Ternary lanthanum sulfide selenides α-LaS2-xSex (022- (X=S, Se)

Article abstract of DOI:10.1016/j.jssc.2011.10.039

Mixed lanthanum sulfide selenides LaS_2-xSe_x (02-xSe_x compounds crystallize in space group P2 ₁/a, no. 14, and adopt the α-LnS₂ (Ln=Y, LaLu) structure type with a pronounced site preference for the chalcogen atoms. The mixed chalcogenides form a complete miscible series with lattice parameters a=820849 pm, b=413425 pm and c=822857 pm (β≈90°) following Vegard's rule. Raman signals indicate the presence of mixed X₂^2- dianions, a species rarely evidenced in literature, besides the well known anions S₂^2- and Se₂^2-. The band gaps of the LaS_2-xSe_x compounds, determined by optical spectroscopy, decrease nearly linearly with increasing amount of selenium.

Full text of DOI:10.1016/j.jssc.2011.10.039

Journal of Solid State Chemistry 185 (2012) 101–106
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Ternary lanthanum sulfide selenides
a-LaS2 Se (0oxo2) with mixed
ꢀx
x
2
ꢀ
dichalcogenide anions X
(X¼S, Se)
2
n
Christian Bartsch, Thomas Doert
Department of Chemistry and Food Chemistry, Technische Universit a¨ t Dresden, D-01062 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Mixed lanthanum sulfide selenides LaS2 Se (0oxo2) were obtained by metathesis reactions starting
ꢀx
x
Received 25 July 2011
Received in revised form
from anhydrous lanthanum chloride and alkali metal polychalcogenides. The LaS
ꢀx x
Se compounds
2
crystallize in space group P2
pronounced site preference for the chalcogen atoms. The mixed chalcogenides form a complete
miscible series with lattice parameters a¼820–849 pm, b¼413–425 pm and c¼822–857 pm ( E901)
dianions, a species rarely
. The band gaps of the LaS2ꢀxSe
x
1
/a, no. 14, and adopt the
a-LnS
2
(Ln¼Y, La–Lu) structure type with a
1
9 October 2011
Accepted 24 October 2011
Available online 10 November 2011
b
2
2
ꢀ
following Vegard’s rule. Raman signals indicate the presence of mixed X
Keywords:
2ꢀ
2ꢀ
evidenced in literature, besides the well known anions S
2
and Se
2
Polychalcogenides
Rare earth metal
Crystal structure
Raman spectra
compounds, determined by optical spectroscopy, decrease nearly linearly with increasing amount of
selenium.
&
2011 Elsevier Inc. All rights reserved.
1
. Introduction
We were interested in investigating mixed polychalcogenides
of the trivalent rare earth metals in order to check if a fine-tuning
of the optical and electronic properties can be achieved in
substitution series. The results for the compounds of the ternary
Rare earth polychalcogenides LnX2ꢀ
dr0.3) are in the focus of scientists for decades because of
d
(Ln¼Y, La–Lu, X¼S, Se,
Te;
their interesting structural, optical and electrical properties. They
are, e. g., discussed for applications as far-infrared transmitting
materials, lasers, phosphors, scintillators, and magneto-optical
devices [1]. The optical and electrical properties vary with the
lanthanum dichalcogenide series LaS2ꢀxSe (0oxo2) are pre-
x
sented in the following.
2. Experimental
constituting elements as well as with the chalcogen deficiency
2–4]. All binary polysulfides and polyselenides of trivalent rare
earth metals investigated so far are semiconductors [3,4].
d
[
2.1. Synthesis
Detailed structure investigations revealed that the aforemen-
tioned polychalcogenides crystallize in superstructures of the
tetragonal ZrSSi type (space group P4/nmm, aE400 pm,
All manipulations with the starting reagents were carried out
under purified argon (Air Liquide, 99.999%). Similar to binary rare
earth metal disulfides [5], the ternary LaS2ꢀxSe compounds
x
bE400 pm, cE800 pm). The non-deficient disulfides LnS
diselenides LnSe either adopt a twofold superstructure, the
monoclinic -LnS – sometimes also called CeSe type – (P2 /a,
aE800 pm, bE400 pm, cE800 pm, E901) or the orthorhombic
-LaS type (Pnma, aE800 pm, bE1600 pm, cE400 pm). Dif-
ferent other kinds of superstructures are formed for compounds
LnX2ꢀ
2
and
(
0oxo2) can be obtained by metathesis reactions of adequate
2
3
amounts of anhydrous lanthanum chloride (LaCl : 99.9% Strem
a
2
2
1
Chemicals GmbH, Kehl, Germany) and the alkaline metal poly-
b
1
chalcogenides Na
tion scheme:
2 2 2 3
S and K Se according to the following reac-
b
2
d
with 0o
d
r0.3 due to the ordering of vacancies in the
LaCl
3
þð1:5ꢀ0:75xÞNa þð0:75xÞK
2
S
2
2 3
Se
chalcogen layers [4c–e].
-LaS Se þ3Na K0:5xClþð120:5xÞSþ1:25xSe:
2
ꢀx
x
ð1ꢀ0:5xÞ
K
2
Se
amounts of potassium (ingot, 99.99%, Chempur, Karlsruhe,
Germany) with selenium in liquid ammonia at 210 K; Na was
obtained by reaction of sodium (99.9%, Fluka), H S (99.9%,
3
has been prepared by reaction of stoichiometric
2 2
S
n
Corresponding author. Fax: þ49 351 463 37287.
2
E-mail address: thomas.doert@chemie.tu-dresden.de (T. Doert).
1
Messer-Griesheim) and sulfur in ethanol (99%, Merck; dried with
sodium and diethyl terephthalate prior to use). The reaction
mixtures were filled in glassy carbon crucibles which were loaded
2 2
b-LaS is a stacking variant of the a-LnS type and not a superstructure of
ZrSSi in the strict sense of group theory, i.e. there is no group-subgroup relation
between the structures of ZrSSi and -LnS
b
2
.
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.10.039

1
02
C. Bartsch, T. Doert / Journal of Solid State Chemistry 185 (2012) 101–106
in silica ampoules. The ampoules were flame sealed and placed in
a chamber furnace which was then heated up to 850 1C with a
heating ratio of 2 K/min. After one week the samples were cooled
down to room temperature and rinsed with a 1:1 mixture of
ethanol and water. The products consist of plate-like, lustrous
crystals which show a red to black coloring in accordance with
the respective selenium amount. Mixed lanthanum sulfide sele-
nides can also be obtained by flux reactions starting from the
elements in alkaline metal halides, like NaCl, using the same
temperature program. Following both reaction routes, a slight
excess of the respective chalcogenides is beneficial in order to
avoid the formation of chalcogen deficient phases, above all
LnX1.9. Red selenium which is formed as one by-product dissolves
in the ethanol/water mixture and can thus be easily removed
from the target product. Sulfur, the other by-product, is not
visible in optical microscopy and in X-ray powder diagrams. It
probably deposits amorphously upon cooling the reaction mix-
tures and is rinsed off during the lavation process of the products.
white standard DIN 5033) as standard. Measurements were per-
formed in the range from 350 nm to 900 nm with a Varian Praying
Mantis device for diffuse reflectometry. The band gap was deter-
mined as the onset-wave-length at the absorption edge.
2.6. Electron microscopy
Transmission electron microscopy (TEM) and electron diffrac-
tion investigations were performed using a FEI Tecnai 10 electron
microscope (FEI company, Eindhoven, The Netherlands) with a
LaB -source at 100 kV acceleration voltage and at a camera length
6
of 380 mm. Images were recorded with a Tietz slow scan CCD
F224HD TVIPS camera (2k ꢁ 2k pixels, pixel size 24
mm, digitiza-
tion 16 bit) with an active area of 49 mm ꢁ 49 mm (Tietz Video
and Image Processing Systems GmbH, Gauting, Germany). The
analyses of the TEM images were realized by means of the Digital
Micrograph software (Gatan, USA).
3. Results and discussion
2.2. Phase analyses
3.1. Phase analyses
Phase purity of the reaction products was checked by X-ray
a
1
powder diffraction (Panalytical, X’Pert PRO, CuK radiation,
According to the powder X-ray data, single phase samples were
Bragg-Brentano setup). Using silicon as internal standard and
the LeBail method [6] the lattice parameters of all single phase
samples were determined by profile fitting with the program
obtained from the metathesis reactions. The refined lattice para-
meters increase with increasing amount of selenium in accordance
to Vegard’s rule, Fig. 1. Despite the observed site preference of the
chalcogen atoms (vide infra) no hints for the formation of an ordered
superstructure, at the composition LaSSe, e. g., are observed in X-ray
and TEM investigations (cf. Supplementary information).
J
ANA2006 [7].
2.3. Chemical analyses
The sulfur and selenium contents of selected single phase
samples were independently determined via ICP-OES analysis.
Approximately 10 mg of the substances were molten into 0.7 mm
capillaries, which were cracked in an autoclave with polytetra-
3.2. Chemical analyses
The results of the chemical analyses via ICP-OES are in reason-
able agreement with the initial element ratios as well as with the
results from structure refinement on single crystal data, cf.
Table 1.
3
fluoroethylene (PTFE) inlay and digested using a 4 M HNO /HCl
solution. The standard reference solutions for the rare-earth
metals and sulfur were taken from Johnson Matthey.
2.4. Single crystal structure determination
3.3. Crystal structure
Complete X-ray data sets of suitable crystals were recorded on
The mixed lanthanum sulfide selenides LaS2 Sex (0oxo2)
ꢀx
image plate diffractometers (IPDS 1 or IPDS 2, Stoe & Cie., Darm-
stadt, Germany) at ambient temperature. LP corrections were
performed with the program X-Red32 [8]. Numerical absorption
corrections were applied using the refined crystal shape [8,9]. One
data set was collected on a Bruker Apex2 CCD diffractometer, again
at ambient temperature. LP corrections were performed with the
2 2
crystallize in the a-LnS (or CeSe ) structure type in space group
P2 /a (no. 14; non-standard setting of P2 /c) which is adopted by
1
1
the majority of the binary disulfides and all known diselenides of
the trivalent rare earth metals [4,13]. The structure can be
described as a twofold superstructure of the aristotype ZrSSi by
APEX SUITE software [10] and a multi-scan absorption correction was
applied in this case [11]. Structure refinements were performed
with SHELX-97 [12] or JANA2006 program packages [7]. The occupancy
factors of the mixed occupied chalcogen positions X1 and X2 were
coupled to 1 as no indications for deficiencies were obtained from
chemical analyses. Further crystallographic information can be
obtained from the FIZ-Karlsruhe (http://www.fiz-karlsruhe.de/obtai
ning_crystal_structure_data.html) on quoting the depository num-
bers stated in Table 2. As for the binary
2 2
a-LnS and LnSe com-
pounds, all investigated crystals of the mixed series were found to
be twinned [4a,5b,13].
2.5. Optical spectroscopy
Raman spectra were recorded on single crystals with a
Renishaw RM 2000 Raman microscope equipped with a 633 nm
He/Ne Laser in the range from 100 nm to 500 nm.
The optical band gaps were determined by UV/Vis spectroscopy
with a Varian Cary 4000 on powdered samples using BaSO
4
(Merck,
x
Fig. 1. Vegard plot of the lattice parameters of the LaS2ꢀxSe samples.

C. Bartsch, T. Doert / Journal of Solid State Chemistry 185 (2012) 101–106
103
following a three stepped symmetry reduction from P4/nmm to
P2 /a [13d]. The results of the structure analyses are briefly
summarized in Table 2 and the atomic site parameters are given
in Table 3 for LaS1.47(1)Se0.53(1) as one example. As already stated,
no indications for the formation of a superstructure were
observed from the experimental data.
are found: at zE0.63 for the X1 atoms of the double slab and at
zE0 for the X2 atoms of the planar layer. Assuming mixed
occupied positions for X1 and X2 and the usual constrains for
site and displacement parameters and a coupling of the site
occupancy factors s.o.f.(Se)¼1–s.o.f.(S) the refinements proceed
smoothly to the results stated in Table 2.
1
According to these refinements, the structure can be described
as an alternating stacking of a puckered double slab of the
The metal ions in this structure are surrounded by four X2
chalcogen atoms of the puckered double slab and four X1
chalcogen atoms of three different X2 anions in a capped slightly
distorted square antiprism. As expected, the average Ln–X1,
the Ln–X2 and the X2–X2 distances increase with increasing Se
amount of the compound. However, the average distances Ln–X1
are slightly shorter than the Ln–X2 distances. In the investigated
þ
2ꢀ
composition [LaX] and a planar polychalcogenide layer, consist-
2
ꢀ
ing of a herringbone type pattern of dichalcogenide anions X
,
2
þ
2ꢀ
Fig. 2. The structural formula can be written as ([LaX]
)
2
[X ].
2
Accordingly, two crystallographically independent chalcogen sites
x
series of mixed lanthanum sulfide selenides LaS2ꢀxSe , these two
Table 1
different chalcogen positions are mixed occupied by sulfur and
selenium with a pronounced site preference. In accordance to
its higher electrophilic behavior the sulfur atoms favor the X1
position which is closer to the lanthanum site. In contrast the X2
positions in the planar layer are favored by the selenium atoms.
Due to this preferential occupation a characteristic elemental
distribution across the whole range of the solid solution is
observed, Fig. 3. However, in a complete miscible series without
Chalcogen amounts of selected compounds as determined by X-ray structure
refinement and ICP-OES.
Starting composition
Sulfur amount
Selenium amount
X-ray
ICP-OES
X-ray
ICP-OES
LaS1.5Se0.5
LaS1.4Se0.6
LaS1.3Se0.7
LaS1.2Se0.8
LaSSe
1.47(1)
1.42(1)
1.29(1)
1.08(2)
0.94(4)
0.73(2)
0.29(1)
1.47(2)
1.38(4)
1.26(1)
1.13(1)
0.98(2)
0.75(2)
0.28(1)
0.53(1)
0.58(1)
0.71(1)
0.92(2)
1.06(4)
1.27(2)
1.71(1)
0.56(1)
0.62(1)
0.74(1)
0.93(1)
1.03(2)
1.28(3)
2
2
ꢀ
rigorous S/Se ordering, mixed ðS12ySe
y
Þ
dianions with 0oyo1
should occur. The increasing X2–X2 distances with increasing Se
amount of the compounds can be taken as one hint for these
mixed dianions, cf. Fig. 4.
LaS0.6Se1.4
LaS0.2Se1.8
a
1.72
Considering the quite different sizes of sulfur and selenium, the
question arises, if a structure model with two distinct positions for S
a
Se amount re-calculated according to x(Se)¼2ꢀx(S).
Table 2
Crystallographic data of mixed lanthanum sulfide selenides
a
-LaS2ꢀxSe
x
(0rxr2).
Density/gcm
–
3
Compound
CSD-no. Lattice parameters
Unique
Residual factors/%
Goodness Resid. el. density
/e ꢁ 10ꢀ6 pm3
ꢀ
reflections/
parameters
of fit
of averaging
for all data
wR
a/pm
b/pm
c/pm
b/1
R
s
R
int
R
1
2
Min.
Max.
LaS1.83Se0.17
LaS1.80Se0.20
LaS1.47Se0.53
LaS1.42Se0.58
LaS1.29Se0.71
LaS1.08Se0.92
LaS1.02Se0.98
LaS0.94Se1.06
LaS0.73Se1.27
LaS0.57Se1.43
LaS0.54Se1.46
LaS0.44Se1.56
LaS0.37Se1.63
LaS0.29Se1.71
LaS0.23Se1.77
LaS0.21Se1.79
LaS0.14Se1.86
LaS0.08Se1.92
423,307 820.23(2) 413.12(1) 821.82(1) 90.05(1) 5.04
423,308 820.76(1) 413.44(1) 822.59(1) 90.06(1) 5.05
423,309 825.85(1) 414.52(1) 828.40(1) 90.06(1) 5.33
423,310 829.00(6) 413.97(3) 831.84(6) 90.22(1) 5.35
423,311 830.16(2) 415.79(1) 832.67(7) 90.08(1) 5.46
423,312 833.37(2) 416.91(2) 836.10(1) 90.13(1) 5.63
423,313 835.24(2) 417.67(1) 837.64(1) 90.13(1) 5.66
423,314 835.72(2) 417.99(2) 838.55(2) 90.12(1) 5.73
423,315 838.68(2) 419.90(1) 842.44(1) 90.21(1) 5.88
423,316 842.01(2) 420.70(1) 845.55(1) 90.14(1) 5.99
423,317 841.60(7) 420.98(4) 845.65(2) 90.14(1) 6.02
423,318 845.91(6) 421.85(2) 849.02(2) 90.29(1) 6.05
704/32
1209/32
477/32
478/30
531/32
536/32
1249/32
373/30
1071/34
1437/32
1673/34
623/31
1332/34
1151/32
1897/31
928/32
859/33
661/31
3.48
5.25
1.11
1.50
1.71
3.60
4.47
4.44
1.22
3.61
1.74
4.00
1.16
2.27
2.92
1.43
1.27
3.75
5.32
8.84
5.43
2.33
2.51
4.57
6.84
5.96
7.36
5.66
4.93
7.59
9.21
3.00
2.93
2.92
4.19
5.48
3.00 8.38 2.31
4.35 6.82 1.24
2.18 5.11 1.18
2.31 3.55 1.45
1.93 4.33 1.12
3.95 5.30 1.20
4.19 5.89 1.31
4.59 7.29 1.14
3.51 8.12 2.55
4.26 5.38 1.12
4.06 7.95 2.41
3.34 5.64 1.18
4.51 9.04 3.05
2.44 5.11 1.09
2.92 4.09 0.94
4.07 6.30 1.90
3.39 6.72 2.20
3.74 7.61 1.08
ꢀ1.31
ꢀ2.77
ꢀ0.61
ꢀ1.00
ꢀ0.90
ꢀ1.58
ꢀ2.18
ꢀ1.57
ꢀ1.88
ꢀ1.60
ꢀ4.76
ꢀ2.35
ꢀ3.20
ꢀ1.70
ꢀ1.08
ꢀ0.98
ꢀ2.74
ꢀ1.40
0.96
2.97
0.47
1.00
0.73
1.52
2.39
1.19
1.54
1.66
2.00
1.13
2.81
1.14
1.32
1.06
2.24
1.47
a
423,319 844.5(2)
422.25(6) 848.0(2)
90.06(1) 6.14
423,320 847.70(3) 423.53(1) 852.17(1) 90.15(1) 6.14
423,321 846.12(3) 423.46(1) 852.27(3) 90.10(1) 6.22
a
a
a
a
423,322 845.3(2)
423,323 849.4(3)
423,324 848.7(1)
424.23(7) 852.9(2)
425.1(2) 852.3(3)
424.78(5) 857.2(2)
90.24(2) 6.23
90.03(3) 6.26
90.04(1) 6.30
a
Lattice parameters from single crystal refinement.
Table 3
Site, occupancy and displacement parameters (10 pm ) for LaS1.47(1)Se0.53(1) as an example; structure model with mixed S/Se positions.
4
2
Atom
site
x
y
z
occ.
U
11
U
22
U
33
La1
S1
4e
4e
4e
4e
4e
0.1285(1)
0.1249(4)
0.1249(4)
0.3865(2)
0.3865(2)
0.2151(1)
0.2398(4)
0.2398(4)
0.1704(3)
0.1704(3)
0.2802(1)
0.6351(2)
0.6351(2)
0.9979(2)
0.9979(2)
1
69(3)
67(10)
67(10)
97(7)
77(3)
74(10)
74(10)
120(8)
120(8)
93(3)
88(10)
88(10)
78(5)
0.945(8)
0.055(8)
0.529(9)
0.471(9)
Se1
S2
Se2
97(7)
78(5)

1
04
C. Bartsch, T. Doert / Journal of Solid State Chemistry 185 (2012) 101–106
Fig. 2. Crystal structure of LaS1.47(1)Se0.53(1), the Se amount of the mixed occupied
positions is given in the upper panel, the distances in the dianion in the lower
panel; atoms are shown on a 99% probability level.
ꢀ
6
3
Fig. 5. Fourier map (Fobs, top, contour lines in steps of 5e /10 pm ) and Fourier
ꢀ
6
3
difference map (Fobs–Fcalc, bottom, contour lines in steps of 0.2e /10 pm ) for
LaS1.47(1)Se0.53(1) for z¼0, negative values are indicated by dashed lines.
and Se in the puckered double slab and/or in the planar layer can be
2
2
ꢀ
2ꢀ
refined. There is no any evidence for an ordering into S and Se2
anions, e.g., from the respective Fourier and Fourier difference maps
as can be seen in Fig. 5 for LaS1.47(1)Se0.53(1) as an example. In some
structure models, however, somewhat elongated displacement
parameters may be taken as a hint for such an S/Se ordering. A free
refinement of such a model with S/Se ordering is not possible as it
suffers from large correlations, resulting in large standard deviations
and physically meaningless displacement parameters, e.g. Never-
theless, such a model can be constructed and refined using con-
strained isotropic displacement parameters. In doing so, the residual
values and the electron density hardly change (cf. Supplementary
information for the example of LaS1.47(1)Se0.53(1)). The result for a
calculation with distinct S and Se sites in the planar chalcogen layer
is shown in Table 4 and Fig. 6 for the example of LaS1.47(1)Se0.53(1)
again.
Fig. 3. Occupation of the two different chalcogen positions as function of the
selenium content.
The bond distances within the dichalcogenide anions are then
computed to 215.3(17) pm for the disulfide, 243.0(8) pm for the
2
2
ꢀ
diselenide and 229.1(13) pm for the mixed ðS12ySe
y
Þ
dianions,
respectively. Despite the large standard deviations, the bond
distances for the homonuclear dianions are in good agreement
with those found in binary disulfides and diselenides of the rare
earth metals [4,13]. A sketch of a possible ordering pattern in the
planar chalcogen layer is given in Fig. 6 (the number of S and Se
atoms in the sketch do not match the calculated occupancies
precisely).
3.4. Optical spectroscopy
Raman spectra were recorded to confirm of the existence of mixed
2
2
ꢀ
dichalcogenide anions X . As the respective Raman spectra of the
2ꢀ
binaries
2 2
a-LaS [3,14] and LaSe [15] are well investigated, a detailed
Fig. 4. Interatomic distances within the dichalcogenide anions X2 as function of
the selenium content.
discussion can be skipped here, the respective data are taken as

C. Bartsch, T. Doert / Journal of Solid State Chemistry 185 (2012) 101–106
105
references. The Raman signal for the Se–Se stretching mode of the
[3,14,15]. Between these two signals, an additional Raman signal is
2ꢀ
ꢀ1
Se2 dianion at about 215 cm can clearly be observed for samples
found only for the mixed chalcogenides (0oxo2) which can be
ꢀ
1
2ꢀ
with xZ0.1, Fig. 7. At 400 cm , the doublet signal for the S–S
attributed to the stretching mode of a mixed ðS12ySe
y
Þ
dimer
2
2
2
ꢀ
stretching mode (S dianions) is found for samples with xr1.7.
(0oyo1). The intensity of this signal is quite pronounced for low
selenium contents. This is reasonable as the Raman intensity of a
stretching mode mainly depends on its symmetry and on the
polarizability of the atoms. A small amount of the more polarizable
Se atoms in a sulfur-rich environment should thus yield compara-
tively high Raman intensities. Consequently, the intensity of this
stretching mode decreases for higher amounts of Se and is zero in
both binaries. Moreover, the S–Se stretching mode shows a clear
wavelength shift in accordance to the selenium amount of the
These wave numbers are in good agreement with literature data
Table 4
4
2
Site, occupancy and displacement parameters (10 pm ) for LaS1.47(1)Se0.53(1) as an
example; structure model with distinct
chalcogen layer.
S and Se positions in the planar
Atom site
x
y
z
occ.
U
11
U
22
U
33
La1
S1
4e 0.1285(1) 0.2151(1) 0.2802(1)
1
67(3) 77(3) 91(3)
4e 0.1250(5) 0.2398(4) 0.6349(2) 0.940(8) 70(10) 77(10) 84(9)
4e 0.1250(5) 0.2398(4) 0.6349(2) 0.060(8) 70(10) 77(10) 84(9)
Se1
S2
a
4e 0.392(2) 0.149(2) 0.998(1) 0.525(9) 81(6)
a
Se2
4e 0.3836(8) 0.1793(9) 0.9979(5) 0.475(9) 81(6)
a
U
iso (isotropic refinement only).
Fig. 6. Sketch of a possible ordering pattern in the planar chalcogen layer of
Fig. 8. Band gap determination of LaS1.47(1)Se0.53(1) from UV/vis-spectra as
LaS1.47(1)Se0.53(1); atoms are shown on a 99% probability level.
example.
2
ꢀ
ꢀ1
2ꢀ
ꢀ1
2ꢀ
Fig. 7. Raman spectra of selected LaS2ꢀxSe
x
compounds; the ranges for the signals of the stretching modes of Se2 at E215 cm , (S1ꢀy
S
y
)
2
at E300 cm and S2 at
ꢀ
1
E400 cm
are highlighted.

1
06
C. Bartsch, T. Doert / Journal of Solid State Chemistry 185 (2012) 101–106
Dresden) for Raman and UV/Vis measurements and for practical
support, respectively. Special thanks to Dr. G. Auffermann and
Dr. P. Simon (Max Planck Institute for Chemical Physics of Solids,
Dresden) for chemical analyses and TEM investigations.
Appendix A. Supporting information
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2011.10.039.
References
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Fig. 9. Band gaps as a function of the selenium content.
[
[
[
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ꢀ
1
sample. This signal is found at about 320 cm for low Se contents
ꢀ
1
and at ca. 290 cm for higher Se amounts, Fig. 7. A similar behavior
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dianion in solid state bulk material.
Finally the optical band gaps of selected samples were deter-
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linearly with the selenium amount between 500 nm (2.48 eV for
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3] (a) I.G. Vasilyeva, in: K.A. Gschneidner Jr., L. Eyring, G.H. Lander (Eds.),
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4
. Conclusions
(
6
A complete series of crystalline mixed lanthanum sulfide selenides
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Palatinus, Prague, 2010.
LaS2ꢀxSe
x
(0rxr2) was obtained via metathesis reaction. The
compounds have been identified via powder and single crystal
X-ray data and chemical analyses. The ternary compounds adopt
[8] X-Red32, Program for data reduction and absorption correction, Stoe & Cie.,
Darmstadt, Germany, 1998.
the
a
-LnS
2
structure with a pronounced site preference of the two
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[
[
[
crystallographically independent chalcogen positions. The lattice
parameters a, b and c, and hence the unit cell volumes, increase with
the selenium content of the compounds according to Vegard’s rule.
On the other hand, the optical band gaps decrease with increasing
[
13] (a) S. B e´ nazeth, D. Carr e´ , P. Laruelle, Acta Crystallogr. B38 (1982) 33;
(
(
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Z. Kristallogr. 215 (2000) 272;
selenium content. Raman measurements evidence
a
mixed
2ꢀ
2
2
ꢀ
ðS12ySe
y
Þ
chalcogenide (0oyo1) besides the well known S2 and
2
2
ꢀ
(
(
d) Th. Doert, Ch. Graf, Z. Anorg. Allg. Chem. 631 (2005) 1101;
e) C.J. M u¨ ller, Th. Doert, U. Schwarz, Z. Kristallogr. 226 (2011) 646.
Se species.
[
[
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(
(
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Acknowledgment
2
1
This work was financially supported by the Deutsche Forschungs-
gemeinschaft (DFG, Bonn Germany). We thank Ch. Ziegler, A. Klausch,
and E. Ahrens (Department of Chemistry and Food Chemistry, TU
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Chem. Solids 57 (1996) (1625).
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