Ba4Ga2Se8: A ternary selenide containing chains and discrete Se22− units

Journal of Solid State Chemistry 237 (2016) 144–149

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Ba4Ga2Se8: A ternary selenide containing chains and discrete Se22 − units Wenlong Yin a,b, Abishek K. Iyer a, Xinsong Lin a,c, Arthur Mar a,n a b c

Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, People's Republic of China Centre for Oil Sands Sustainability, Northern Alberta Institute of Technology, Edmonton, Alberta, Canada T6N 1E5

art ic l e i nf o

a b s t r a c t

Article history: Received 24 December 2015 Received in revised form 4 February 2016 Accepted 7 February 2016 Available online 8 February 2016

The ternary selenide Ba4Ga2Se8 has been synthesized by reaction of BaSe, Ga2Se3, and Se at 1023 K. Single-crystal X-ray diffraction analysis revealed a monoclinic structure (space group P21/c, Z¼4, a¼ 13.2393(5) Å, b¼ 6.4305(2) Å, c ¼20.6432(8) Å, β ¼ 104.3148(6)°) featuring one-dimensional chains of corner-sharing Ga-centered tetrahedra and discrete Se22 − anionic units, with charge-compensating Ba2 þ cations located between them. The UV/vis/NIR diffuse reflectance spectrum reveals an optical band gap of 1.63(2) eV, which is consistent with the black color of the crystals and agrees with a calculated gap of 1.51 eV obtained from band structure calculations. The presence of the Se22 − units narrows the band gap in Ba4Ga2Se8 relative to other Ba–Ga–Se phases. & 2016 Elsevier Inc. All rights reserved.

Keywords: Selenide Crystal structure Electronic structure Optical properties

1. Introduction Exploratory synthesis of multinary metal chalcogenides over the last few decades has revealed a remarkable diversity of structures and properties. Some notable examples among numerous compounds that have received attention for their potential applications include: BaFe2S3 which undergoes pressure-induced superconductivity [1], CsBi4Te6 which shows excellent thermoelectric properties at low temperatures [2], and Cs2Hg6S7 which is a promising material for X-ray and γ-ray detection [3]. Given the semiconducting behavior found in many of these chalcogenides, a particular focus is to examine their potential as nonlinear optical (NLO) materials in the infrared range. Dozens of candidates have now been identified, including Rb3Ta2AsS11 [4], La4InSbS9 [5], γ-NaAsSe2 [6], BaGa2GeSe6 [7,8], Ba23Ga8Sb2S38 [9], Ba4CuGa5Se12 [10], BaGa4S7 [11], BaGa2SnSe6 [12], and Ba2Ga8GeS16 [13], all of which exhibit strong second harmonic generation (SHG) responses. Among these, the Ga-containing compounds take advantage of asymmetric GaS4 or GaSe4 tetrahedral units that are arranged to give noncentrosymmetric structures. Because the ternary Ba–Ga–S and Ba–Ga–Se systems have still not been thoroughly examined, it is important to investigate what other compounds occur in these systems. Within the Ba–Ga–Se system, four compounds have been reported so far and all are semiconductors. Ba5Ga2Se8 (with a 2.5 eV band gap) contains isolated GaSe4 tetrahedra [14]. BaGa2Se4 (with n

Corresponding author. E-mail address: [email protected] (A. Mar).

http://dx.doi.org/10.1016/j.jssc.2016.02.014 0022-4596/& 2016 Elsevier Inc. All rights reserved.

a 3.2 eV band gap) contains chains of edge-sharing GaSe4 tetrahedra and has been tested as a host for luminescent materials [15– 17]. Ba5Ga4Se10 (with a 2.2 eV band gap) contains unusual discrete anionic [Ga4Se10]10  clusters with a Ga–Ga bond [18]. BaGa4Se7 (with a 2.6 eV band gap) contains a three-dimensional framework of corner-sharing GaSe4 tetrahedra and is a newly developed infrared NLO material [19–24]. Here, we report the preparation of Ba4Ga2Se8, which is a new member in this family, and describe its crystal structure, optical properties, and electronic structure.

2. Experimental 2.1. Synthesis The following reagents were used as obtained: Ba shot (99%, Sigma-Aldrich), Ag powder (99.99%, Sigma-Aldrich), Ga shot (99.99%, Cerac), and Se powder (99.99%, Sigma-Aldrich). The binary starting materials, BaSe and Ga2Se3, were prepared by stoichiometric reaction of the elements at high temperatures (1173 K for BaSe and 1223 K for Ga2Se3) in sealed fused-silica tubes. Crystals of Ba4Ga2Se8 were initially obtained from a reaction of BaSe, Ga2Se3, Ag, and Se in the molar ratio of 6:1:2:1 in an attempt to prepare a quaternary selenide “Ba3AgGaSe5” to expand on the previously known representatives Ba4AgGa5Se12 [25], Ba7AgGa5Se15 [26], and Ba4AgGaSe6 [27] in the Ba–Ag–Ga–Se system. The mixture of starting materials (BaSe, 195 mg, 0.90 mmol; Ga2Se3, 57 mg, 0.15 mmol; Ag, 33 mg, 0.30 mmol; Se, 12 mg, 0.15 mmol) was finely ground and loaded into a fused-silica tube. The tube was evacuated and flame-sealed, and then placed in

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20 mA. The experimental XRD pattern is in excellent agreement with the simulated pattern based on the single-crystal structure (Fig. 1b). Crystal growth experiments were performed by subjecting the pure polycrystalline sample of Ba4Ga2Se8 to the same heat treatment and slow-cooling profile described above. From these experiments, we can infer that Ba4Ga2Se8 melts congruently below 1023 K, but further investigations (such as differential thermal analysis) are required to characterize the thermal properties more accurately. Black needle-shaped crystals (typically 0.1– 0.2 mm in their longest dimension) obtained in the tube after the reaction were selected and verified by EDX analysis (observed: 34 (4)% Ba, 11(2)% Ga, 55(6)% Se averaged over 4 crystals; expected: 29% Ba, 14% Ga, 57% Se) and unit cell measurements to be the desired compound. 2.2. Structure determination Single-crystal X-ray diffraction data were collected at room temperature on a Bruker PLATFORM diffractometer equipped with a SMART APEX II CCD area detector and a graphite-monochromated Mo Kα radiation source, using ω scans at six different ϕ angles with a frame width of 0.3° and an exposure time of 20 s per frame. Face-indexed numerical absorption corrections were applied. Structure solution and refinement were carried out with use of the SHELXTL (version 6.12) program package [28]. Initial atomic positions were located by direct methods and refinements proceeded in a straightforward fashion. All atomic sites were confirmed to be fully occupied and exhibited reasonable displacement parameters. Atomic coordinates were standardized with the program STRUCTURE TIDY [29]. The final refinement included anisotropic displacement parameters and a secondary extinction correction. Crystal data and further details are listed in Table 1, positional and equivalent isotropic displacement parameters in Table 2, and interatomic distances in Table 3. Further data (including anisotropic displacement parameters), in CIF format, have been sent to Fachinformationszentrum Karlsruhe, Abt. PROKA, 76344 Table 1 Crystallographic data for Ba4Ga2Se8.

Fig. 1. (a) SEM image of Ba4Ga2Se8 crystal. (b) Powder XRD pattern of Ba4Ga2Se8.

a computer-controlled furnace. The reaction mixture was heated to 1223 K over 20 h, kept at that temperature for 48 h, cooled to 923 K over 4 d, cooled to 723 K over 2 d, and then cooled to room temperature by shutting off the furnace. Black needle-shaped crystals, subsequently identified as Ba4Ga2Se8, were found in the tube. Selected crystals were examined on a JEOL JSM-6010LA InTouchScope scanning electron microscope (Fig. 1a) and energydispersive X-ray (EDX) analysis revealed the presence of Ba, Ga, and Se in the approximate ratio of 4:2:8. The crystals are stable in air for months. After the composition was established from the structure determination, Ba4Ga2Se8 was prepared rationally through stoichiometric reaction of BaSe (0.216 g), Ga2Se3 (0.094 g), and Se (0.020 g) in the molar ratio of 4:1:1. As before, the mixture was finely ground and loaded into a fused-silica tube, which was evacuated and sealed. The tube was heated to 1023 K over 15 h, kept at that temperature for 96 h, and then cooled to room temperature by shutting off the furnace. The sample was analyzed by powder X-ray diffraction (XRD), performed on an Inel powder diffractometer equipped with a curved position-sensitive detector (CPS 120) and a Cu Kα1 radiation source operated at 40 kV and

Ba4Ga2Se8 1320.48 P21/c (No. 14) 13.2393(5) 6.4305(2) 20.6432(8) 104.3148(6) 1702.90(11) 4 5.151 296(2) 0.10  0.03  0.03 Graphite monochromated Mo Kα, λ ¼0.71073 Å μ(Mo Kα) (mm  1) 29.29 Transmission factors 0.177–0.545 2θ limits 3.17–66.39° Data collected  20r h r 20,  9r k r9,  31r l r 31 No. of data collected 23,800 No. of unique data, including Fo2 o 0 6462 (Rint ¼ 0.058) No. of unique data, with Fo2 42s(Fo2) 4568 No. of variables 128 R(F) for Fo2 4 2s(Fo2)a 0.036 2 b Rw(Fo ) 0.086 Goodness of fit 1.052 (Δρ)max, (Δρ)min (e Å–3) 3.22,  1.85 Formula Formula mass (amu) Space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z ρcalcd (g cm–3) T (K) Crystal dimensions (mm) Radiation

a

R(F) ¼∑||Fo|  |Fc||/∑|Fo|. Rw(Fo2) ¼[∑[w(Fo2  Fc2)2]/∑wFo4]1/2; p ¼[max(Fo2,0) þ 2Fc2]/3. b

w–1 ¼ [s2(Fo2)þ (Ap)2 þ Bp],

where

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W. Yin et al. / Journal of Solid State Chemistry 237 (2016) 144–149

Table 2 Atomic coordinates and equivalent isotropic displacement parameters for Ba4Ga2Se8. Atom

Wyckoff position

x

y

z

Ueq (Å2)a

Ba1 Ba2 Ba3 Ba4 Ga1 Ga2 Se1 Se2 Se3 Se4 Se5 Se6 Se7 Se8

4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e

0.10703(3) 0.14642(3) 0.38150(3) 0.47370(3) 0.11465(6) 0.69640(6) 0.04226(5) 0.11592(5) 0.12759(5) 0.27135(5) 0.31194(5) 0.31287(5) 0.57993(5) 0.60976(5)

0.23543(6) 0.74287(5) 0.23679(5) 0.23184(5) 0.32190(11) 0.31602(11) 0.73538(9) 0.24527(9) 0.69471(10) 0.18871(10) 0.42055(10) 0.04928(10) 0.22635(9) 0.23526(9)

0.21588(2) 0.04884(2) 0.03748(2) 0.31263(2) 0.40173(4) 0.10019(4) 0.17843(3) 0.01461(3) 0.38379(4) 0.37405(3) 0.17469(4) 0.17398(3) 0.16481(3) 0.48230(3)

0.01792(9) 0.01620(8) 0.01630(8) 0.01592(8) 0.01611(15) 0.01523(14) 0.01609(13) 0.01631(13) 0.01900(14) 0.01670(13) 0.01835(14) 0.01811(14) 0.01722(13) 0.01657(13)

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor.

Table 3 Interatomic distances (Å) for Ba4Ga2Se8. Ba1–Se5 Ba1–Se6 Ba1–Se3 Ba1–Se1 Ba1–Se1 Ba1–Se1 Ba1–Se4 Ba2–Se2 Ba2–Se1 Ba2–Se2 Ba2–Se3 Ba2–Se2 Ba2–Se8 Ba2–Se6 Ba2–Se5 Ba3–Se7 Ba3–Se8 Ba3–Se8 Ba3–Se4 Ba3–Se6 Ba3–Se5

3.2620(8) 3.2822(8) 3.2850(8) 3.2862(8) 3.3676(7) 3.3682(7) 3.4599(8) 3.2803(7) 3.2993(8) 3.3104(7) 3.3780(9) 3.3964(8) 3.4441(8) 3.5492(8) 3.6101(8) 3.2250(8) 3.2370(7) 3.2565(7) 3.3624(8) 3.3891(8) 3.9996(8)

Ba3–Se2 Ba3–Se8 Ba4–Se4 Ba4–Se7 Ba4–Se6 Ba4–Se5 Ba4–Se7 Ba4–Se5 Ba4–Se6 Ba4–Se8 Ba4–Se7 Ga1–Se2 Ga1–Se1 Ga1–Se3 Ga1–Se4 Ga2–Se7 Ga2–Se3 Ga2–Se8 Ga2–Se4 Se5–Se6

3.4304(8) 3.4878(8) 3.2466(8) 3.3160(7) 3.3302(8) 3.3394(8) 3.3839(7) 3.4297(8) 3.4412(8) 3.5204(8) 3.6624(8) 2.3658(10) 2.3800(10) 2.4383(10) 2.4393(10) 2.3467(10) 2.4013(10) 2.4428(10) 2.4691(10) 2.3876(9)

over 50 irreducible k points within the first Brillouin zone.

3. Results and discussion The ternary selenide Ba4Ga2Se8 is a new member in the Ba–Ga– Se system; it is isotypic to the sulfide analog Ba4Ga2S8, which was recently reported [32]. Ba4Ga2Se8 can be obtained in pure form by stoichiometric reaction of BaSe, Ga2Se3, and Se at 1023 K. Attempts to prepare other substitutional derivatives, such as In for Ga or Te for Se, were unsuccessful under similar conditions. The monoclinic structure of Ba4Ga2Se8 (space group P21/c) consists of 14 crystallograpically independent sites – four Ba, two Ga, and eight Se – all in Wyckoff position 4e. The structure contains Ba2 þ cations, one-dimensional anionic chains [GaSe3]3  , and discrete Se22 − units (Fig. 2). (Although Ba2GaSe4 would be the simplest empirical formula, it is more informative to express the formula as Ba4Ga2Se6(Se2) or Ba4Ga2Se8 to highlight the presence of these units.) The chains are built from corner-sharing GaSe4 tetrahedra, alternately centered by Ga1 and Ga2 atoms, that extend in a zigzag fashion along the b-direction. Within individual chains, the tetrahedra all point in the same direction, but the

Eggenstein-Leopoldshafen, Germany, as Supplementary material No. CSD-430648 and can be obtained by contacting FIZ (quoting the article details and the corresponding CSD numbers). 2.3. Diffuse reflectance spectroscopy A Cary 5000 UV–vis–NIR spectrophotometer equipped with a diffuse reflectance accessory was used to collect the spectrum of Ba4Ga2Se8 over the range of 350 nm (3.54 eV) to 2500 nm (0.50 eV). The powder sample was spread on a compacted base of BaSO4, used as a 100% reflectance standard. The optical absorption spectrum was converted from the diffuse reflectance spectrum using the Kubelka–Munk function, α/S ¼(1–R)2/2 R, where α is the Kubelka–Munk absorption coefficient and S is the scattering coefficient [30]. 2.4. Band structure calculation Tight-binding linear muffin tin orbital band structure calculations were performed on Ba4Ga2Se8 within the local density and atomic spheres approximation with use of the Stuttgart TB-LMTOASA program (version 4.7) [31]. The basis set consisted of Ba 6s/6p/ 5d, Ga 6s/4p/4d, and Se 4s/4p/4d orbitals, with the Ba 6p, Ga 4d, and Se 4d orbitals being downfolded. Integrations in reciprocal space were carried out with an improved tetrahedron method

Fig. 2. (a) Structure of Ba4Ga2Se8 viewed down the b-direction. (b) A slice perpendicular to (001) showing chains of up- and down-pointing GaSe4 tetrahedra, as well as Se22 − units; Ba atoms are omitted for clarity.

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overall structure is nonpolar because the dipole moments of chains containing up-pointing tetrahedra cancel with those containing down-pointing tetrahedra. The tetrahedra are slightly distorted, with angles of 101.90(3)–115.76(4)° around Ga1 and 100.10(3)–126.42(4)° around Ga2. The Ga–Se distances in Ba4Ga2Se8 (2.3658(10)–2.4393(10) Å around Ga1 and 2.3467(10)– 2.4691(10) Å around Ga2) are similar to those found in Ba4Ga4SnSe12 (2.387(2)–2.451(2) Å) [33] and Ba4AgGa5Se12 (2.362 (1)–2.427(1) Å) [25], where zigzag [GaSe3]3  chains form fragments of three-dimensional networks. In Ba4Ga4SnSe12, the Sn atoms serve as linkages between the [GaSe3]3  chains so that they all point in the same direction, giving rise to a noncentrosymmetric structure; however, in Ba4AgGa5Se12, the connecting Ag atoms fail to bring about an overall polarity and the chains point in both directions. The Se22 − units are also aligned along the b-direction, parallel to the [GaSe3]3  chains. They are formed from Se5 and Se6 atoms bonded at a distance of 2.3876(9) Å. This short distance is typical of a Se–Se single bond, comparable to the 2.392 (1)–2.407(1) distances found in the recent example of Ba8PdU2Se12(Se2)2 [34]. Three Ba sites share the Se22 − unit (η2) around their coordination environments, whereas the fourth Ba site is coordinated exclusively through single Se atoms (η1) (Fig. 3). The geometries around these Ba sites are rather irregular (with local site symmetry of 1), roughly monocapped trigonal prismatic (CN7) around Ba1, bicapped trigonal prismatic (CN8) around Ba2 and Ba3, and tricapped trigonal prismatic (CN9) around Ba4. The Ba–Se distances of 3.2466(8)–3.6624(8) Å agree well with those found in Ba8PdU2Se12(Se2)2 (3.1625(10)–3.7114(10) Å) [34] and Ba6Sn6Se13 (3.228(1)–3.770(1) Å) [35]. The charge-balanced formulation (Ba2 þ )4(Ga3 þ )2(Se2  )6( Se22 −) is supported by calculations of bond valence sums [36], which are 2.0–2.3 for the Ba atoms, 3.0 for the Ga atoms, and 2.0–2.2 for the isolated Se atoms; the Se5 and Se6 atoms would have bond valence sums of 1.3 if the Se–Se bond is neglected but they are restored to a more reasonable value of 2.2 if included. The interesting point of comparison is between Ba4Ga2Se8 and the isostructural sulfide, Ba4Ga2S8, which has been reported to be a semiconductor of a yellow color with a band gap of 2.55 eV (experimental) or 1.79 eV (calculated) [32]. The black color of Ba4Ga2Se8 implies that it has a smaller band gap than Ba4Ga2S8.

Fig. 3. Coordination environments around Ba atoms in Ba4Ga2Se8. The η2 coordination of the Se22 − dimer around Ba1, Ba3, and Ba4 atoms is highlighted by the yellow bonds. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

147

Fig. 4. Optical absorption spectrum of Ba4Ga2Se8.

This is confirmed by the measured optical band gap of 1.63(2) eV, deduced by the extrapolation method [37] from the absorption spectrum of Ba4Ga2Se8, which was converted from the diffuse reflectance spectrum using the Kubelka–Munk function (Fig. 4). The band gap in Ba4Ga2Se8 is considerably less than those in other Ba–Ga–Se phases (ranging from 2.2 eV in Ba5Ga4Se10 [18] to 3.2 eV in BaGa2Se4 [15]), as described earlier, suggesting that the presence of the Se22 − units has a profound effect on the electronic structure. It would be desirable to measure the electrical band gap but unfortunately the crystals of Ba4Ga2Se8 were too small (typically 0.1–0.2 mm in longest dimension) to permit single-crystal resistivity measurements. A band structure calculation performed on Ba4Ga2Se8 corroborates this point (Fig. 5). The density of states (DOS) curve shows valence and conduction bands separated by a gap of 1.51 eV, in good agreement with the measured gap of 1.63 eV. Empty Babased levels are mostly found above the Fermi level, while filled Ga- and Se-based levels are mostly found below, consistent with the notion of electron transfer from electropositive Ba atoms to an anionic Ga–Se framework. Strong covalent Ga–Se interactions are indicated by the mixing of Se 4p states with Ga 4s (from  6.2 to  4.9 eV) and 4p states (from  3.6 to 0 eV). As seen in the crystal orbital Hamilton population (COHP) curves, these Ga–Se interactions are optimized, with all bonding and no antibonding levels occupied, resulting in an integrated COHP (–ICOHP) value of 2.75 eV/bond. However, there is also considerable mixing of Ba and Se states in the valence (from 3.6 to 0 eV) and conduction bands (from þ 1.5 eV upwards). Covalent character provides a small but non-negligible contribution to Ba–Se bonding, giving an –ICOHP value of 0.48 eV/bond. The Se–Se COHP curve clearly shows the occupation of bonding and antibonding levels that are derived from the Se22 − dimers present in the structure: low-lying s4s and s*4s levels (at 13 and  11 eV, not shown), s4p and π4p levels (  3.6 to  1.6 eV), and π*4p levels (  1.6 to 0 eV). With the s*4p levels (þ1.5 eV upwards) remaining unoccupied, the electronic structure corresponds to the molecular orbital diagram for Se22 −, isoelectronic to Br2. This Se–Se bond has an –ICOHP value of 2.82 eV/bond. Close inspection reveals that the top of the valence band and the bottom of the conduction bands are dominated by Se states involved in Se–Se bonding, and secondarily by Ba states involved in Ba–Se bonding. In the absence of Se22 −dimers, as occurs

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Fig. 5. Density of states (DOS) and crystal orbital Hamilton population (–COHP) curves for Ba4Ga2Se8.

in other Ba–Ga–Se phases, the band gap would be greater than 2 eV, which corresponds to the energy in the conduction band above which Ga–Se antibonding levels only start to become prominent.

4. Conclusions The ternary selenide Ba4Ga2Se8 is a new member in the Ba–Ga– Se system in which chains of corner-sharing GaSe4 tetrahedra are formed but point in opposing directions so that an overall polar structure could not be attained. The anionic Se22 − units play an important role in decreasing the band gap to 1.6 eV, about 1 eV lower than in other Ba–Ga–Se phases that do not contain these units. Given that the corresponding sulfide Ba4Ga2S8 has a measured band gap of 2.6 eV, the band gap is also narrowed by a similar magnitude upon substitution of S by Se, as a result of weaker π-bonding interactions in the Se22 − units.

Acknowledgments This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the National Natural Science Foundation of China (No. 51402270).

Appendix A. Supplementary material Supplementary data associated with this article can be found in

the online version at http://dx.doi.org/10.1016/j.jssc.2016.02.014.

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