Preparation, crystal structure, physical properties and electronic band structure of TlScQ2 (Q = S, Se and Te)

Article abstract of DOI:10.1002/zaac.200700440

The title compounds were prepared by reaction of Tl₂Q (Q = S, Se and Te) Sc and Q in the temperature range of 200 to 500 °C. The structures of the selenide and the telluride adopt the α-NaFeO₂ type, while TlScS₂ crystallizes in the β-RbScO₂ type structure. The space group is R3m for TlScSe₂ and TlScTe ₂ with a = 3.9370(4) A, c = 23.194(5) A, and a = 4.2129(4) A, c = 24.099(3) A, respectively. The sulphide crystallizes in P6₃/mmc with a = 3.761(3) A and c = 14.942(4) A. The crystal chemical relations between the three chalcogenides are discussed. According to the electrical measurements and the band structure calculations, the compounds are semiconductors or poor metals.

Full text of DOI:10.1002/zaac.200700440

DOI: 10.1002/zaac.200700440
Preparation, Crystal Structure, Physical Properties and Electronic Band
Structure of TlScQ2 (Q ؍ S, Se and Te)
Christoph L. Teskea, Wolfgang Benscha,*, Sergey Mankovskyb, and Hubert Ebertb
a Kiel, Institut für Anorganische Chemie, Christian-Albrechts-Universität
b München, Department Chemie und Biochemie Ϫ Physikalische Chemie, Ludwig-Maximilians-Universität
Received July 11th, 2007.
˚
˚
Abstract. The title compounds were prepared by reaction of Tl2Q
(Q ϭ S, Se and Te) Sc and Q in the temperature range of 200 to
500 °C. The structures of the selenide and the telluride adopt the
α-NaFeO2 type, while TlScS2 crystallizes in the β-RbScO2 type
a ϭ 3.761(3) A and c ϭ 14.942(4) A. The crystal chemical relations
between the three chalcogenides are discussed. According to the
electrical measurements and the band structure calculations, the
compounds are semiconductors or poor metals.
¯
structure. The space group is R3m for TlScSe2 and TlScTe2 with
˚
˚
˚
a ϭ 3.9370(4) A, c ϭ 23.194(5) A, and a ϭ 4.2129(4) A, c ϭ
Keywords: Thallium; Scandium; Chalcogenides; Crystal structure;
Electronic band structure
˚
24.099(3) A, respectively. The sulphide crystallizes in P63/mmc with
Introduction
onal modification forming the β-RbScO2 type structure
[18], which is also observed for CsYbSe2 [16]. This is ac-
companied by a slight distortion of the octahedral sur-
rounding of the Aϩ cation in the α-NaFeO2 type towards
a trigonal-prismatic coordination and can be discussed in
connection with the ratio R of ionic radii (R ϭ r(Ln3ϩ)/
r(Aϩ)) [14], which amounts to 0.52 for CsYbSe2 (with
Among the simply composed ternary compounds rare earth
chalcogenides like ALnQ2 (A ϭ alkaline metal, Ln ϭ lan-
thanoid and Q ϭ S, Se, Te) are subject of interest e. g. be-
cause of the possible coupling of magnetic moments in
these structures and the resulting practical applications. The
first compounds with A ϭ Li, Na, and K were prepared by
Ballestracci and Bertaut [1, 2], sulphides and selenides with
A ϭ Li by Ohtani et al. [3]. The series of compounds
NaLnS2 (Ln ϭ Ho, Er, Tm and Lu) were reported by
Schleid and Lissner [4]. Sato et al. also reported on NaLnS2
(Ln ϭ La, Nd, Pr and Gd) [5, 6]. Further extensive investi-
gations were undertaken by Bronger et al. (A ϭ Rb and
Cs) [7Ϫ9]. Plug and Verschoor determined the structure of
KCeS2 [10, 11], and KErTe2 was investigated by Keane and
Ibers [12]. More recent investigations were carried out by
Deng et al. on RbLnSe2 (Ln ϭ La to Er and Lu) [13]. Stöwe
intensively investigated the tellurides ALnTe2 (with A ϭ K,
Rb, Cs and Ln ϭ La, Pr, Gd; Ce, Nd and Nd) [14]. The
crystal structure of KYbSe2 was determined by Gray, Mar-
tin and Dorhout [15], and Deng and Ibers recently published
their results on CsYbSe2 [16].
r(Yb3ϩ) ϭ 0.868 and r(Cs1ϩ) ϭ 1.67 A [19]). These obser-
˚
vations indicate a rough relation between chemical compo-
sition, atomic volume and possible crystal structures. A
similar trend should be observed for homologous chalcog-
enides ABQ2 composed of the same A and B cation re-
sulting in an identical ratio R of the ionic radii r but con-
taining different Q atoms (Q ϭ S, Se or Te). For instance,
the substitution of Se for S is accompanied by a reduction
of the partial molar volume and this may be viewed as an
increase of the so-called “chemical pressure“. This could
induce a slight distortion necessary for stabilising the β-
RbScO2 type structure. In this context application of exter-
nal pressure seems also likely to have the same influence.
To learn more about this hypothesis we selected the hom-
ologous compounds TlScQ2 (Q ϭ S, Se, Te) having a very
similar value of R ϭ 0.5 (with r(Sc3ϩ) ϭ 0.745 and
Among these over hundred compounds the most com-
monly adopted structure is the α-NaFeO2 type. Some mate-
rials with A ϭ Li or Na [1Ϫ3, 5] exhibit disordered struc-
tures which adopt the rock-salt type. An additional
exception is NaPrTe2 [17], which crystallizes in a new cubic
form. A few sulfides like CsLnS2 are reported in a hexag-
r(Tl1ϩ) ϭ 1.5 A [19]) as CsYbSe2. Here we report on the
˚
preparation, crystal structures, physical properties and elec-
tronic band structure of the series TlScQ2 (Q ϭ S, Se, Te).
Experimental Section
a) Preparation
Starting materials were the elements Tl (99.99 % Johnson Mat-
they), Sc (Stanford Materials Corporation 99.999 %), S (sublimed),
Se (Riedel-De Hae¨n 99.5 %) and Te (Fluka 99.7 %). Tl2S was pre-
pared by decomposition of Tl2CS3 as described elsewhere [20].
Tl2Se and Tl2Te were synthesized from the elements in sealed glass
* Prof. Dr. W. Bensch
Inst. f. Anorg. Chemie der Universität
Otto-Hahn-Platz 6/7
D-24098 Kiel
E-mail: wbensch@ac.uni-kiel.de
Z. Anorg. Allg. Chem. 2008, 634, 445Ϫ451
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
445

C. L. Teske, W. Bensch, S. Mankovsky, H. Ebert
ampoules (DURAN®) under reduced Ar-pressure (p Х 8 mbar) in
the temperature rage of 200 to 500 °C. The ternary compounds
TlScQ2 (with Q ϭ S, Se and Te) were prepared by solid-state-reac-
tion from stoichiometric mixtures of Tl2Q and the elements in a
sequence of several steps always in sealed ampoules under reduced
Ar-pressure.
1. The mixtures were heated slowly (3 K/min) to T ഠ 500 °C and
the temperature kept for 18 to 24 h. After the heat treatment the
initial melt of Tl chalcogenides had solidified and the ampoules
(DURAN®) were taken out of the furnace.
2. The samples were homogenized and heated again at T ഠ 530 °C
(in DURAN®) for at least one day.
3. Progress of reaction was controlled by X-Ray powder diffraction
˚
(SIEMENS D5000 diffractometer, Cu Kα1 λ ϭ 1.54056 A). Step 2
was repeated until no reflections of the starting materials could be
detected in the patterns (Fig 1.) Finally micro-crystalline powders
of TlScS2 (dark brown), TlScSe2 (black) and TlScTe2 (black) were
obtained. The samples are stable in dry air for a long period.
4. Single crystals for X-Ray structure investigation were grown in
sealed silica glass ampoules in Ar atmosphere during a heating per-
iod of at last one week in the temperature range of T ഠ 800 to
840 °C without any surplus of chalcogen.
An excess of chalcogen might lead to decomposition at these elev-
ated temperatures, which was quite obvious for TlScS2 where Tl2Sx
condensed at the colder parts of the ampoule and yellow-orange
Sc2S3 was detected in the residue of the sample.
b) Electrical conductivity
For electrical measurements sintered (argon atmosphere, T about
470 °C, 3 d) pellets of polycrystalline TlScQ3 (diameter ഠ 8 mm,
thickness ഠ 2 mm) were used. Contacts were made with silver con-
ducting paste. The resistance was recorded in the temperature range
20 K Յ T Յ 300 K applying the four-probe method in the dc
mode [21].
Fig. 1 X-ray powder diffraction patterns of TlScQ2 (Q ϭ S, Se
and Te). (a) Circles observed intensities, line calculated intensities,
(b) peak positions, (c) Iobs-Icalc
.
c) Single crystal X-ray investigations
stadt) was employed. The spectra were transformed into absorption
spectra through the Kubelka-Munk function α/S ϭ (1-R)2/2R,
(α ϭ absorption coefficient, R ϭ reflectance with given wave length
and S ϭ scattering coefficient) [25]. The optical band gap was esti-
mated from the intersection point between the abscissa and the
tangent to the linear part of the absorption edge in the plot α/S ϭ
f(Eg). BaSO4 powder was used as white standard.
The intensity data for single crystal X-ray investigations were col-
lected on a STOE AED II diffractometer and the raw data were
corrected for Lorentz and polarization effects. An absorption cor-
rection was applied using X-Red [22] and X-Shape [23]. The refine-
ments were done against F2 employing SHELXL-97 [24]. Technical
details of the data acquisition and some selected results of the re-
finements are summarized in Table 1. All atoms were refined with
anisotropic displacement parameters. The final atomic co-ordinates
as well as the equivalent isotropic displacement parameters are
listed in Table 2. Table 3 shows a comparison of the shortest in-
e) Band structure calculations
To get a more detailed insight into the electronic structure of the
investigated systems, additional calculations have been performed
within the local-density approximation to the density functional
theory using the relativistic linear-muffin-tin orbital (LMTO)
method in the atomic-sphere approximation (ASA) [26]. The von
Barth-Hedin [27] exchange-correlation has been used. A cut-off of
lmax ϭ 3 in decomposition of the wave functions has been used.
The Brillouin zone (BZ) integrations were performed using the
tetrahedron method on a grid of 189 k-points in 1/12 (irreducible
part) of the BZ for TlScSe2 and TlScTe2 and 144 k-points in 1/24
(irreducible part) of BZ for TlScS2.
˚
teratomic distances in the range of 2.5Ϫ3.5 A. Further details of
the crystal structure investigations may be obtained from the Fach-
informationszentrum Karlsruhe D-76344 Eggenstein-Leopolds-
hafen, Germany on quoting the depository numbers CSD-418474
(QϭS), CSD-418475 (QϭSe) and CSD-418476 (QϭTe).
d) UV-Vis spectroscopy
For the investigation of the reflectance spectra an UV-VIS-NIR-
two-channel spectrometer Cary 5 (Varian Techtron Pty., Darm-
446
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© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2008, 445Ϫ451

TlScQ2 (Q ϭ S, Se and Te)
Results and Discussion
Table 1 Crystal data and structure refinement results for TlScQ2
(standard deviations in parentheses).
a) Crystal structures
Empirical formula
Formula weight
Temperature
TlScS2
313.45
TlScSe2
407.25
TlScTe2
504.53
The X-Ray investigations revealed that TlScS2 adopts the
hexagonal β-RbScO2 type structure, while the homologous
TlScSe2 and TlScTe2 crystallize in the α-NaFeO2 structure
type. In the latter the anions are densely packed and the
cations occupy the octahedral voids thus forming ordered
alternating layers stacked along [001]. The surrounding of
Sc3ϩ is only slightly distorted with all six Sc-Q distances
being identical. Interestingly, the Sc-Se bond length is
slightly larger than the sum Σr of the ionic radii [19] (Sc-
293(2) K
hexagonal
P63/mmc
3.761(3)
14.942(4)
183.0(2)
2
5.687
46.683
2.73Ϫ30.04
540
293(2) K
trigonal
293(2) K
trigonal
Crystal system
Space group
¯
¯
R3m
R3m
˚
a /A
3.9370(4)
23.194(5)
311.33(8)
3
4.2129(4)
24.099(3)
370.42(7)
3
˚
c /A
3
˚
Volume /A
Z
ρ (calcd.) / g cmϪ3
6.516
6.785
μ / mmϪ1
57.700
2.63Ϫ27.83
1152
121
0.1016
45.338
2.54Ϫ29.95
483
171
0.0699
Θ range /°
Reflections collected
Independent reflections 135
Rint
˚
˚
Se ϭ 2.728(1) A, Σr ϭ 2.725 A) whereas the corresponding
0.0519
˚
Sc-Te bond is shorter by about 0.021 A (Sc-Te
ϭ
Refinement method
Full-matrix
least-squares on F2
135 / 9
1.122
0.0308
0.0671
0.014(3)
1.823/Ϫ1.297
˚
˚
2.9341(6) A, Σr ϭ 2.955 A). The angles Q-Sc-Q in the basal
planes are in the range from 87.64(5) to 92.36(5)° (Q ϭ
Se) and 88.24(2) to 91.76(2)° (Q ϭ Te), respectively. The
surrounding of Tlϩ is markedly more distorted. All dis-
tances are equal and significantly shorter than the sum of
Data / parameters
121 / 8
1.245
0.0483
0.1215
Ϫ
171 / 8
1.047
0.0478
0.0817
Ϫ
GOF (F2)
R1
wR2
Extinction coefficient
3
˚
δF / e/A
3.203/Ϫ2.978 2.260/Ϫ1.556
˚
the ionic radii [19] (Tl-Se ϭ 3.274(2), Σr ϭ 3.48 A, Tl-Te ϭ
˚
˚
3.4000(8) A, Σr ϭ 3.71 A) indicative for a pronounced co-
valent character of the Tl-Se/Te bonds. The angles Q-Tl-Q
in the basal planes range from 73.91(4) to 106.09(4)° for
TlScSe2 and are between 76.57(2) and 103.43(2)° for
TlScTe2. The structural parameters indicate a slight in-
crease of the distortion within the homologous series
TlScQ2 (Q ϭ Te, Se) with decreasing radius of Q2Ϫ
Table 2 Atomic co-ordinates (z ϫ 104) and equivalent isotropic
displacement parameters (A ·10 ) for TlScQ2. Ueq is defined as
one third of the trace of the orthogonalized Uij tensor.
2
3
˚
In the space group P63/mmc (No. 194) the atoms occupy the positi-
¯
¯
ons: Tl (2d) 6m2, Sc (2a) 3m and S (4e) 3m and in the space group
(r(Te2Ϫ) ϭ 2.21 and r(Se2Ϫ) ϭ 1.98 A [19]). For the sulfide
˚
¯
¯
¯
R3m (No. 166) Tl (3b) 3m, Sc (3a) 3m and Se or Te (6c) 3m, respec-
the distortion around Sc3ϩ is similar small with S-Sc-S
tively (standard deviations in parentheses).
angles in the range from 86.95(7) to 93.05(7)°. The Sc-S
˚
TlScS2
bond length of 2.591(2) A is longer than the sum of the
˚
x
y
z
U(eq)
ionic radii of 2.585 A. A severe alteration as compared to
the Se/Te compounds of the environment around Tlϩ is evi-
dent leading to a trigonal prismatic coordination (Fig. 2,
left) rather than an octahedral surrounding. Again, the Tl-
S bond lengths are shorter than the sum of the ionic radii
Tl
Sc
S
1/3
0
1/3
2/3
0
2/3
1/4
1/2
5947(2)
31(1)
17(1)
17(1)
TlScSe2
x
y
z
U(eq)
of Tlϩ and S2Ϫ (Tl-S: ϭ 3.1782(2) A, Σr ϭ 3.34 A). The two
opposite triangular faces of the octahedron in the Ͱ-
NaFeO2 type being perpendicular to [001] are twisted (Fig.
2) to form the two corresponding gable faces of the prism.
Hence, the anion layers neighboring to Tlϩ are no longer
densely packed along [001] but instead a bcc-type arrange-
ment is realized to suite the space requirement of Tlϩ in a
surrounding of the smaller sulfur atoms. In this connection
the adopted β-RbScO2 type structure of TlScS2 is very
likely to be the high pressure form of the selenide and tellu-
ride.
˚
˚
Tl
Sc
Se
0
0
0
0
0
0
1/2
0
2683(1)
29(1)
16(1)
15(1)
TlScTe2
x
y
z
U(eq)
Tl
Sc
Te
0
0
0
0
0
0
1/2
0
2652(1)
34(1)
20(1)
20(1)
˚
Table 3 Shortest inter atomic distances /A for TlScQ2 (standard
deviations in parentheses).
TlScS2
Tl-S
Sc-S
b) Electrical conductivity
3.178(2) (6ϫ)
2.591(2) (6ϫ)
The electrical measurement (Fig. 3) revealed that the com-
pounds are semiconductors or poor metals, respectively.
TlScS2 behaves like a semiconductor in the temperature re-
gion between 130 and 300 K while below T ഠ 102 K the
resistivity is decreasing with decreasing temperature. This
behavior may indicate a semiconductor to metal transition
TlScSe2
Tl-Se
Sc-Se
3.2743(15) (6ϫ)
2.7282(11) (6ϫ)
TlScTe2
Tl-Te
Sc-Te
3.4000(8) (6ϫ)
2.9341(6) (6ϫ)
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447

C. L. Teske, W. Bensch, S. Mankovsky, H. Ebert
Fig. 2 Crystal structure of TlScS2 (β-RbScO2 type) left and
TlScSe2 or TlScTe2, respectively (Ͱ-NaFeO2 type) right.
in the low temperature region. The selenide and the tellu-
ride behave different with a resistivity of the latter being
about two magnitudes lower than that of the former com-
pound. But the temperature dependence of the resistivity of
the selenide and the telluride is very similar showing hardly
any change of the resistivity with decreasing temperature in
the range from 300 to 12 K. There is a very slight increase
of the resistivity at low temperatures to be seen better for
the telluride (Fig. 3).
Fig. 3 Resistivity as a function of temperature for TlScQ2 (Q ϭ
S, Se and Te).
c) Electronic band structure
become narrower going from S to Te. In the Te compound
this band is separated from the rest of the VB, i.e., it is a
semi-core band. From a chemical point of view the Tl 6s2
electron lone-pair is part of the VB in TlScS2 and TlScSe2
whereas in the Te compound the lone-pair is fully sepa-
rated.
In the conduction band (CB) Sc d states as well as Tl p
states are the main components with a minor contribution
from Tl d states. The DOS of Tl based states in the VB and
CB exhibits a more structured shape for TlScS2 than for the
other two compounds which may be due to the more coval-
ent character of the bonds in the latter two chalcogenides.
The DOS at the Fermi energy (EF) in TlScTe2 comes from
Te based states and Sc d states. One explanation may be
that Te atoms with the lowest electronegativity donate elec-
trons to empty Sc d states.
A detailed picture about the bonding properties is gained
from the energy dispersion relation E(k). Figure 5 shows
the electron energy band structure of the ground states of
the compounds. For all compounds the bands exhibit
strong dispersions along distinct high symmetry directions
reflecting strong bonding interactions between the atoms
along these directions. For TlScS2 the top of the VB is lo-
cated at the Γ point whereas the bottom of the CB is at the
M point, i.e., the compound is an indirect semiconductor.
The total as well as component and angular momentum
resolved density of states (DOS) curves for the three com-
pounds are displayed in Figure 4. As can be seen from the
drawings the sulfur and selenium compounds are semi-
conductors with a very small band gap or they are poor
metals, whereas the tellurium compound is calculated to be
a metal, but conductivity or spectroscopic investigations
suggest that it is a semiconductor. The values for the optical
band gaps estimated from reflectance spectra are 1.51 eV
for TlScS2 and 0.79 eV and 0.85 eV for TlScSe2 and
TlScTe2, respectively. For all three compounds the valence
band (VB) between 0 and Ϫ5 eV is dominated by hy-
bridized chalcogen p and Sc d states. An admixture of small
contributions of Tl based s, p, and d states is also seen in
this region. Within an ionic picture one would expect that
Sc based states are empty but according to the calculations
there is relatively large contribution of Sc d states to the
DOS in the VB. The largest contribution in the VB shifts
slightly to higher energies from about Ϫ3 eV for TlScS2 to
Ϫ2.3 eV for TlSTe2 in agreement with the alteration of the
electronegativity of the chalcogen atoms going from S to
Te. In addition, at the bottom of the VB around Ϫ6 eV
mainly Tl s state contributions are located. Interestingly,
these Tl s states shift to slightly lower energy values and
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TlScQ2 (Q ϭ S, Se and Te)
Fig. 4 The density of states curves for TlScQ2
(top: Q ϭ S; middle: Q ϭ Se; bottom: Q ϭ Te).
Parts of the bands along Γ-M, Γ-K and Γ-A are just below
EF , whereas all other bands are lower in energy. A pro-
nounced dispersion of the bands along Γ-M and Γ-K gives
clear evidence for strong bonding interactions for the atoms
within the a1-a2 plane. For the Se compound the top of the
VB is located at the Z point and the bottom of the CB at
UN. Hence, this compound is also an indirect semicon-
ductor. Strong dispersions are found along Γ-Z and Z-UN
which is caused by strong bonding interactions, especially
between Sc and Se. Finally, the dispersion relation for
TlScTe2 is very similar to that of the Se compound. The
main difference is that the bands along Γ-Z and Z-UN now
cross the Fermi level, i.e., the calculations suggest that this
compound should be a metal.
Z. Anorg. Allg. Chem. 2008, 445Ϫ451
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449

C. L. Teske, W. Bensch, S. Mankovsky, H. Ebert
S the β-RbScO2 type structure is adopted, while the selenide
and the telluride crystallize in the α-NaFeO2 type. This ob-
servation is explained applying the rough relation between
chemical composition, atomic volume and possible crystal
structures. Hence, the substitution of Se or Te for S is ac-
companied by a reduction of the partial molar volume and
this may be viewed as an increase of the so-called “chemical
pressure“. The latter may be responsible for a distortion of
the polyhedron around Tlϩ and for stabilising the β-
RbScO2 type structure. The physical measurements indicate
that the new compounds are semiconductors or poor met-
als. According to the electronic band structure calculations
TlScS2 and TlScSe2 are semiconductors with small band
gaps whereas the telluride should be a metal. The contradic-
tion between the experimental findings and the results of
the theoretical work is not surprising, because it is well
known that DFT based methods underestimate the value
for the electronic band gap. One should also keep in mind
that the resistivity measurements were performed on
pressed pellets and grain boundary effects may affect the
experimental values. Nevertheless, the results of the band
structure calculations yield a detailed picture of the bond-
ing situation. As expected the covalent character of the
bonds increases from S to Te, being in accordance with the
decreasing electronegativity going from S via Se to Te. In
the same direction the metallic character of the homologous
compounds increases. It is a challenging task to prove ex-
perimentally that TlScSe2 and TlScTe2 can adopt the β-
RbScO2 type structure under high pressure conditions as
suggested by the results of the present work. If this tran-
sition can be experimentally realized it is of great interest
to learn more about the pressure dependent behavior of the
electronic states applying appropriate band structure calcu-
lations. The temperature dependence of the resistivity of
TlScS2 requires low temperature X-ray diffraction experi-
ments which may show whether the transition from typical
semiconductor to a metallic like behavior is accompanied
by a structural phase transition.
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Conclusions
The homologous series TlScQ2 (Qϭ S, Se and Te) was pre-
pared and the crystal structures were determined. For Q ϭ
450
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TlScQ2 (Q ϭ S, Se and Te)
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