Diverse Hf-Q Chains Existing in the Ternary Eu-Hf-Q (Q = S, Se) System

1. INTRODUCTION

Rare-earth chalcogenides (RECh) have been investigated extensively in the past years in view of their rich structures and versatile application potentials^[1]. Recently, more and more RECh compounds are updated to this interesting family, including BaRE₂In₂Q₇ (RE = La-Nd; Q = S, Se)^[2], RE₄S₄Te₃ (RE = Gd, Ho, Er, Tm)^[3], Ba₃La₄Ga₂Sb₂S₁₅^[4], Yb₆Ga₄S₁₅^[5], MgRE₆Si₂S₁₄^[6], BaRESn₂Q₆ (RE = Ce, Pr, Nd; Q = S; RE = Ce, Q = Se)^[7], RE₃S₃BO₃ (RE = Gd, Sm)^{[8, 9]} and RE₃GaS₆ (RE = Dy, Y)^[10]. Apart from their structures, these RECh compounds demonstrate attractive photocatalytic, second-order nonlinear optic (NLO), magnetic, photoluminescent, and photocurrent responsive activities.

Pushed by these progresses, it still makes sense to explore more new RECh compounds. On the other hand, divalent rare-earth metal Eu exhibits similar coordination style and ionic radius with alkali-earth (AE) metals, and AE chalcogenides behave great potentials as NLO^{[11, 12]}, photovoltaic^[13], and thermoelectric materials^[14]. Combining these considerations together, it is interesting to obtain new Eu-based chalcogenides (EuCh). Following our recent achievements on EuCh compounds, viz. EuZnGeS₄^[15], α-EuZrS₃^[16], Eu_0.81Ga₂Te₄^[17], Eu₉MgS₂B₂₀O₄₁^[18], Cu₂EuMQ₄ (M = Si, Ge; Q = S, Se)^[19], and Eu₈In_17.33S₃₄^[20], two new ternary RECh compounds were synthesized by us recently, namely, EuHfSe₃ (1) and Eu₅Hf₃S₁₂ (2). Here, we report their crystal structures, especially the diverse Hf-Q chains and the electronic structure of 1.

2. EXPERIMENTAL

2.1 Syntheses and analyses

All starting materials were used as received without further purification. Single crystals of 1 and 2 were obtained by solid-state reactions with KI (99 %) as flux. The starting materials are Eu₂O₃ (99.99%), S or Se (99.999%), HfO₂ (99.9%), and B powder (99%) with the stoichiometric ratios. A standard synthetic process is like this. Each sample has a total mass of 500 and 400 mg KI additional. The mixture of starting materials was ground into fine powder in an agate mortar and pressed into a pellet, followed by being loaded into a quartz tube. The tube was evacuated to be 1 × 10^–4 torr and flame-sealed. The sample was placed into a muffle furnace, heated from room temperature to 573 K in 5 h and maintained for 5 h, followed by heating to 873 K in 5 h and maintained for 5 h, then heated to 1223 K in 5 h and maintained for 5 days, finally cooled down to 573 K in 5 days and powered off. The crystals of 1 and 2 stable in moisture and air were obtained, which was then washed using ethanol and hot water under ultrasonic wave.

Semiquantitative microscope element analysis on the as-prepared single crystals was performed on a field-emission scanning electron microscope (FESEM, Zeiss-Supra55) equipped with an energy dispersive X-ray spectroscope (EDS, Bruker, Quantax), which confirmed the presence of Eu, Hf, and S/Se with the approximate compositions of 1 and 2, respectively, and no other elements were detected. The exact compositions were established from the X-ray structure determinations.

2.2 Structure determination

The intensity data sets were collected on a Bruker D8 QUEST X-ray diffractometer with graphite-monochromated MoKα radiation (λ = 0.71073 Å). The structures were solved by direct methods and refined by full-matrix least-squares techniques on F² with anisotropic thermal parameters for all atoms. All the calculations were performed with Siemens SHELXL-2015 package of crystallographic software^[21]. The final refinement included anisotropic displacement parameters for all atoms and a secondary extinction correction. Compound 1 crystallizes in orthorhombic space group Pnma with Z = 4, a = 8.8767(9), b = 3.9333(4), c = 14.3988(16) Å, V = 502.73(9) ų, D_c = 7.496 g/cm³, μ = 54.526 mm^–1, S = 1.051, (Δ/σ)_max = 0.091, (Δρ)_max = 1.29, (Δρ)_min = –1.32 e/ų, the final R = 0.0195 and wR = 0.0334. Compound 2 crystallizes in hexagonal space group P$\overline 6$2m with Z = 1, a = 11.6376(3), c = 3.9335(2) Å, V = 461.36(3) ų, D_c = 6.047 g/cm³, μ = 34.850 mm^–1, S = 1.083, (Δ/σ)_max = 0.024, (Δρ)_max = 2.39, (Δρ)_min = –1.54 e/ų, the final R = 0.0298 and wR = 0.0722. The bond lengths of 1 and 2 are listed in Table 1.

Table 1

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

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EuHfSe3 (1)
Bond	Dist.		Bond	Dist.
Eu(1)–Se(1)	3.171(1)		Hf(1)–Se(1)	2.723(1)
Eu(1)–Se(2)#5	3.373(1)		Hf(1)–Se(1)#1	2.696(1)
Eu(1)–Se(2)#6	3.206(1)		Hf(1)–Se(1)#2	2.696(1)
Eu(1)–Se(2)#7	3.206(1)		Hf(1)–Se(2)	2.675(1)
Eu(1)–Se(3)#1	3.201(1)		Hf(1)–Se(2)#3	2.675(1)
Eu(1)–Se(3)#2	3.201(1)		Hf(1)–Se(3)	2.561(1)
Eu(1)–Se(3)#8	3.161(1)
Eu5Hf3S12 (2)
Bond	Dist.		Bond	Dist.
Eu(1)–S(1)	2.846(4)		Eu(2)–S(1)#11	3.029(4)
Eu(1)–S(1)#3	2.846(4)		Eu(2)–S(1)#12	3.029(4)
Eu(1)–S(1)#5	2.846(4)		Eu(2)–S(2)	3.380(1)
Eu(1)–S(1)#7	2.846(4)		Eu(2)–S(2)#8	3.380(1)
Eu(1)–S(2)#8	2.917(8)		Eu(2)–S(2)#12	3.380(1)
Eu(1)–S(3)	2.871(2)		Hf(1)–S(1)	2.529(5)
Eu(1)–S(3)#9	2.871(2)		Hf(1)–S(1)#1	2.529(5)
Eu(2)–S(1)	3.029(4)		Hf(1)–S(2)	2.543(5)
Eu(2)–S(1)#5	3.029(4)		Hf(1)–S(2)#2	2.543(5)
Eu(2)–S(1)#8	3.029(4)		Hf(1)–S(3)	2.638(6)
Eu(2)–S(1)#10	3.029(4)		Hf(1)–S(3)#2	2.638(6)
Symmetry transformations used to generate the equivalent atoms: #1: –x, 1–y, 1–z; #2: –x, –y, 1–z; #3: x, 1+y, z; #4: x, –1+y, z;#5: 1/2–x, –y, 1/2+z; #6: –1/2+x, y, 3/2–z; #7: –1/2+x, 1+y, 3/2–z; #8: 1/2–x, 1–y, 1/2+z for 1. #1: 2–x, 1–x+y, 1–z;#2: x, y, 1+z; #3: –y+x, –y, –z; #4: –y+x, –y, z; #5: x, y, –1+z; #6: x, y, –z; #7: –y+x, –y, 1–z; #8: 1+y–x, 1–x, z;#9: 1–y, –1+x–y, z; #10: 1+y–x, 1–x, –1+z; #11: 1–y, x–y, –1+z; #12: 1–y, x–y, z for 2

2.3 Theory calculation

Since compound 2 contains trivalent Eu³⁺ ions, the efforts to calculate their electronic structures failed. Therefore, only calculation on 1 was performed. The calculation model was built directly from its single-crystal diffraction data, and no further geometry optimization was performed. The electronic structure calculation including band structure and density of states based on density functional theory (DFT) was performed using the CASTEP code in software Material Studio^[22]. The generalized gradient approximation (GGA) was chosen as the exchange-correlation functional and a plane wave basis with the projector-augmented wave (PAW) potentials used. The plane-wave cutoff energy of 480 eV and the threshold of 10^–5 eV were set for the self-consistent-field convergence of the total electronic energy. The valence electronic configurations for Eu, Hf, and Se were 4f⁷6s², 5d²6s² and 4s²4p⁴, respectively. The numerical integration of the Brillouin zone was performed using 3 × 6 × 2 Monkhorst-Pack k-point meshes and the Fermi level (E_f = 0 eV) was selected as the reference^[23].

3. RESULTS AND DISCUSSION

Compound 1 is a new member belonging to the M^Ⅱ-M^Ⅳ-Q₃ (M^Ⅱ = divalent metal Ca, Sr, Ba, Pb, Sn, Eu, Yb; M^Ⅳ = Ti, Zr, Hf; Q = S, Se) family with the perovskite structure, and it is also the first M^Ⅱ-Hf-Se₃ compound. It crystallizes in orthorhombic space group Pnma, isostructural with the previously studied α-EuZrS₃^[16]. There are one Eu, one Hf, and three Se atoms in the crystallographically independent unit. The 3D structure is constructed by EuSe₈ bicapped trigonal prisms and HfSe₆ octahedra (Fig. 1). The latter are linked together via sharing Se edges with four neighbouring HfSe₆ units to form ladder-like {[HfSe₃]^2–}_∞ chains along the b-axis. This chain also can be viewed as built by pseudo-cubane Hf₃Se₄ units, and each such unit contains three Se(1) and one Se(2) atoms. The {[HfSe₃]^2–}_∞ chain can be recognized as a double-chain structure unit, too, comprised of two parallel {[HfSe₄]^4–}_∞ chains, which are linked together via Eu–Se(1) bonds with the longest distance of 2.723(1) Å.

Figure 1

Figure 1.  (a) Coordination geometry of 1. (b) Ladder-like {[HfSe₃]^2–}_∞ chains shown in the structure of 1. For clarity, the Eu–S bonds are omitted. Symmetry transformations used to generate the equivalent atoms: #1: –x, 1–y, 1–z; #2: –x, –y, 1–z; #3: x, 1+y, z; #4: x, –1+y, z; #5: 1/2–x, –y, 1/2+z; #6: –1/2+x, y, 3/2–z; #7: –1/2+x, 1+y, 3/2–z; #8: 1/2–x, 1–y, 1/2+z

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Compound 2 is a new selenide crystallized in the hexagonal space group P$\overline 6$2m, isostructural with Eu₅Zr₃S₁₂^[24]. Different from 1, there are two types of Eu ions in the crystallographically independent unit, and they are ninefold (Eu(1))- or sevenfold (Eu(2))-coordinated with S atoms to form tricapped and monocapped trigonal prisms, respectively (Fig. 2). In the EuS₉ unit, there are only two kinds of Eu–S bond lengths, 3.029(3) (× 6) and 3.380(1) (× 3) Å, while in the EuS₇ unit, three types of Eu–S distances can be found, 2.846(2) (× 4), 2.872(1) (× 2) and 2.917(5) (× 1) Å. Each Hf atom bonds with six S atoms to form HfS₆ octahedron geometry, and these HfS₆ units connect with each other to form linear {[HfS₄]^4–}_∞ chains along the c-axis.

Figure 2

Figure 2.  (a) Coordination geometry of 2. (b) {[HfS₄]^4–}_∞ chains shown in the structure of 2. For clarity, the Eu–S bonds are omitted. Symmetry transformations used to generate the equivalent atoms: #1: 2–x, 1–x+y, 1–z; #2: x, y, 1+z; #3: –y+x, –y, –z; #4: –y+x, –y, z; #5: x, y, –1+z; #6: x, y, –z; #7: –y+x, –y, 1–z; #8: 1+y–x, 1–x, z; #9: 1–y, –1+x–y, z; #10: 1+y–x, 1–x, –1+z; #11: 1–y, x–y, –1+z; #12: 1–y, x–y, z

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It is interesting that both the two compounds contain Hf–Q constructed chains, namely, linear single one in 2 and linear ladder-like or double-one in 1, though the coordination geometries of Hf are the same, viz. octahedra in 1 and 2. The oxidation states of Eu ions can be easily assigned to Eu²⁺ in 1, and Eu²⁺ and Eu³⁺ with the molar ratio of 3:2 coexisted in 2 to make the whole molecules electro-neutral. To date, only Eu_0.4ZrSe₂^[25], LaTi_1.667Se_4.334^[26] and La_1.2Ti₂Se_5.2^[26] can be found for the RE-M^Ⅳ-Q₃ (RE = rare-earth metal; M^Ⅳ = Ti, Zr, Hf; Q = S, Se) system from the Inorganic Crystal Structure Database (ICSD) apart from the structures studied here. However, there should be more Eu, M^Ⅳ-based chalcogenides existing based on the similarity of Eu²⁺ and AE²⁺ ions. As far as we know, except for the AE-M^Ⅳ-Q₃ system, there are Ba₂Zr₃S₇, Ba₂ZrS₄, Ba₃Zr₂S₇, Ba₄Zr₃S₁₀, Ba₅Hf₄S₁₃, Ba₆Hf₅S₁₆, Ba₁₅Zr₁₄Se₄₂, and Sr₂₁Ti₁₉Se₅₇ found in the ICSD. Therefore, there is a great opportunity to obtain more ternary Eu-M^Ⅳ-Q compounds.

The yields of 1 and 2 are really low, even though hard efforts have been made to optimize the synthetic method, which impedes our interest to explore their potential physical properties, such as magnetic and photoluminescent data. On the other hand, hitherto, it is still a great challenge to calculate the electronic structures of compounds containing RE metal ions. Usually, RE metal ions with f⁰, f⁷, or f¹⁴ electronic configuration can be calculated. Therefore, only the electronic structure of 1 is computed.

To investigate the electronic structure of 1, the band structure and density of states (DOS) computations based on the DFT theory were performed using Materials Studio software. The calculated band structure along high symmetry points of the first Brillouin zone is shown in Fig. 3a, from which it can be seen that both the bottom of conduction band and the top of valence band locate at the G point, indicating that 1 has a direct band gap of 0.226 eV. This value is possibly lower than the experimental one in view of the limitation of DFT. The valence orbitals are mainly contributed by Se-4p and Eu-5p ones (Fig. 3b).

Figure 3

Figure 3.  Electronic band structure (a) and DOS (b) of 1. The Fermi level (red line) is set at 0 eV

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