Ba3TM2Se9 (TM = Nb, Ta): Synthesis and Characterization of New Polyselenides

Ba

₃

TM

₂

Se

₉

(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

*

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 ∞

3_[BaSe

3]3.33−that incorporated TM atoms at octahedral sites 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 Ba₃Nb₂Se₉and 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.

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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. Concerning polysulfides, the S_n2−(n > 1) units exhibited 2c− 2e bonds forming one-dimensional (1D) structures. For example, the binary polysulfides Rb2S2,3 Rb2S3,4 and Rb2S55 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 Se_nx−(n =

2−6; x = 2, 4). Examples of the Se₃4− unit with an

approximately linear conformation and 3c−4e bonds were

found in the crystal structures of Rb12Nb6Se356and Ba2Ag4Se5.7 In addition, building units of an infinity chain and a network of polyselenide were discovered in La₂U2Se9

8

and Cs3Se22,

9

respectively. Polytellurides typically exhibit complex coordina-tion modes because of their secondary coordinacoordina-tion inter-actions between tellurium atoms. Tenx− units, including V-shaped∞1[Te44−] and planar∞2[Te63−] moieties, are found in the crystal structures of CsCe3Te8

10

and Cs3Te22,

11

respectively. 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.

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 contains a superstructure arising from ordered Se22‑ and Se2‑ 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.

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Syntheses. All operations on chemicals and compounds were performed in a glovebox in a nitrogen atmosphere. Chemicals (Alfa Received: June 8, 2013

Published: December 23, 2013

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 subsequentlyflame-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 Ae3TM2Q9with 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

comparisons of their calculated patterns. Ba3Nb2Se9is unstable in the

presence of O2and H2O and should be stored in a vacuum, whereas

Ba3Ta2Se9is stable in moist air for more than 1 month.

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 Al2O3crucibles. 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 Ba}

3Ta2Se9was annealed at 673 K

for 1 week before the measurement, and the resistivity data of Ba3Nb2Se9were 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 glassfibers 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 package23APEX 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.

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 Figure 1. Experimental and calculated patterns of powder X-ray

diffraction for Ba3Nb2Se9and Ba3Ta2Se9.

Table 1. Crystal Data and Structure Refinement for Compounds Ba3TM2Se9(TM = Nb, Ta)

refined composition Ba3Nb2Se9 Ba3Ta2Se9

temperature (K) 273(2) 273(2)

wavelength (Å) 0.71073 0.71073

crystal system monoclinic monoclinic

space group, Z P 21/c (14),2 P 21/c (14), 2 a (Å) 7.359(2) 7.3579(8) b (Å) 12.383(3) 12.374(1) c (Å) 17.574(4) 17.593(2) β (deg) 97.147(5) 97.359(2) V (Å3_{) 1589.0(7) 1588.6(3)}

θmin,θmax(deg) 2.02 to 25.03 2.02 to 25.11

independent reflns (Rint) 2807 (0.0376) 2825 (0.0386) obsd reflns 13794 13765 dcalcd(mg m−3) 5.47 6.207 abs coeff (mm−1) 29.302 41.631 GOF on iF2 _{0.984 1.026} R1, wR2 (I > 2σ(I)) 0.0278, 0.0565 0.0319, 0.0837 R1, wR2 (all data)a _{0.0455, 0.0620 0.0398, 0.0882} largest diff. peak and hole (e Å−3) 1.238,−1.051 1.609,−2.483

a_{R1 =Σ||F}

structure using information from densities of states (DOS), band structure,28 and crystal orbital Hamiltonian population (COHP) curves.29

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Structure Description. Ba3TM2Se9 (TM = Nb, Ta)

crystallize in a new structure type in the monoclinic space

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 Ba3Ta2Se9along 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 atomsfill one-sixth of the octahedral holes to form distorted TaSe6octahedra. These units share a face to form a [Ta2Se9]6‑ unit, which contains a Se₂2‑diselenide fragment (Figure 2). The TaSe6octahedra 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 short Se−Se distance indicates the formation of a Se₂2‑ unit. Similar Se22‑units were reported for Sr4Sn2Se931(dSe−Se= 2.46 Å). The Ta atoms are in a distorted octahedral coordination

environment caused by the short Se−Se contact and

interactions between Se₂2‑ 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−).

The crystal structure of Ba3TM2Se9 (TM = Nb, Ta) was compared to related compounds, namely, Sr_0.9La_0.1MnO₃,32 BaNb0.8Se3,33and Cs3Ho2Br934in Figure 3. These compounds contain close-packed layers of atoms with varied sequences and occupied sites for incorporated atoms. For PuAl3,356H-SiC,36 and Sr_0.9La_0.1MnO₃,32 the crystal structures of these phases contain the same HCP sequence as the title compound. Taking Sr_0.9La_0.1MnO₃32(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 with Mn atoms. The crystal structure of BaNb0.8Se333(Figure 3b) indicates that the close-packing sequence is ABAB for ∞

3_[BaSe

3]−4and the Nb atoms partiallyfill the octahedral holes (80%). The structure contains 1D chains of face-shared ∞

1_[NbSe

3]−2units (Figure 3b). The last example Cs3Ho2Br934 exhibits the same elemental ratio as Ba₃TM₂Se₉ (TM = Nb, Ta), but the crystal structure contained a packing sequence ABAB for_∞3_[CsBr

3]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 Ba₃Nb₂Se₉and

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. Ba3Nb2Se9was 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

Figure 2.Perspective view of Ba3Ta2Se9structure along the a-axis with

its ellipsoid plot of the Ta2Se9−6unit. The red bonds indicate the short

Se−Se contacts.

Figure 3.Related structures of Sr0.9La0.1MnO3(a), BaNb0.8Se3(b), and Cs3Ho2Br9(c).

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 Ba3Nb2Se9and 1.6 eV for Ba3Ta2Se9. The sharp peaks at 1.38 eV (λ = 900 nm) originated from the changes of the detectors

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

Ba₃Nb₂Se₉and 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−1for Ba₃Ta₂Se₉. In both compounds, shifts in the range of 210−250 cm−1were assigned as Se₂2− stretching vibrations. According to the published spectra of various polyselenides, the Se22−stretching vibrations are in the range of 215−266 cm−1depending on the coordination environment of the Se atom and metal atoms bonded to Se. For example, Rb2BaNb2Se11

37

and Tl4Ta2Se11

38

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−1range 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 Ba₃Ta₂Se₉to evaluate its properties and bonding characteristics. The total and partial DOS diagrams are

Figure 5.UV−vis reflectance spectra and temperature dependence of electronic resistivity of Ba3Nb2Se9(black) and Ba3Ta2Se9(red).

Figure 6.Raman spectra of Ba3Nb2Se9(black) and Ba3Ta2Se9(red).

Figure 7.Total and partial densities of states for Ba3Ta2Se9(a): crystal orbital-Hamiltonian population curves for selected Se−Se (b) and for average

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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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 contains a coordinated diselenide group Se22−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.

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*

(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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Corresponding Author

*E-mail: [email protected].

Notes

The authors declare no competingfinancial interest.

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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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