Synthesis and Characterization of Ba2Ag2Se2 (Se2)

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Synthesis and Characterization of Ba2Ag2Se2(Se2) Subhendu Jana,† Mohd Ishtiyak,† Adel Mesbah,‡,§ Sébastien Lebègue,⊥ Jai Prakash,†,‡ Christos D. Malliakas,‡ and James A. Ibers*,‡ †

Department of Chemistry, Indian Institute of Technology (IIT) Hyderabad, Kandi, Sangareddy, 502285 Telangana, India Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States § ICSM, UMR 5257, CEA CNRS, ENSCM, Université de Montpellier, Site de Marcoule-Bât. 426, BP 17171, 30207 Bagnols-sur-Cèze, France ⊥ Laboratoire de Physique et Chimie Théoriques (LPCT, UMR CNRS 7019), Institut Jean Barriol, Université de Lorraine, BP 239, Boulevard des Aiguillettes, Vandoeuvre-lès-Nancy 54506, France

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ABSTRACT: Single crystals and a polycrystalline sample of Ba2Ag2Se2(Se2) were synthesized by standard solid-state chemistry methods at 1173 and 973 K, respectively. The crystal structure of this ternary compound was established by single-crystal X-ray diffraction studies at 100(2) K. The superstructure of this compound is commensurate and crystallizes in the space group P21/c of a monoclinic system with cell constants of a = 6.1766(2) Å, b = 6.1788(2) Å, c = 21.5784(8) Å, and β = 90.02(1)° (Z = 4). The asymmetric unit of the superstructure comprises eight atoms occupying general positions: two Ba atoms, two Ag atoms, and four Se atoms. In this structure, each Ag atom is tetrahedrally coordinated with four adjacent Se atoms to form distorted AgSe4 units that share edges with the neighboring tetrahedra to form a two-dimensional [AgSe4/4]− layer. These layers are separated by Ba2+ and Se22− units. The presence of the Se22− unit is also supported by an intense band at around 247 cm−1 in the Raman spectrum of Ba2Ag2Se2(Se2). A density functional theory study shows that the compound is a semiconductor with a calculated band gap of 1.1 eV. As determined by UV−visible spectroscopy, the direct and indirect band gaps are 1.23(2) and 1.10(2) eV, respectively, in good agreement with the theory and consistent with the black color of the compound. A temperature-dependent resistivity study also confirms the semiconducting nature of Ba2Ag2Se2(Se2).

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INTRODUCTION The structure−property relationships in transition-metal chalcogenides have been extensively studied in the past because they offer intriguing physical properties including magnetism [e.g., BaMS3 (M = V, Nb, Ti),1 Ba2MnS3,2 Ba2MnSe3,2 AgCrSe2,3 and FeCr2S44], superconductivity (e.g., PbMo6S8,5 K0.8Fe2Se2,6 and FeSe7,8), thermoelectric properties (e.g., Tl2Ag12Te7.4,9 Cu11MnSb4S13,10 BaCu6−xSeTe6,11 and CuInTe212), magnetoresistance (e.g., FeCr2S4,4 Ag2Te,13 Ag2Se,14 and CdCr2Se415), electrochemical properties (e.g., MgCr2S416), and optical properties [e.g., AgMQ2 (M = Ga, In; Q = S, Se, Te)].17 These diverse properties of transition-metal chalcogenides result from the stabilization of a variety of structure types, ranging from simple three-dimensional structures to complex two- and one-dimensional structures. These diverse structures are possible because of the condensation of various types of MxQy (M = transition metal; Q = S, Se, Te) polyhedra that control the dimensionality of the crystal structure. The catenation tendency of chalcogen atoms along with intermediate Q−Q interactions has resulted in many intricate crystal structures with a variety of Q−Q bonding interactions, such as simple Q22− dumbbells (e.g., ZrTe3,18 K2Se2,19 and Na2Se219), (Qn)m− units (n > 2, e.g., the Se34− unit in Ba2Ag4Se520), infinite Q chains (e.g., CuTe21), and two-dimensional nets of Q atoms (e.g., NbTe422). These Q−Q interactions often help to stabilize new structure types that cannot be predicted in © XXXX American Chemical Society

advance. Hence, exploratory synthesis plays a major role in the discovery of unprecedented structure types. Recently, considerable activity in the exploratory synthesis of new Ag+ and Cu+ ternary and quaternary chalcogenides Ak− Cu/Ag−Q (Ak = Mg, Ca, Sr, Ba; Q = S, Se, Te) has produced compounds such as BaCu2Se2,23 Ba4Cu8Se13,24 BaCu4S3,25 BaCu5.65S4.5,26 BaCu2S2,23 BaCu2Te2,27 BaCu6−xSeTe6,11 BaAg2S2,28 BaAg2Te2,29 BaAg8S5,30 Ba2Ag4Se5,20 Ba2Ag8S7,31 and Ba3Cu14−xTe12.32 Interest has evolved because metal chalcogenides containing Ag+ and Cu+, such as β-Cu1.98Se,33 Ag1.54Te,34 BaCu6−xSTe6,11 and BaCu6−xSeTe611 have been studied for thermoelectric applications.11 The mobility of Ag+ and Cu+ ions in these structures is responsible for their low lattice thermal conductivity, which is favorable for thermoelectric applications. However, there was only one ternary compound (Ba2Ag4Se520) known for the Ak−Ag−Se system. Here we present the synthesis and structure of a semiconducting ternary polyselenide, Ba2Ag2Se2(Se2), a new member of the Ak−Ag− Se system that crystallizes in its own structure type. We have also presented its optical, Raman, and ultraviolet (UV) spectroscopies, resistivity, and electronic structure in detail. Received: February 20, 2019

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DOI: 10.1021/acs.inorgchem.9b00506 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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performed with use of the program SADABS.38 Precession images of the data sets indicated that the structure has a commensurate supercell. Subsequently, the structure of the subcell was solved and refined in a straightforward manner with use of the SHELX-15 algorithms of the SHELXT program package.38,39 The program STRUCTURE TIDY40 in PLATON41 was used to standardize the atomic positions. Further details are given in Tables 1 and 2.

EXPERIMENTAL METHODS

Syntheses and Analyses. Caution!232Th is an α-emitting radioisotope and as such is considered to be a health risk. Experiments using radioisotopes require appropriate infrastructure and personnel trained in the handling of radioactive materials. The following reactants were used as supplied: Ba rod (Alfa Aesar, 99.5%), Ag flakes (Aldrich, >99.9%), Th powder (MP Biomedicals, 99.1%), BaSe (Aldrich, >99.5%), and Se powder (Aldrich, 99.5%). Synthesis of Single Crystals of Ba2Ag2Se2(Se2). Black irregularly shaped crystals of Ba2Ag2Se2(Se2) suitable for crystalstructure analysis were initially obtained from the reaction of Ba (47.3 mg, 0.344 mmol), Th (10 mg, 0.043 mmol), Ag (9.3 mg, 0.086 mmol), and Se (54.4 mg, 0.689 mmol) in the search for a new quaternary compound in the Ba−Th−Ag−Se system. The reaction mixture was loaded into a 6 mm carbon-coated fused-silica tube in an Ar-filled glovebox. Then the tube was evacuated to 10−4 Torr, flamesealed, and transferred into a computer-controlled furnace. The temperature of the reaction mixture was first ramped up to 1073 K (in 36 h) and held there for 15 h before the temperature was raised to 1173 K in 24 h. It was kept at 1173 K for 198 h, followed by slow cooling to 573 K in 195 h, and finally the furnace was switched off. The resultant single crystals were analyzed by energy-dispersive X-ray (EDX) analysis with the use of a Hitachi S3400 scanning electron microscope. The EDX study on black irregularly shaped crystals showed the presence of Ba2Ag2Se2(Se2) (Ba:Ag:Se ≈ 1:2:2). Other secondary phases present in the reaction product were orange plates of ThOSe (Th:Se ≈ 1:1)35 and red blocks of BaSe (Ba:Se ≈ 1:1).36 The synthesis of single crystals of Ba2Ag2Se2(Se2) was further carried out by the stoichiometric reaction of BaSe (40.1 mg, 1.85 mmol), Ag (20 mg, 1.85 mmol), and Se (14.6 mg, 1.85 mmol) powders in an evacuated carbon-coated fused-silica tube. The temperature of the reaction tube was first raised to 873 K in 12 h and held there for 96 h, followed by cooling of the furnace to 373 K at a rate of 5 K/h. A black lump was obtained that was then smashed to reveal very small black crystals of Ba2Ag2Se2(Se2) (Ba:Ag:Se ≈ 1:2:2) with small amounts of BaSe (Ba:Se ≈ 1:1) and Ag2Se (Ag:Se ≈ 2:1) as side products. As discussed earlier, the crystals of Ba2Ag2Se2(Se2) were first obtained accidently while exploring the quaternary Ba−Th−Ag−Se system. Later, Ba2Ag2Se2(Se2) was reproduced using the stoichiometric reaction of the corresponding elements or binary reactants, but it yielded only very small crystals. The synthesis with Th may be the only way to obtain crystals suitable for structure analysis. Synthesis of Polycrystalline Ba2Ag2Se2(Se2). Stoichiometric amounts of Ba (201.2 mg, 1.465 mmol), Ag (158.0 mg, 1.465 mmol), and Se (231.4 mg, 2.930 mmol) were placed in a 10 mm (inner diameter) carbon-coated fused-silica tube, which was then evacuated to 2σ(Fo2). cRw(Fo2) = {∑[w(Fo2 − Fc2)2]/∑wFo4}1/2. For Fo2 < 0, w−1 = σ2(Fo2); for Fo2 ≥ 0, w−1 = σ2(Fo2) + qFo2, where q = 0.0004 for the superstructure and 0.1223 for the substructure. a

The superstructure was refined with JANA2006 software.42−44 The apparent subcell could be indexed with an orthorhombic unit cell of a = 4.3684(1) Å, b = 4.3684(1) Å, and c = 21.5784(9) Å and space group Immm (Z = 2) when the weak diagonal supercell reflections at 1 /2a* + 1/2b* were ignored. The asymmetric unit of the orthorhombic substructure contains five crystallographically independent sites: Ag1,

Table 2. Selected Interatomic Distances (Å) and Angles (deg) for Ba2Ag2Se2(Se2)a distances and angles Ag1−Se2 Ag1−Se3 Ag2−Se2 Ag2−Se3 Se1−Se4 Ba1−Se1 Ba1−Se2 Ba1−Se3 Ba1−Se4 Ba2−Se1 Ba2−Se2 Ba2−Se3 Ba2−Se4 Ag1···Ag1 Ag2···Ag2 Ag1···Ag2 Se2−Ag1−Se2 Se2−Ag1−Se3 Se3−Ag1−Se3 Se2−Ag2−Se2 Se2−Ag2−Se3 Se3−Ag2−Se3

2.772(1), 2.754(1) 2.737(1), 2.762(1) 2.771(1), 2.764(1) 2.743(1), 2.758(1) 2.378(2) 3.326(1) × 2 3.388(1), 3.394(1) 3.394(1), 3.403(1) 3.329(1), 3.326(1) 3.328(1), 3.325(1) 3.387(1), 3.397(1) 3.396(1), 3.340(1) 3.323(1), 3.328(1) 3.083(1), 3.096(1) 3.079(1), 3.100(1) 3.087(1), 3.090(1) 111.83(6) 104.81(5), 111.51(6),112.44(6),104.61(5) 111.82(6) 111.87(6) 104.90(5), 112.36(6), 111.26(6), 104.69(5) 111.93(6)

a Some entries have been rounded to three significant figures to facilitate comparisons.

B


Article

Inorganic Chemistry Ba1, and Se1 of site symmetry mm2 are fully occupied, and Se2 of site symmetry mm2 and Se3 of site symmetry 2mm are half-occupied and disordered. Ag atoms in the orthorhombic substructure are bonded to four Se1 atoms to form distorted AgSe4 tetrahedra that share edges with the neighboring tetrahedra to form a two-dimensional [AgSe4/4]− layer. These layers are separated by layers of Ba2+ cations and disordered Se22− units. A twin law of 90° rotation down the long c axis [0 1 0 0 1 0 0 0 −1] was used with a refined twin fraction of 0.696(2). Because the supercell is commensurate with respect to the subcell, the threedimensional space group P21/c was used for the description of the supercell with a unit cell of a = 6.1766(2) Å, b = 6.1788(2) Å, c = 21.5784(8) Å, and β = 90.02(1)°. PXRD Study. The phase purity of the polycrystalline Ba2Ag2Se2(Se2) sample was evaluated by PXRD studies at 298(2) K. A Cu tube was used as an X-ray radiation source in the diffractometer (model: X’Pert Pro-PAN analytical), and data were recorded with the use of X’Pert High Score software. The polycrystalline sample was fixed at a flat sample stage. The scan angle, step size, and scan time were 10−70°, 0.01°, and 50 min, respectively. An Xcelerator detector was used to collect the reflections. The PXRD pattern thus obtained was analyzed using the Match3 software suite (version 3.6.1). Raman Spectroscopy of Polycrystalline Ba2Ag2Se2(Se2). Raman spectra of a compacted circular disk of polycrystalline Ba2Ag2Se2(Se2) were collected at 298(2) K using a Sentara II Compact Raman microscope. To check the homogeneity of the sample, Raman data were collected at various regions on the disk. There were no noticeable variations. Solid-State UV−Visible Spectroscopy of Ba2Ag2Se2(Se2) Powder. An optical absorption study on the polycrystalline sample of Ba2Ag2Se2(Se2) was carried out at 298(2) K using a UV−visible spectrophotometer (JASCO V-770) with dry BaSO4 as a standard reference. The wavelength of the radiation was varied from 200 to 1200 nm. The band gap of Ba2Ag2Se2(Se2) was investigated using the following equation:

(αhν)n = A(hν − Eg )

relaxation of the structure, whereas for the HSE calculation, a default cutoff was used.

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RESULTS AND DISCUSSION Synthesis. While exploring the quaternary Ba−Th−Ag−Se system by employing a standard solid-state reaction of the Ba,

Figure 1. PXRD pattern of the polycrystalline Ba2Ag2Se2(Se2) compound. The asterisk shows a reflection of the secondary phase BaSe.

(1)

Here α, h, ν, A, and Eg are respectively the absorption coefficient, Planck constant, frequency of light, proportionality constant, and band gap. When n = 1/2, the band gap is indirect; when n = 2, the band gap is direct.45,46 Temperature-Dependent Resistivity Study. We collected resistivity (ρ) data as a function of the temperature in the range of 75−300 K by the four-probe method using a closed-cycle cryostat (Advanced Research System model CS202AB). A sintered rectangular-shaped pellet of Ba2Ag2Se2(Se2) was used, and four contacts were made using Cu wires of 0.025 mm thickness (Omega) with Ag paste. A constant direct current of 10 mA was applied with the help of a Keithley model 2400 current source, and the resulting potential drop was measured using a Keithley model 2400 nanovoltmeter. The measurement temperature was controlled by a Lakeshore 335 temperature controller. Data acquisition was accomplished by custom-written software. Theoretical Calculations. The calculations were conducted with the Vienna ab Initio Simulation Package (VASP 5.4)47,48 code implementing density functional theory (DFT)49,50 within the projector-augmented-wave method.51 We have proceeded in two steps: first, to keep the calculations computationally affordable, we have relaxed the experimental crystal structure with the Perdew−Burke−Ernzerhof (PBE) functional.52 We have found equilibrium parameters of a = b = 6.32 Å and c = 21.87 Å, which are slightly larger than the experimental values, as expected with the PBE functional. Then, using the relaxed structure, in order to have a realistic band gap, we carried out a calculation with the Heyd− Scuseria−Ernzerhof (HSE) functional.53−55 For both calculations, the Brillouin zone was sampled with a 10 × 10 × 3 mesh. For the PBE calculation, a cutoff of 1000 eV for the planewave part of the wave function was used in order to reduce the effect56 of Pulay stress on

Figure 2. View of unit cells of (a) the monoclinic commensurate superstructure of Ba2Ag2Se2(Se2) and (b) tetragonal ThCr2Si2.68 The Ba, Ag, Th, Cr, Si, and Se atoms are shown in blue, green, black, pink, yellow, and orange, respectively. For clarity, the Ba−Se and Th−Si interactions are not shown and the Se−Se bonds are shown in red.

Ag, Th, and Se elements at 1173 K, we discovered serendipitously a new ternary selenide, Ba2Ag2Se2(Se2). Very small single crystals of Ba2Ag2Se2(Se2) were further synthesized by a Th-free stoichiometric reaction of BaSe, Ag, and Se powders in high yield (about 90% based on Ag) along with small amounts of unreacted BaSe and Ag2Se phases. The single crystals of Ba2Ag2Se2(Se2) were stable in air for about 1 week, as judged from the unit cell determination of the crystals. The synthesis of an almost single-phase polycrystalline Ba2Ag2Se2(Se2) compound was reproduced by a stoichiometric reaction of the elements using the sealed tube method. The polycrystalline product was stable under ambient conditions C


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Inorganic Chemistry Table 3. Ag−Se Distances in Some Related Compounds with 4-Coordinated Ag Atomsa compound

structure

Ag−Se (Å)

ref

Ba2Ag2Se2(Se2) SrAgSeF BaAgSeF AgHoSe2 CsAg5Se3 CsAgSe4 RbAgSe4

layered layered layered three-dimensional three-dimensional one-dimensional one-dimensional

2.737(1)−2.772(1) 2.761(1) 2.753(1) 2.697(5−2.748(5)) 2.585(1)−2.954(1) 2.614(2)−2.940(2) 2.612(2)−2.865(2)

this work 57 57 73 74 75 75

a

Some distances have been rounded for comparison. All of the compounds contain Ag+. Figure 4. Raman spectrum of polycrystalline Ba2Ag2Se2(Se2).

for at least 1 day because there was no noticeable change in the PXRD pattern after exposure of the product to air. Figure 1 shows the PXRD pattern of the polycrystalline product obtained by a two-step reaction of the elements at 973 K. The experimental PXRD pattern may be satisfactorily indexed based on the simulated PXRD pattern obtained using single-crystal data of Ba2Ag2Se2(Se2). A minute amount of the secondary phase BaSe can be seen in the PXRD pattern. Further attempts to synthesize Sr and Ca analogues of Ba2Ag2Se2(Se2) by reaction of the corresponding elements were carried out under various temperature conditions, but the reactions were not successful. The PXRD patterns of the final products showed the presence of a mixture of binary selenides, AkSe (Ak = Ca, Sr), Ag2Se, and Se. Crystal Structure. Commensurate Superstructure of Ba2Ag2Se2(Se2). The superstructure of this new structure type, Ba2Ag2Se2(Se2), was found to be commensurate. It crystallizes in the space group P21/c of the monoclinic crystal system with four formula units with a cell of a = 6.1766(2) Å, b = 6.1788(2) Å, c = 21.5784(8) Å, and β = 90.02(1)°. The unit cell structure of Ba2Ag2Se2(Se2) is shown in Figure 2a, and important metrical data are tabulated in Table 2. The asymmetric unit of the superstructure comprises eight atoms occupying general positions: two Ba atoms, two Ag atoms, and four Se atoms. Each Ag atom in this structure is coordinated to two Se2 and two Se3 atoms in a distorted tetrahedral fashion. These AgSe4 tetrahedra are connected to neighboring tetrahedra by the sharing of edges to form the two-dimensional layers of [AgSe4/4]− that are perpendicular to the c axis. These

tetrahedral layers are very similar to the tetrahedral electrically neutral [FeSe4/4] layers found in the FeSe structure.7,8 The arrangement of atoms in these two-dimensional layers of [AgSe4/4]− and [FeSe4/4] in the Ba2Ag2Se2(Se2) and FeSe structures, respectively, is of the anti-PbO-type.7,8 The Ba1, Ba2, Se1, and Se4 atoms are sandwiched between these layers. The Ag1−Se and Ag2−Se interatomic distances in this structure are 2.737(1)−2.772(1) and 2.743(1)−2.771(1) Å, respectively. These distances are in good agreement with the Ag+−Se distances observed in some related structures (Table 3). The Ag atoms of the [AgSe4/4]− tetrahedral layers are arranged in a distorted square net-like fashion with four types of interactions ranging from 3.079(1) to 3.100(1) Å (Figure 3a). These Ag−Ag interactions are in good agreement with the corresponding distances in BaAgSeF [4 × 3.072(1) Å] and SrAgSeF [4 × 2.945(1) Å].57 The Se1 and Se4 atoms of the superstructure form Se1−Se4 dimers, i.e., Se22− units. The Se1−Se4 distance of 2.378(2) Å is in good agreement with the single Se−Se bond distances found in orthorhombic CuSe [2.341(1) Å],58 KAuSe5 [2.362(1)− 2.343(1) Å],59 Cs4Ge2Se8 [2.350(1) Å],60 CsAuSe3 [2.384(1) Å],61 Ba8PdU2Se12(Se2)2 [2.392(1) and 2.407(1) Å],62 Cs 2 U 2 P 2 Se 9 (Se 2 ) 2 [2.403(1) and 2.409(1) Å], 63 and Ba3ThSe3(Se2)2 [2.374(1) and 2.377(1) Å].46 Ba atoms in the superstructure of Ba2Ag2Se2(Se2) are surrounded by eight Se atoms (two each of the Se1, Se2, Se3, and Se4 atoms) with Ba−Se distances ranging from 3.323(1) to 3.403(1) Å. The coordination geometry formed by eight Te atoms around the Ba atom can be described as a distorted

Figure 3. Views of (a) the Ag part of the Ba2Ag2Se2(Se2) structure and (b) Ba coordination environments in Ba2Ag2Se2(Se2). The Se−Se bond is shown in red. D


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Inorganic Chemistry

Figure 5. (a) Absorption spectra and (b) temperature dependence of the resistivity of polycrystalline Ba2Ag2Se2(Se2).

Figure 6. Plots of the total and partial DOSs for the commensurate monoclinic superstructure of Ba2Ag2Se2(Se2). The Fermi level is set at 0.

Charge balance in Ba2Ag2Se2(Se2) can be achieved by 2 × Ba2+, 2 × Ag+, 2 × Se2−, and 1 × Se22− species. Hence, the structural formula is written as Ba2Ag2Se2(Se2) instead of BaAgSe2. Raman Spectroscopy of Ba2Ag2Se2(Se2). A Raman spectrum of Ba2Ag2Se2(Se2) collected at 298(2) K is shown in Figure 4. The most intense band at ∼247 cm−1 corresponds to vibration of the Se22− units, in good agreement with that of Se22− in Na2Se2 (∼252 cm−1),19 K2Se2 (∼253 cm−1),19 EuSe2 (∼256 cm−1),69 and Ba4SiSb2Se11 (∼247 cm−1).70 The band at ∼178 cm−1 results from Ag−Se bond stretching and compares well with that of Ag−Se (∼178 cm−1) in chalcopyrite AgGaSe2.71 The bands at ∼80 and ∼97 cm−1 correspond to Ba−Se interactions.72 Optical Band Gap of Ba2Ag2Se2(Se2). An absorption spectrum in the visible range was collected at 298(2) K on a powdered sample of Ba2Ag2Se2(Se2). It shows a broad absorption edge around 1.1 eV, consistent with the black color of the compound. Analysis of the square and square root of the absorbance data as a function of the energy (Figure 5a) gives a direct band gap of 1.23(2) eV and an indirect band gap of 1.10(2) eV. Resistivity Study. The temperature dependence of the resistivity of Ba2Ag2Se2(Se2) between 75 and 300 K is shown in

square antiprism (Figure 3b). Each Se22− unit is shared with Ba1 and Ba2 atoms, as shown in Figure 3b. The Ba−Se distances are close to the corresponding distances observed in other known compounds where Ba atoms are in a similar coordination environment. Examples are Ba2SnSe4 [3.197(5)− 3.792(9) Å],64 Ba2In2Se5 [3.211(1)−3.781(1) Å],65 Ba2ZnSe3 [3.197(1)−3.423(1) Å],66 and Ba0.5CuZrSe3 [3.165(1)− 3.470(1) Å].67 This new Ba2Ag2Se2(Se2) structure type shows some resemblance to the well-known tetragonal ThCr2Si2 structure type (space group I4/mmm),68 which is also layered, as shown in Figure 2b. The Cr atoms (site symmetry −4m2) in the ThCr2Si2 structure are bonded to four Si atoms, forming the distorted CrSi4 units with four equidistant Cr−Si bonds. These CrSi4 units form two-dimensional layers of [Cr2Si2]4− via the edge sharing of neighboring CrSi4 units similar to the [AgSe4/4]− layers in the Ba2Ag2Se2(Se2) structure. The packing of atoms in the [Cr2Si2]4− layers can also be described as the anti-PbO type akin to the [AgSe 4/4 ] − layers in the Ba2Ag2Se2(Se2) structure. The coordination geometry of the Th atom in the ThCr2Si2 structure type is square-antiprismatic, similar to that of the Ba atoms in Ba2Ag2Se2(Se2).68 However, the main difference between these two structures is that the ThCr2Si2 structure type has no anionic bonding. E


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Inorganic Chemistry Figure 5b. The resistivity at 300 K, found to be ∼650 mΩ·cm, increases to ∼1020 mΩ·cm at 75 K, consistent with semiconducting behavior. Electronic Band Structure of Ba2Ag2Se2(Se2). The computed HSE total (upper plot) and partial (lower plots) densities of states (DOSs) for Ba2Ag2Se2(Se2) are presented in Figure 6. If we ignore the small structure that appears around +0.6 eV in the DOSs and contributes negligibly to the optical spectra, then a gap in the energy of about 1 eV around the Fermi level is observed. From our partial DOS, the top of the valence bands consist mostly of Se- and Ba-derived states, while all of the atoms contribute to the bottom of the conduction bands.

thank Ministry of Human Research Development, IIT Hyderabad, and DST India, respectively, for research fellowships. Use was made of the Integrated Molecular Structure Education and Research Center X-ray Facility at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental Resource (NSF Grant ECCS-1542205), the State of Illinois, and the International Institute for Nanotechnology. The authors also thank Dr. Surya Jammalamadaka and Dwipak P. Sahu from the Department of Physics, IIT Hyderabad, for their help in collecting temperature-dependent resistivity data.

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CONCLUSIONS A new ternary polyselenide, Ba2Ag2Se2(Se2), was obtained by a solid-state sealed-tube reaction at 1173 K. A single-crystal Xray diffraction study at 100(2) K shows that this compound crystallizes in a new structure type. The commensurate superstructure of Ba2Ag2Se2(Se2) was solved in the monoclinic space group P21/c with four formula units per cell. There are eight crystallographically unique atoms in one unit cell: two Ba atoms, two Ag atoms, and four Se atoms all at general sites. Each Ag atom in this structure is coordinated to four Se atoms in a distorted tetrahedral arrangement, and these AgSe4 tetrahedra form a two-dimensional [AgSe4/4]− layer by edge sharing with the neighboring tetrahedra. The Ba2+ and Se22− units separate these two-dimensional layers. The Raman spectrum of polycrystalline Ba2Ag2Se2(Se2) shows an intense band at ∼247 cm−1 corresponding to Se22− units, and the Ag− Se bond stretching band appears at ∼178 cm−1. A semiconducting behavior with a band gap of 1.1 eV consistent with the resistivity study is predicted by the DFT study. The optical study by UV−visible spectroscopy shows that the direct and indirect band gaps of the compound are 1.23(2) and 1.10(2) eV, respectively, which are in good agreement with the theoretical study and consistent with the black color of the compound.

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CCDC 1893662 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Adel Mesbah: 0000-0002-6905-2402 James A. Ibers: 0000-0002-5418-3645 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.P. thanks the Department of Science and Technology (DST), Government of India, for financial support under a Early Career Research award (Grant ECR/2017/000822) and IIT Hyderabad for seed grant and research facilities. S.J. and M.I. F


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Inorganic Chemistry

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Synthesis and Characterization of Ba2Ag2Se2 (Se2)

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