SUN Zong-Dong GUO Sheng-Ping

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Two Ternary Europium Chalcogenides Eu1-xGa2Te4(≈ 0.19) and EuY2Se4, Experimental and Theoretical Investigations①

SUN Zong-Dong GUO Sheng-Ping②

(225002)

Two ternary europium chalcogenides, Eu1-xGa2Te4(≈ 0.19) (1) and EuY2Se4(2), have been synthesized by a facile solid-state route using boron as the reducing reagent. Eu1-xGa2Te4crystallizes in the tetragonal space group4/with= 8.2880(9),= 6.7439(12) ?,= 463.24(13) ?3, and= 2. EuY2Se4crystallizes in the orthorhombic space groupwith= 12.4726(16),= 4.1204(6),= 14.849(2) ?,= 763.11(19) ?3, and= 4. Eu1-xGa2Te4belongs to the TlSe-type 3closed structure, while EuY2Se4adopts the CaFe2O4-type 3channel structure. The optical band gap of Eu1-xGa2Te4is determined to be 0.48 eV. Electronic structures of 1 and 2 are calculated using TB-LMTO software.

europium chalcogenide, solid-state reaction, crystal structure, band gap;

1 INTRODUCTION

Europium chalcogenides have been investigated extensively recently because of their diverse crystal structures and rich physical properties[1, 2]. As divalent rare-earth metal Eu2+and Yb2+ions have similar coordination and bonding habits with diva- lent alkali-earth metal ions (Mg2+, Ca2+, Sr2+, and Ba2+), novel europium chalcogenides can be derived from the corresponding alkali-earth chalcogenides. Since multinary barium-based chalcogenides are recently explored as second-order nonlinear optical (NLO) materials in the infrared (IR) region[3], it is interesting to plan to design several potential Eu-based chalcogenides as NLO materials. Based on this consideration, many efforts are made to synthesize novel multinary Eu chalcogenides by us in recent years. In this work, two ternary Eu chalcogenides, Eu1-xGa2Te4(≈ 0.19) (1) and EuY2Se4(2), were obtained. The former is a new polymorph of EuGa2Te4, and the latter is structure re-determination of EuY2Se4. Here, their syntheses, crystal structures, electronic structures, and optical properties are presented.

2 EXPERIMENTAL

2. 1 Syntheses and analyses

All starting materials were used as received without further purification. Single crystals of the title compound were obtained by solid-state reaction with KI (99 %) as flux[4-8]. The starting materials are Eu2O3(99.9%), Eu2O3(99.9%), Ga2O3(99.9%), Se (99.999%), Te (99.999%), and boron powder (99%). Each sample has a total mass of 500 mg and 400 mg KI (99%) additional, and the molar ratios of Eu:Ga:Te:B or Eu:Y:Se:B are 1:2:4:6. The mixture of starting materials was ground into fine powder in an agate mortar and pressed into one pellet, followed by being loaded into quartz tubes. The tubes were evacuated to be 1′10–4torr and flame-sealed. The samples were placed into a muffle furnace, heated from room temperature to 1223 K for several intermediate holding zones, then kept for 5 days, finally cooled down to 573 K with the speed of 5 K/h, and powered off. Black crystals of 1 and 2 stable in air and water were obtained. The exact compositions were established from X-ray structure determination. The purity of powder sample of 1 was confirmed by powder X-ray diffraction (PXRD) study. The PXRD pattern was collected with a PANalytical X'Pert Pro diffractometer at 40 kV and 40 mA for Cu-radiation (= 1.5406 ?) with a scan speed of 5°/min at room temperature. The simulated pattern was produced using the Mercury v2.3 program provided by the Cambridge Crys- tallographic Data Center (CCDC) and single-crystal reflection data. The PXRD pattern of 1 (Fig. 1) matches well with the simulated one, indicating the picked out sample is pure, which was subsequently sent to measure its optical property.

Fig. 1. Powder X-ray diffraction pattern of 1

2. 2 Structure determination

Theintensity data sets were collected on a Bruker D8 QUEST diffractometer with graphite-monochro- mated Mo-radiation (= 0.71073 ?). The structures of 1 and 2 were solved by direct methods and refined by full-matrix least-squares techniques on2with anisotropic thermal parameters for all atoms. All the calculations were performed using Shelxtl-2014[9]through the Olex2[10]interface. The final refinements included anisotropic displacement parameters for all atoms and a secondary extinction correction. Compound 1 crystallizes in the tetragonal space group4/with= 2,= 8.2880(9),= 6.7439(12) ?, and= 463.24(13) ?3.The finalandvalues for all data are 0.0327 and 0.0708, respectively. Compound 2 crystallizes in the orthorhombic space groupwith= 4,= 12.4726(16),= 4.1204(6),= 14.849(2) ? and= 763.11(19) ?3.The finalandvalues for all data are 0.0277 and 0.0410, respectively. The bond lengths of both crystals are listed in Table 1.

2. 3 Optical property

The diffuse reflectance spectrum of 1 was recor- ded at room temperature on a computer-controlled Varian Cary 5000 UV-Vis-NIR spectrometer equip- ped with an integrating sphere. As the yield of 2 is too low, its diffuse reflectance spectrum was not measured. The measurement wavelength was set in the range of 300～1700 nm. A BaSO4plate was used as a reference, on which the finely ground powdery sample was coated. The absorption spec- trum was calculated from reflection spectrum by the Kubelka-Munk function[11, 12].

2. 4 Theory calculation

The calculation models were built directly from the single-crystal structure data of 1 and 2. Their band structures and densities of state were calculated by tight-binding linear muffin-tin orbital (TB-LMTO) software, using the LMTO47 program[13]. Employing this program, the electronic structures of several earlier compounds obtained by us have been successfully calculated[14-17]. This package uses the atomic sphere approximation (ASA) method, in which space is filled with overlapping Wigner-Seitz (WS) atomic spheres[18].The symmetry of the potential is considered spherical inside each WS sphere, and a combined correction is used to take into account the overlapping part[19]. The radii of WS spheres were obtained by requiring that the overlapping potential be the best possible approxi- mation to the full potential and were determined by an automatic procedure. Exchange and correlation were treated by the local density approximation[20]. The WS radii are as follows: Eu = 3.72 ?, Ga = 2.75 ?, Te = 3.12 ? for 1, and Eu = 3.93 ?, Y = 3.23～3.26 ?, Se = 2.93 ? for 2. The-space integrations were conducted by the tetrahedron method, and the self-consistent charge densities were obtained using4 × 4 × 4points for 1 and 2 × 6 × 2points for 2 in the Brillouin zones. The Eu 6, Ga 3, Te 5, Se 4, and Y 5orbitals were treated using the Lo?wdin downfolding technique.

Table 1. Bond Lengths (?) for 1 and 2

3 RESULTS AND DISCUSSION

Compounds 1 and 2 crystallize in the tetragonal space group4/and orthorhombic space group, respectively. The structure of 1 belongs to the TlSe-type 3closed structure, while the structure of 2 adopts the CaFe2O4-type 3channel structure. When checking known related compounds with 1, there is no RE–M2–Q4(RE = rare earth; M = trivalent metal; Q = S, Se, Te) compounds adopting the tetragonal4/structure, and only several tellurides in AE–M2–Q4(AE = Mg, Ca, Sr, Ba; M = Al, Ga, In; Q = S, Se, Te) compounds with this structure are reported. Most of the RE–M2–Q4or AE–M2–Q4compounds crystallize in the space group,, or. EuGa2Te4was firstly reported as early as in 1980[21], which crystallizes in the orthorhombic space group. The tetragonal phase of EuGa2Te4is firstly investigated here. While for 2, which is firstly determined by Souleau in 1968 using powder X-ray diffraction data[22]. Here, single-crystal X-ray diffraction data are obtained and used to determine the exact structure of 2 and further calculation.

There are one Eu, one Ga, and one Te atoms in the crystallographically independent unit in the structure of 1. Eu and Ga atoms are coordinated with eight or four Te atoms to constitute a EuTe8decahedron and a GaTe4tetrahedron, respectively. There are one Eu, two Y, and four Se atoms in the crystallographically independent unit in the structure of 2. Eu and Y atoms connect with eight or six Se atoms to build a EuSe8bicapped trigonal prism () and a YSe6octahedron, respectively (Fig. 2).

Fig. 2. Coordination geometries of 1 (a) and 2 (b)

The 3crystal structure of 1 is shown in Fig. 3. EuTe8decahedra connect with each other to con- struct the 3framework, in which Ga atoms occupy the tetrahedral cavities. Each EuTe8decahedron shares corners (one Te atoms), edges (two Te atoms), and faces (four Te atoms) with eight, four, and two neighboring EuTe8decahedra, respectively. Each GaTe4tetrahedron shares edges with two neighboring GaTe4tetrahedra to build a [(GaTe4)5–]chain along the-axis. Different from the EuTe8decahedron in 1, the EuSe8unit in 2 forms a-type coordination geometry. According to Fig. 4, EuSe8link together to form starfish-like chains by sharing corners along the-axis. YSe6octahedra connect with each other to build chains along the-axis. The structure motif constructed by Y–Se bonds can be described to Y3Se4-type pseudo-cubane lacking one corner, a similar motif with that in-EuZrS3[1]- and3S3BO3[23, 24]. Due to two different crystallographic Y atoms, there are four types of Y3Se4pseudo-cubanes, as highlighted with black lines in Fig. 4. All the pseudo-cubanes are comprised of three Y and four Se atoms.

Fig. 3. Crystal structure of 1. (a) 3-D view of the structure along theaxis; (b) Connection between each EuTe8octahedron and its neighboring GaTe4tetrahedra viewed along theaxis; (c) [(GaTe4)5–]chain built by GaTe4tetrahedra via sharing edges along theaxis

Fig. 4. Crystal structure of 2. (a) Connection between EuSe8viewed along theaxis;

(b) Pseudo-cubane units constructed by three Y and four Se atoms viewed along theaxis

The selected bond distances for 1 and 2 are listed in Table 1. The Eu–Te and Ga–Te bond distances in 1 are 3.4996(5) and 2.6354(11) ?, similar with 3.490(1)～3.595(1) and 2.622(1)～2.766(1) ? discovered in AE–Ga2–Te4compounds, respectively. Eu–Se and Y–Se bond distances are 3.1793(3)～3.4053(8) and 2.7907(6)～2.8759(6) ?. Similar with isostructural compounds EuLn2Se4(Ln = Tb–Lu)[25], the Eu–Se and Y–Se bond lengths in 2 are in the ranges of 3.1469(7)～3.4382(2) and 2.7473(6)～2.8955(9) ?, respectively.

The band structure and density of states (DOS) of 1 and 2 are obtained by TB-LMTO-ASA electronic structure calculations. The calculated band structure along high symmetry points of the first Brillouin zone are shown in Fig. 5. It can be seen that the band gaps of 1 and 2 arecalculatedto be 0.46 and 1.33 eV, respectively, with the former close to its experi- mental value obtained from the ultraviolet diffuse reflection spectrum at room temperture (Fig. 6). The lowest conduction band (CB) and the highest valence band (VB) of 1 are located at Z and G points, respectively, while for 2, both of them are located at the G points, inidcating that the band gaps of 1 and 2 are indirect and direct, respectively.

Fig. 5. Calculated band structures and DOS of 1 and 2. The Fermi level is chosen as the energy reference at 0 eV

Fig. 6. UV-Vis-NIR spectroscopy measurement for 1 with an extracted optical band gap of ~0.48 eV at room temperature

The total and partial DOS (TDOS and PDOS) of 1 and 2 are drawn in Fig. 5. As for 1, the highest valence band (HOMO) is mainly constituted by Te-5orbitals, and the lowest conducation band (LUMO) is primarily constituted by Ga-3orbitals. For 2, the HOMO and LUMO are mainly consituted by Se-4and Y-3orbitals, respectively. The vanlence bands of 1 between –8 and –2 eV are caused mainly formed by Te-5, Eu-4, and Ga-4orbitals, and the conduction bands between –1 to 8 eV are mianly created by Eu-4, Te-5, and Ga-4orbitals. For 2, the vanlence bands between –5 to 0 eV are mainly caused by Se-4, Y-4, and Eu-4orbitals, and the conduction bands in the range of 2～8 eV are consitituted by Y-4, Eu-4, and Se-4orbitals. Therefore, it is clear the optical absorptions of 1 and 2 are caused by the charge transfer from Te-5to Ga-3and from Se-4to Y-3, respectively.

Here, two ternary europium chalcogenides are reported with their solid-state syntheses, crystal and electronic structures, together with the optical property. It is obvious that more Eu-based chalco- genides can be designed from the corresponding alkali-earth chalcogenides. It is supposed that more such compounds can be studied with rich structural chemistry and physical performance.

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13 December 2017;

13 March 2018 (ICSD 433862 for 1 and 433861 for 2)

① This research was supported by NNSFC (21771159), NSF of Yangzhou (YZ2016122), and State Key Lab of Structural Chemistry Fund (20150009)

. Tel: +86-514-87975244, E-mail: spguo@yzu.edu.cn

10.14102/j.cnki.0254-5861.2011-1924