Ba3TM2Se9 (TM = Nb, Ta): Synthesis and characterization of new polyselenides

Article abstract of DOI:10.1021/ic4013763

New ternary polyselenides Ba₃TM₂Se₉ (TM = Nb, Ta) were synthesized through a solid-state reaction, and their structures were characterized using single-crystal X-ray diffraction. These compounds crystallized into a new structural type with a monoclinic space group P2 ₁/c. The structures were constructed from distorted, close-packed layers of ∞³[BaSe₃]^3.33- that incorporated TM atoms at octahedral sites and contained [(TM⁵⁺) ₂(Se^2-)₇(Se₂^2-)] units. Diffuse reflectance spectra and electronic resistivity measurements indicated semiconducting properties and optical band gaps of 1.3 eV for Ba ₃Nb₂Se₉ and 1.6 eV for Ba₃Ta ₂Se₉. Raman spectra were used to investigate the interatomic interactions. Calculations of the electronic structure verified the semiconducting behavior and bonding interaction of short Se-Se contacts.

Full text of DOI:10.1021/ic4013763

Article
pubs.acs.org/IC
Ba3TM2Se9 (TM = Nb, Ta): Synthesis and Characterization of New
Polyselenides
Ming-Yan Chung and Chi-Shen Lee*
Department of Applied Chemistry, National Chiao Tung University, 1001 University Road, Hsinchu 30010, Taiwan
S
* Supporting Information
ABSTRACT: New ternary polyselenides Ba3TM2Se9 (TM =
Nb, Ta) were synthesized through a solid-state reaction, and
their structures were characterized using single-crystal X-ray
diffraction. These compounds crystallized into a new structural
type with a monoclinic space group P21/c. The structures were
constructed from distorted, close-packed layers of
∞[BaSe3]3.33− that incorporated TM atoms at octahedral sites
3
and contained [(TM5+)2(Se2−)7(Se22−)] units. Diffuse reflec-
tance spectra and electronic resistivity measurements indicated
semiconducting properties and optical band gaps of 1.3 eV for
Ba3Nb2Se9 and 1.6 eV for Ba3Ta2Se9. Raman spectra were used
to investigate the interatomic interactions. Calculations of the electronic structure verified the semiconducting behavior and
bonding interaction of short Se−Se contacts.
Alkaline-earth polychalcogenides are less common than alkali
metal polychalcogenides, and known examples include
Sr6Sb6S17,16 Ba9Nb4S21,17 K4Ba2(Nb2S11)2,18 K2BaTa2S11,19
Ba4SiSb2Se12,20 and Ba2Sn2Te5,21 which contain coordinated
oligomeric motifs. Some polyselenides contain mixed alkali and
rare-earth metals, such as ATh2Se6 (A = K, Rb),22 which
INTRODUCTION
■
Polychalcogenides are chalcogen-rich compounds that contain
partially reduced chalcogen atoms with homonuclear chalcogen
bonds in various shapes and lengths. These materials have
received interest for decades because of their unique structural
features and potential use in applications such as thermoelectric
devices1 and ion-exchange.2 The homonuclear bonds of the
polychalcogenides contained hypervalent interactions, which
indicated the diversity of structures and coordination.
contains a superstructure arising from ordered Se2 and Se2‑
2‑
units.
Based on a review of the literature, no previous report has
focused on compounds in the ternary polychalcogenide system
Ae−TM−Q (Ae = alkaline-earth element; TM = V, Nb, Ta). In
this study, we investigated the ternary chalcogenide system in
the Ae−TM−Q system by varying the relative compositions of
alkaline-earth (Sr, Ba), early transition-metal (V, Nb, Ta), and
group-16 elements (S, Se, Te) to establish new solid-state
compounds. In our exploratory study of alkaline-earth
polychalcogenides, we prepared and characterized new
polychalcogenides of the form Ba3TM2Se9 (TM = Nb or Ta),
which feature a new structural type. Single-crystal and powder
X-ray diffraction analyses were performed to understand the
crystal structure. Measurements of diffuse reflectance and
electrical resistivity were used to determine the band gap.
Raman spectra were recorded, and the electronic structure was
calculated for these compounds to understand the structure and
bonding in the system.
2−
Concerning polysulfides, the Sn (n > 1) units exhibited 2c−
2e bonds forming one-dimensional (1D) structures. For
example, the binary polysulfides Rb2S2,3 Rb2S3,4 and Rb2S5
5
contained dumbbell, bent, and zigzag shaped anions,
respectively. For polyselenides, interactions of the types 2c−
2e and 3c−4e were observed in various 1D motifs of Senx− (n =
4−
2−6; x = 2, 4). Examples of the Se3 unit with an
approximately linear conformation and 3c−4e bonds were
found in the crystal structures of Rb12Nb6Se356 and Ba2Ag4Se5.7
In addition, building units of an infinity chain and a network of
polyselenide were discovered in La2U2Se9 and Cs3Se22,9
8
respectively. Polytellurides typically exhibit complex coordina-
tion modes because of their secondary coordination inter-
x−
actions between tellurium atoms. Ten units, including V-
shaped 1∞[Te44−] and planar ∞2 [Te36−] moieties, are found in the
crystal structures of CsCe3Te8 and Cs3Te22,11 respectively.
10
Multinary polychalcogenides typically contain one or more
electropositive elements, which might include alkali, alkaline-
earth, and rare-earth metals. Multinary alkali polychalcogenides
have been widely investigated since the development of the
reactive flux method.12−14 Examples include A4TM2S11 and
A6Nb4Q22 (A = K, Rb; TM = Nb, Ta),15 which contain
TM2Q11 units and polychalcogen units of various shapes.
EXPERIMENTS
■
Syntheses. All operations on chemicals and compounds were
performed in a glovebox in a nitrogen atmosphere. Chemicals (Alfa
Received: June 8, 2013
© XXXX American Chemical Society
A
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Aesar) were used as obtained: Sr, 99.00%, chunks; Ba, 99.00%, chunks;
Nb, 99.90%, powder; Ta, 99.50%, powder; Sb, 99.90%, powder; S,
99.5%, powder; Se, 99.95%, powder; and Te 99.99%, powder. In
typical reactions, the reaction mixtures in stoichiometric proportions
(total mass = ∼0.5 g) were placed in silica tubes, which were
subsequently flame-sealed in a vacuum. These tubes were placed in a
computer-controlled furnace for reaction. The elemental mixtures
were heated to 1023 K within 1 day, maintained for 1 day, and slowly
cooled to 673 K within 1 day. BaSe was synthesized as a precursor,
which was obtained from the stoichemical mixture of the elements Ba
(chunks) and Se (powder), annealed at 823 K for more than 1 week,
and spontaneously cooled to room temperature.
An unknown phase was initially observed in the powder X-ray
diffraction (PXRD) pattern of the reaction product Ba/Ta/Sb/Se = 4/
1/1/16.2. Suitable pieces were selected for single-crystal X-ray
diffraction (SXRD). The SXRD refinement confirmed the formula
to be Ba3Ta2Se9, featuring a new structural type. The elements Ba, Ta,
and Se were mixed in a mmol ratio of 3:2:9, heated by using the
aforementioned temperature program, and yielded a pure phase of
Ba3Ta2Se9. We also prepared analogues of Ae3TM2Q9 with Ae = Sr,
Ba; TM = Nb, Ta; Q = S, Se, Te. Characterization using PXRD, the
“Ba3Nb2Se9” reaction produced a mixture of BaSe2 and unknown
phases that exhibited a PXRD pattern similar to that of Ba3Ta2Se9. The
remaining reactions yielded known compounds, listed in Supporting
Information Table S1. After the SXRD refinement was completed, the
unknown phase was identified as Ba3Nb2Se9, which was obtained in a
pure phase of slowly heating reactive mixtures of BaSe, Nb, and Se in
the molar ratio BaSe/Nb/Se = 3:2:6 to 823 K over 1 week and
annealing for 1 month. After switching off the power of the furnace,
the product was cooled to room temperature. The PXRD patterns of
the title compounds are illustrated in Figure 1, which includes
for 1 week before the measurement, and the resistivity data of
Ba3Nb2Se9 were directly collected on the cold-pressed bar to prevent
decomposition of the compound. Five scans of the Raman spectra
(Bruker RSF-100 FT-Raman spectrometer equipped with a Nd/YAG
laser, λ = 1064 nm) on the powder samples placed in silica tubes (i.d.
= 3 mm) were performed to cover a range of 200−400 cm−1.
Crystallographic Measurements. Single crystals of Ba3Nb2Se9
and Ba3Ta2Se9 were obtained on crushing polycrystalline samples.
Suitable crystals with a metallic luster were mounted on glass fibers
with epoxy resin glue for SXRD measurement. The intensity data were
collected (Bruker X8 APEX diffractometer using graphite-mono-
chromatic Mo−Kα radiation, λ = 0.71073 Å) at 273 K, and the
distance between the sample crystal and the detector was 4.0 cm. The
2θ values cover the range from 4.04° and 50.22°. The diffraction
intensities of all frame data were used to determine the parameters of
the unit cell. The program package23 APEX 2 was used to determine
and refine the structure. Multiscan absorption corrections were applied
using the SADABS program. The structure solution provided two
possible models with space groups P21/c (14) and C2/c (15), on
which the refinement in the space group C2/c provided a smaller R
value than did that with the space group P21/c. To verify this
conclusion, a recombination of the zone images of Ba3Nb2Se9 and
Ba3Ta2Se9 are respectively illustrated in Supporting Information
Figures S1a and S1b. The diffraction signals resulting from the (h0l)
(h = 2n+1, l = 2n′) planes were present but weak in both cases, and
corresponded to the systematic absence of space group P21/c. The
direct method provided 14 unique sites: five metal and nine Se sites.
The determinations of the coordination and electronic density
revealed that three Ba sites, two TM (TM = Nb or Ta) sites, and
nine Se sites could be satisfactorily refined using reasonable isotropic
displacement parameters. After refinement of the anisotropic displace-
ment parameters for all atoms, the R-value was approximately 0.02 and
the residual electron density was less than 2 e Å−3. The SXRD
refinement results are listed in Table 1 and Supporting Information
Table S2.
Table 1. Crystal Data and Structure Refinement for
Compounds Ba3TM2Se9 (TM = Nb, Ta)
refined composition
temperature (K)
wavelength (Å)
crystal system
space group, Z
a (Å)
Ba3Nb2Se9
273(2)
Ba3Ta2Se9
273(2)
0.71073
0.71073
monoclinic
P 21/c (14),2
7.359(2)
monoclinic
P 21/c (14), 2
7.3579(8)
12.374(1)
17.593(2)
97.359(2)
1588.6(3)
2.02 to 25.11
2825 (0.0386)
13765
b (Å)
12.383(3)
17.574(4)
97.147(5)
1589.0(7)
2.02 to 25.03
2807 (0.0376)
13794
c (Å)
β (deg)
V (Å3)
Figure 1. Experimental and calculated patterns of powder X-ray
diffraction for Ba3Nb2Se9 and Ba3Ta2Se9.
θmin, θmax (deg)
independent reflns (Rint)
obsd reflns
comparisons of their calculated patterns. Ba3Nb2Se9 is unstable in the
presence of O2 and H2O and should be stored in a vacuum, whereas
Ba3Ta2Se9 is stable in moist air for more than 1 month.
d
calcd (mg m−3
)
5.47
6.207
abs coeff (mm−1
)
29.302
41.631
Characterization. PXRD (Bruker D8 Advance diffractometer
operated at 40 kV and 40 mA, Cu Kα radiation, λ = 1.5418 Å) data
were obtained in a 2θ range from 7° to 60° by using a step size of
0.016° and an exposure interval of 0.1 s/step. Differential thermal
analysis (DTA) was performed with a Netzsch STA 409PC device.
Powder samples were placed into flame-sealed quartz capsules
contained in Al2O3 crucibles. After heating to 1073 K at a rate of 10
K/min, the samples were cooled to 673 at 10 K/min under a constant
flow of N2. The diffuse reflectance was measured using a UV−visible
spectrophotometer (Jasco V-570) near 298 K, and an integrating
sphere was used to measure the spectra over a range of 200−2400 nm.
Ground powder samples were pressed onto a thin quartz slide holder,
and a BaSO4 plate was used as the reference. The electric resistivity
was measured following a standard four-probe method on cold-pressed
bars (1 × 1 × 5 mm3). The sample Ba3Ta2Se9 was annealed at 673 K
GOF on iF2
0.984
1.026
R1, wR2 (I > 2σ(I))
0.0278, 0.0565
0.0455, 0.0620
0.0319, 0.0837
0.0398, 0.0882
a
R1, wR2 (all data)
largest diff. peak and hole (e Å−3
R1 = Σ||F0 − Fc||/Σ|F0. wR2 = {Σ[w(F0 − Fc2)2]/Σ[w(F0 )2]}1/2
)
1.238, −1.051
1.609, −2.483
a
2
2
.
Calculation of Electronic Structure. Tight binding of the linear
muffin-tin orbitals (LMTO) with an atomic-sphere approximation
(ASA) was used to evaluate the electronic structure. In LMTO, we
applied the density-functional theory with local-density approximation
(LDA).24−27 The electronic structure of Ba3Ta2Se9 was calculated
using the SXRD data. A k-space integration was performed on a grid of
more than 300 independent k-points. We analyzed the electronic
B
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structure using information from densities of states (DOS), band
structure,28 and crystal orbital Hamiltonian population (COHP)
curves.29
RESULTS AND DISCUSSION
Structure Description. Ba3TM2Se9 (TM = Nb, Ta)
crystallize in a new structure type in the monoclinic space
■
Figure 4. DTA curves for Ba3Nb2Se9 (black) and Ba3Ta2Se9 (red).
The crystal structure of Ba3TM2Se9 (TM = Nb, Ta) was
compared to related compounds, namely, Sr0.9La0.1MnO3,32
Figure 2. Perspective view of Ba3Ta2Se9 structure along the a-axis with
its ellipsoid plot of the Ta2Se9−6 unit. The red bonds indicate the short
Se−Se contacts.
34
BaNb0.8Se3,33 and Cs3Ho2Br9 in Figure 3. These compounds
contain close-packed layers of atoms with varied sequences and
occupied sites for incorporated atoms. For PuAl3,35 6H-SiC,36
and Sr0.9La0.1MnO3,32 the crystal structures of these phases
contain the same HCP sequence as the title compound. Taking
Sr0.9La0.1MnO332 (Figure 3a) as an example, the atomic ratio of
metal (0.9 Sr2+ + 0.1 La3+) to oxygen atoms in the HCP layers
is 1:3. Of the interstitial octahedral sites, a quarter is occupied
group P21/c (14). The compounds comprised 14 independent
crystallographic sites: three sites for Ba, two sites for TM (TM
= Nb, Ta), and nine sites for Se. Figure 2 displays a perspective
view of the structure of Ba3Ta2Se9 along the b-axis to facilitate
discussion on its structure. The structure may be described as a
distorted hexagonal close-packed (HCP) structure comprising
Ba and Se atoms in a Ba:Se ratio of 1:3. The HCP layers are
arranged in an ABCACB sequence, and within these layers, the
Ta atoms fill one-sixth of the octahedral holes to form distorted
TaSe6 octahedra. These units share a face to form a [Ta2Se9]6‑
unit, which contains a Se22‑ diselenide fragment (Figure 2). The
TaSe6 octahedra are characterized by four short (2.39−2.55 Å)
and two long (2.74−2.94 Å) Ta−Se contacts with Se−Ta−Se
angles from 78° to 104°, and is comparable with the Ta−Se
contacts and the Se−Ta−Se angles in TlTaSe3.30 The
bioctahedral Ta2Se9 unit contains one short homonuclear
Se−Se contact with a length of 2.50 Å. Compared with the
longer Se···Se distance in a range 2.95−3.92 Å in Ba3Ta2Se9, the
33
with Mn atoms. The crystal structure of BaNb0.8Se3 (Figure
3b) indicates that the close-packing sequence is ABAB for
3
∞[BaSe3]−4 and the Nb atoms partially fill the octahedral holes
(80%). The structure contains 1D chains of face-shared
34
∞[NbSe3]−2 units (Figure 3b). The last example Cs3Ho2Br9
1
exhibits the same elemental ratio as Ba3TM2Se9 (TM = Nb,
Ta), but the crystal structure contained a packing sequence
ABAB for 3∞[CsBr3]2− and the arrangement of the edge-shared
units [Ho2Br9]3‑ differed from that of Ba3TM2Se9 (Figure 3c).
Physical Properties. We applied DTA to investigate
thermal stability, and the curves are illustrated in Figure 4.
Overall, for both compounds two endothermic signals are
observed that correspond to the melting and decomposition,
which began respectively at 788 and 845 K for Ba3Nb2Se9 and
810 and 1032 K for Ba3Ta2Se9. To understand the
decomposition, we heated the as-prepared samples to the
selected temperatures, and subsequently quenched them in a
water bath. PXRD was conducted, and the results are displayed
in Supporting Information Figures S2 and S3. Ba3Nb2Se9 was
identified after quenching at 823 K, whereas the product of the
quenching reaction at 998 K contained mixtures of BaNb0.8Se3
and BaSe2 (Supporting Information Figure S2). Similar results
2‑
short Se−Se distance indicates the formation of a Se2 unit.
31
Similar Se22‑ units were reported for Sr4Sn2Se9 (dSe−Se = 2.46
Å). The Ta atoms are in a distorted octahedral coordination
environment caused by the short Se−Se contact and
2‑
interactions between Se2 and Ta. The Ta2Se9 unit carries a
charge of −6, which is balanced by three Ba2+ cations. The Ba−
Se contacts were in the range of 3.30 to 4.37 Å, with a
coordination number of 11 or 12. The charge-balanced formula
could therefore be written as (Ba2+)3 (Ta5+)2 (Se2−)7 (Se22−).
Figure 3. Related structures of Sr0.9La0.1MnO3 (a), BaNb0.8Se3 (b), and Cs3Ho2Br9 (c).
C
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Figure 5. UV−vis reflectance spectra and temperature dependence of electronic resistivity of Ba3Nb2Se9 (black) and Ba3Ta2Se9 (red).
and grating. The electronic resistivity was measured, and the
plots of the temperature dependence of resistivity indicated that
the resistivity shows a decrease as the temperatures increases,
indicating semiconducting behavior (Figure 5b). The activation
energies for both compounds were evaluated using the
Arrhenius’ equation, and the results were 0.69 eV for
Ba3Nb2Se9 and 1.05 eV for Ba3Ta2Se9, as shown in Supporting
Information Figure S4. This trend is consistent with the
measured optical band gaps.
The Raman spectrum of the title compounds in Figure 6
shows four distinct lines at 216, 238, 263, and 276 cm−1 for
Ba3Nb2Se9, and at 224, 248, 287, and 305 cm−1 for Ba3Ta2Se9.
In both compounds, shifts in the range of 210−250 cm−1 were
2−
assigned as Se2 stretching vibrations. According to the
2−
published spectra of various polyselenides, the Se2 stretching
vibrations are in the range of 215−266 cm−1 depending on the
coordination environment of the Se atom and metal atoms
37
38
bonded to Se. For example, Rb2BaNb2Se11 and Tl4Ta2Se11
Figure 6. Raman spectra of Ba3Nb2Se9 (black) and Ba3Ta2Se9 (red).
sharing similar building units, [TM2Se11]4− (TM = Nb, Ta), but
their Raman shifts appeared at 235 and 266 cm−1 for
Rb2BaNb2Se11, and at 215, 236, 247 cm−1 for Tl4Ta2Se11.
The remaining shifts in the 250−350 cm−1 range were assigned
as vibrations of TM−Se (TM = Nb, Ta) contacts, and are
comparable to the Raman spectra of the reported com-
pounds.37
Calculations of Electronic Structure. We calculated the
electronic structure of Ba3Ta2Se9 to evaluate its properties and
bonding characteristics. The total and partial DOS diagrams are
were discovered in the Ba3Ta2Se9 (Supporting Information
Figure S3) sample, supporting our hypothesis.
We determined the optical band gaps for Ba3TM2Se9 (TM =
Nb, Ta) according to UV−vis diffuse-reflectance spectra, as
indicated in Figure 5a. The spectra demonstrated abrupt
absorptions and the measured optical band gaps were 1.3 eV for
Ba3Nb2Se9 and 1.6 eV for Ba3Ta2Se9. The sharp peaks at 1.38
eV (λ = 900 nm) originated from the changes of the detectors
Figure 7. Total and partial densities of states for Ba3Ta2Se9 (a): crystal orbital-Hamiltonian population curves for selected Se−Se (b) and for average
Ta−Se contact (c).
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plotted in Figure 7, and the corresponding band structure is
indicated in Supporting Information Figure S5. According to
the band structure, the title compound exhibits a direct band
gap ∼1.2 eV, which is also seen in the DOS curve. The ns states
and np states of all the elements contributed to the valence
band between −10 to −15 and 0 to −5 eV. Ta 5d, Ba 6s, and
Ba 4f states dominated the conduction-band states in the ranges
of 1−3, 3−10, and 10−20 eV, and minor mixing of Ta 6s and
Se 4p states occurred.
The COHP curves for the selected Se−Se and average Ta−
Se contacts are plotted in Figures 7b and 7c, respectively. The
interatomic interactions of the Ta−Se contacts indicated that
the bonding interactions were optimized at the Fermi level, but
the Se−Se contacts exhibited antibonding characteristics near
the Fermi level, which resulted from the hybridization of Se 4s
and 4p states. The interactions of the Se−Se contacts with
Se1−Se8 contained a large -ICOHP value 1.96 eV/bond
compared with the other Se−Se contacts of which -ICOHP =
∼0.32 (eV/bond), which is indicative of strong bonding
between Se1−Se8.
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CONCLUSION
■
We report the preparation and characterization of a new
polyselenide Ba3TM2Se9 (TM = Nb, Ta) that crystallizes in an
unprecedented structure. A new moiety [TM2Se9]6− that
2−
contains a coordinated diselenide group Se2 was discovered.
The semiconducting property of the as-synthesized compounds
was confirmed according to measurements of diffuse reflectance
spectra and electrical conductivity, as well as calculations of the
electronic structure. The Raman spectra and the calculated
electronic structure supported the bonding characteristics of
the Se−Se contacts in these compounds.
ASSOCIATED CONTENT
* Supporting Information
■
S
(1) X-ray crystallographic files in CIF format, (2) the
reconstructed [h0l] zone axis image of Ba3Nb2Se9 and
Ba3Ta2Se9, (3) variation of PXRD pattern with quenching
temperature for Ba3Nb2Se9, (4) variation of PXRD pattern with
quenching temperature for Ba3Nb2Se9, (5) the Arrhenius plots
for Ba3Nb2Se9 and Ba3Ta2Se9, (6) band structure in energy
window 2 eV of Ba3Ta2Se9, (7) summary of reactions and
products of Ae3TM2Q9 (Ae = Sr, Ba; TM = Nb, Ta; Q = S, Se,
Te), (8) fractional atomic coordinates and equivalent isotropic
atomic displacement parameters of Ba3TM2Se9 (TM = Nb, Ta),
and (9) select bond lengths for Ba3TM2Se9 (TM = Nb, Ta).
These materials are available free of charge via Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: chishen@mail.nctu.edu.tw.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Professor W.-G. Diau for assistance with UV diffuse
reflectance measurements and Dr. R.-J. Wu of the Industrial
Technology Research Institute of Taiwan for the use of FT-
Raman spectrum facilities. The National Science Council,
Taiwan supported this research under grant number 101-2113-
M-009-017-MY3.
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dx.doi.org/10.1021/ic4013763 | Inorg. Chem. XXXX, XXX, XXX−XXX