Two new ternary chalcogenides Ba₂ZnQ₃ (Q = Se, Te) with chains of ZnQ₄ tetrahedra: syntheses, crystal structure, and optical and electronic properties

2.1 Syntheses

Initially, single crystals of Ba₂ZnSe₃ and Ba₂ZnTe₃ were obtained by solid-state reactions in an attempt to discover new quaternary compounds in Ba–Th–Zn–Se and Ba–U–Zn–Te systems. Subsequently, Ba₂ZnSe₃ and Ba₂ZnTe₃ single crystals were produced in high yields at 1173 K from stoichiometric reactions of the elements.

2.2 Crystal structure

The isostructural compounds Ba₂ZnQ₃ (Q = Se, Te) crystallize in the K₂AgI₃ structure type [16] with four formula units in the space group D2h16-Pnma of the orthorhombic crystal system (Table 1). As expected, the unit cell constants for Ba₂ZnTe₃ are larger than those for Ba₂ZnSe₃. The crystal structure of these compounds is shown in Fig. 1. There are six atoms in the asymmetric unit of the structure all with site symmetry.m.: two Ba atoms, one Zn atom, and three Q atoms. The Zn atoms in this structure are coordinated to four Q atoms (2 Q1, 1 Q2, 1 Q3) and these form a distorted tetrahedron around each Zn atom. Each ZnQ₄ tetrahedron shares two corners (formed by Q1 atoms) with neighboring ZnQ₄ tetrahedra resulting in the formation of infinite chains of [ZnQ₄⁴⁻] units along the b axis (Fig. 2). The Ba atoms in this structure act as space fillers and separate these [ZnQ₄⁴⁻] chains. Both the Ba1 and Ba2 atoms are surrounded by seven Q atoms in a monocapped trigonal-prismatic geometry.

Fig. 1:

The Ba₂ZnQ₃ structure viewed along the b axis.

Fig. 2:

Local coordination of the Zn atoms and connectivity of the ZnQ₄ tetrahedra.

The ZnQ₄ tetrahedra in these structures are distorted with three different Zn–Q interatomic distances and four different Q–Zn–Q interatomic angles (Table 2). The ZnTe₄ tetrahedron in Ba₂ZnTe₃ is slightly less distorted than the ZnSe₄ tetrahedron in Ba₂ZnSe₃. The Zn–Se distances in Ba₂ZnSe₃ (2.4541 (6) to 2.5730 (5) Å) are in good agreement with the corresponding distances in ZnIn₂Se₄ (2.544(1) to 2.559(1) Å) [17], ZnGa₂Se₄ (2.438(1) Å) [18], Zn₄Ga₁₆Se₃ (2.375(1) to 2.445(1) Å) [19], CsHoZnSe₃ (2.435(1) to 2.552(1) Å) [20], and Cs₂ZnGe₃Se₈ (2.448(1) to 2.485(1) Å) [21]. The Zn–Te distances in Ba₂ZnTe₃ (2.6432(5) to 2.7482(3) Å) are also consistent with the Zn–Te distances in related compounds such as CsLaZnTe₃ (2.649(1) to 2.806(1) Å) [22], CsSmZnTe₃ (2.646(1) to 2.763(1) Å) [22], and KCuZnTe₂ (2.682(1) Å) [23]. The Ba–Se and Ba–Te distances in Ba₂ZnQ₃ (Q = Se, Te) are also normal. These structures do not feature any short Q–Q interactions and hence their charge balance is Ba₂²⁺Zn²⁺(Q²⁻)₃.

2.3 Optical band gap measurement

The absorption spectrum collected at 298 K on a single crystal of Ba₂ZnTe₃ shows an absorption edge at 2.10(2) eV. Analysis of the square and square root of absorbance data as a function of energy (Fig. 3) gives the same value for the direct and indirect band gap components suggesting that the band gap of Ba₂ZnTe₃ is direct at 2.10(2) eV. Furthermore, the edge of the absorption spectrum is consistent with the dark-red color of the crystals. Crystals of the Se analogue, Ba₂ZnSe₃, are pale yellow in color and their absorption falls outside the range of our spectrophotometer.

Fig. 3:

Optical absorption spectrum of a Ba₂ZnTe₃ single crystal (right) and plots of α² and α^1/2 vs. energy (left).

2.4 DFT calculations

Ba₂ZnSe₃ and Ba₂ZnTe₃ are found to be semiconductors with electronic band gaps of 2.6 and 1.9 eV, respectively, as seen on the computed total density of states (upper plots of Figs. 4 and 5). From the partial density of states (PDOS) plots (lower plots of Figs. 4 and 5), it is seen that the top of the valence states corresponds essentially to p states from the chalcogens (Se in Ba₂ZnSe₃ and Te in Ba₂ZnTe₃), while the bottom of the conduction states in both compounds comprise Ba-d states and Zn-s states. The electronic band gap of Ba₂ZnTe₃ of 1.9 eV is in very good agreement with the measured optical band gap of 2.1 eV.

Fig. 4:

Computed total density of states (upper plot) and partial density of states (lower plots) for Ba₂ZnSe₃.

Fig. 5:

Computed total density of states (upper plot) and partial density of states (lower plots) for Ba₂ZnTe₃.

4.1 Syntheses

The following reactants were used as obtained: Ba (Johnson Matthey, 99.5 %), Zn (Aldrich, 99.5 %), Se (Cerac, 99.999 %), Te (Aldrich, 99.8 %), and Th (MP Biomedicals, 99.1 %). U powder was obtained by hydridization and decomposition of depleted U turnings (IBI Labs) in a modification [24] of a literature method [25]. Reactions were performed in sealed 6 mm carbon-coated fused-silica tubes. Chemical manipulations were performed inside an Ar-filled dry box. The reactants were weighed and transferred into tubes that were then evacuated to 10⁻⁴ Torr, flame sealed, and placed in a computer-controlled furnace. Semi-quantitative EDX analyses of the products of the reactions were obtained with the use of a Hitachi S-3400 SEM microscope.

4.2 Synthesis of Ba₂ZnSe₃

Yellow single crystals of Ba₂ZnSe₃ were obtained in an attempt to synthesize the hypothetical quaternary compound Ba₃ZnThSe₆ of the 2H hexagonal perovskite family [26–28]. The reaction mixture comprised Ba (35 mg, 0.255 mmol), Th (19.7 mg, 0.085 mmol), Zn (5.56 mg, 0.085 mmol), and Se (40.13 mg, 0.508 mmol) in 100 mg of CsCl flux. The loaded tube was heated to 1173 K in 48 h, held there for 4 d, cooled to 673 K at 2.5 K h⁻¹. Then the furnace was turned off. The reaction produced rectangular-shaped yellow single crystals in an approximate yield of 20 wt%. For these crystals the EDX analysis gave Ba:Zn:Se ≈ 2:1:3. Byproducts included BaSe and ThOSe. Afterwards, a rational stoichiometric synthesis provided Ba₂ZnSe₃ in a yield of about 80 wt%. Crystals of BaSe and ZnSe were also detected.

4.3 Synthesis of Ba₂ZnTe₃

Red crystals of Ba₂ZnTe₃ were obtained by a reaction of Ba (35 mg, 0.599 mmol), U (10.1 mg, 0.042 mmol), Zn (5 mg, 0.076 mmol), and Te (86.8 mg, 0.680 mmol). The reaction mixture was heated to 1023 K in 24 h and held there for 10 h. The reaction mixture was then heated to 1173 K in 24 h followed by annealing there for 99 h. It was then cooled to 873 K in 99 h, and then finally to 373 K in 99 h. The reaction product contained red lath-shaped crystals of Ba₂ZnTe₃ (Ba:Zn:Te≈2:1:3), black plate-shaped crystals of UOTe (U:Te ≈1:1) [29], and crystals of BaTe (Ba:Te ≈1:1). A similar reaction with Th powder instead of U powder also produced Ba₂ZnTe₃ crystals in high yield. Ba₂ZnTe₃ can also be synthesized in the absence of U or Th.

4.4 Structure determinations

The crystal structures of both compounds were determined from single-crystal X-ray diffraction data collected with the use of graphite-monochromatized MoK_α radiation (λ = 0.71073 Å) at 100(2) K on a Bruker APEX2 diffractometer. The algorithm Cosmo implemented in the program Apex2 [30] was used to establish the data collection strategy with a series of 0.3° scans in ω and φ. The exposure time was 10 sec per frame and the crystal-to-detector distance was 60 mm. The collection of intensity data as well as cell refinement and data reduction were carried out with the use of the program Apex2 [30]. Face-indexed absorption, incident beam, and decay corrections were performed with the use of the program Sadabs [31]. Both crystal structures were solved and refined in a straightforward manner with the use of the Shelx-14 algorithms of the Shelxtl program package [32]. The program Structure tidy [33] in Platon [34] was used to standardize the atomic positions. Further details are given in Table 1.

Further details of the crystal structure investigations in cif format may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the deposition numbers CSD-430578 (Ba₂ZnTe₃) and 430579 (Ba₂ZnSe₃).

4.5 Optical absorption measurement

A single-crystal absorption spectrum was obtained at 298 K on a Hitachi U-6000 Microscopic FT spectrophotometer mounted on an Olympus BH2-UMA microscope. A crystal of Ba₂ZnTe₃ was placed on a glass slide and positioned over the light source where the transmitted light was recorded from above. The background signal of the glass slide was subtracted from the collected intensity.

4.6 DFT calculations

To conduct the calculations we have used density functional theory [35, 36] as implemented in the Vienna ab Initio Simulation Package (VASP) [37, 38] using projector augmented waves [39] as the basis set. We have used the HSE (Heyd-Scuseria-Ernzerhof) [40–42] functional for the exchange and correlation while the geometry (cells and atomic positions) were taken from the experiments. A 4×8×2 mesh was used to sample the Brillouin zone, and the default cutoff was used for the plane wave part of the wave function.