in the temperature range (150−300 K), and fitting of the data
in the 150−300 K range according to the Curie−Weiss law
yields the value of the effective moment μeff = 3.87 μB and the
paramagnetic Curie temperature (θp) = +2.12 K (Figure 7).
The effective paramagnetic moment of V is slightly less than
the theoretical magnetic moment of 4.54 μB expected for Yb3+
having a 4f13 configuration, and the positive sign and small
magnitude of θp indicate a very weak ferromagnetic coupling
between the Yb3+ ions. The lower than expected magnetic
moment for Yb3+ may indicate the presence of Yb2+ in the
material. However, we did not observe any occupancies of less
than 1 on S sites (mainly on S2, 6c Wyckoff position) or the
presence of both Cl1− and S2− in the non-Ge-bonded S sites
(S2). The absence of Cl was also evident in EDS analysis. The
isothermal magnetization (M) of V measured at 1.8 K exhibits a
steady increase in magnetization with increasing field. The
saturation moment at 5 T is about 1.57 μB/Yb (inset in Figure
7), which is substantially less than that expected for a fully
ordered Yb3+ (gJ = 4 μB/Yb) moment. The plot of the
temperature dependence of the magnetic susceptibility (χ) and
inverse susceptibility (χ−1) of VI in the range 2−300 K is given
in Figure 8. The asymptotic nature of the χ(T) plots at 5000
Oe (1 T = 10 000 Oe) under FC and ZFC conditions indicates
paramagnetic behavior of the compounds, and no magnetic
transitions are observed under these conditions (at an applied
field of 0.5 T). The absence of any transition in VI is also
established by the FC and ZFC χ(T) measurements under low
applied field (10 Oe), which are again asymptotic. The plots of
χ−1(T) at 5000 Oe show linear behavior between 2 and 300 K,
and the data in the range 100−300 K could be fitted to the
Curie−Weiss law, χ = C/(T − θ), thus yielding the values of
the effective moment (μeff) of 7.94 μB and the paramagnetic
Curie temperature (θp) = −3.88. The effective paramagnetic
moment is the same as the theoretical magnetic moment of
7.94 μB expected for Eu in the 2+ oxidation state with seven f
electrons, and the negative sign and small magnitude of θp
indicate a very weak antiferromagnetic correlation in the
paramagnetic region. The isothermal magnetization (M)
measured at 1.8 K for VI does not seem to saturate even at
5 T, and the value of the saturation moment at 5 T is about
6.65 μB/Eu (inset in Figure 8). The saturation moment of 6.65
μB/Eu is close to the theoretical saturation moment of 7 μB/Eu,
indicating that at low temperature and high field (5 T) an
almost parallel spin of the electrons of Eu in VI can be reached.
The paramagnetic moment and magnetization behavior of VI
are similar to those of several Eu2+-containing main-group
chalcogenides, Eu2GeSe4, Eu2Ge2Se5,73 Eu2GeS4,74 Eu2SnS4,75
and Na2EuGeSe4.76
Optical Properties. The absorption data calculated from
DRS using the Kubelka−Munk function for two compounds,
III and V, are shown in Figure 9. The absorption spectra of the
compounds show steep absorptions, from which the optical
gaps are calculated. These were calculated to be around 1.33
and 1.30 eV, for III and V, respectively. Such energy gaps are
within the range of semiconductors and are consistent with the
dark color of the materials. The characteristic f → f transition at
low energies below the band-gap transition would indicate that
the f states should be found in the midgap. On the other hand,
the optical band gap of 2.18 eV in the case of VI can be
tentatively assigned to the f → d transition of EuII and is
consistent with the red color of the compound (Figure 10).
To develop further insight into the nature of the electronic
band gap and other transitions, we calculated the total and
partial densities of states (DOSs), employing the extended
Huckel method within the framework of the tight-binding
̈
approximation (EHTB).77 The DOSs were calculated based on
the crystal structure for 396 k points. The parameters of the
atomic orbitals employed in the calculation are default values
from the CAESAR program,78 which are given in Table 5. For
compounds III and V, calculations indicate that the valence
band maximum near the Fermi level is mostly made up of S 3p
states admixed with a small amount of rare earth 5d and Ge 3p
states (Figure 11). The states at the Fermi level comprise the
empty rare earth 5d states. There is almost a negligible
contribution of Na 3s near the Fermi level. The EHTB method
has some limitations, does not provide reliable band gaps, and
also does not reliably localize the f states of the rare-earth
element. However, it is possible to qualitatively relate the
optical gaps to the charge-transfer transition from predom-
inantly S 3p orbitals to empty rare earth 5d orbitals.
CONCLUSIONS
■
We have extended the family of the La3CuSiS7 (A3BCQ7)
structure type by incorporating Na in the Cu (B) sites. Though
the metathesis-type reaction between LnCl3 and Na2GeS3
favors the formation of NaLn3GeS7 for Ln = Ce, Nd, Sm,
Gd, and Yb, the same reaction condition leads to stabilization
of a rare cyclic trimeric Ge3S9 unit for Eu. This was probably
possible because of the presence of infinite corner-shared
chains of GeS4 tetrahedra in Na2GeS3. Thus, this kind of
metathesis synthetic approach can play an important role in the
discovery of new structure types containing unusual building
blocks. The scope of such reactions is very high because there
are large numbers of ternary chalcogenides of main-group
elements with alkali metals containing various building units.
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(23) Nanjundaswamy, K. S.; Gopalakrishnan, J. J. Solid State Chem.
1983, 49, 51−58.
ASSOCIATED CONTENT
* Supporting Information
Final atomic coordinates for compounds I−VI (Tables S1 and
S2), along with X-ray crystallographic data in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
(24) Rodier, N.; Guittard, M.; Flahaut, J. C. R. Acad. Sci., Ser. II 1983,
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AUTHOR INFORMATION
Corresponding Author
*E-mail: dorhout@ksu.edu.
Present Addresses
■
†A.C.: Department of Chemistry, Missouri University of
Science and Technology, Rolla, MO 65409.
‡P.K.D.: 113 Eisenhower Hall, Kansas State University,
Manhattan, KS 66506-1005.
Notes
The authors declare no competing financial interest.
(34) Mills, A. M.; Ruck, M. Acta Crystallogr. 2004, C60, i71−i72.
(35) Gulay, L. D.; Olekseyuk, I. D. J. Alloys Compd. 2005, 388, 274−
278.
ACKNOWLEDGMENTS
The authors acknowledge financial support provided by the
National Science Foundation (Grant DMR-0343412).
■
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