English

• 1. INTRODUCTION

  Metal chalcogenides have attracted increasing attention because of their unique structures and potential applications in nonlinear optics^[1-5], superconductivity^[6], thermoelectric energyconversion^[7], and so forth. As far as we know, several families of ternary chalcogenides have been investigated, such as Hg-Q-X^[8], M-Q-X^[9], A-M-Q^[10] (M = p-block; A = alkali or alkaline earth metal; Q = S, Se, Te; X = F, Cl, Br, I) and so on. Among them, the type of Hg-Q-X compounds can be classified into the following species based on their different stoichiometric ratio types: 1) 3:2:2 type, such as Hg₃S₂F₂^[11], α-, β-, γ-Hg₃S₂Cl₂^[12-14], α-, β-Hg₃S₂Br₂^[15], Hg₃S₂I₂^[16], Hg₃Se₂Cl₂^[12], Hg₃Se₂Br₂^[17], Hg₃Se₂I₂^{[17, 16]}, Hg₃Te₂Cl₂^[12], Hg₃Te₂Br₂^[12], and Hg₃Te₂I₂^[18]; 2) 2:2:2 type: Hg₂Te₂Br₂ and Hg₂Te₂I₂^[8]; 3) 3:1:4 type: Hg₃TeBr₄^[15] and Hg₃TeI₄^[19]; 4) 2:1:3 type: Hg₂TeBr₃^[20]. Based on single-crystal X-ray diffraction analyses, there are various structural phases in these known Hg₃Q₂X₂ compounds. For example, Hg₃S₂Cl₂ can crystallize in space group I2₁3, Pm-3n and Pbmm. It is worthy to note that the syntheses of new structures in Hg-Q-X system can bring about the discovery of new physical properties. As a continuation of the previous efforts, a lot of work has been carried out in our lab^{[8, 18, 21]}. Interestingly, we found another new ternary structural phase in the system of Hg-Q-X, γ-Hg₃S₂Br₂ (1), which possesses different structures from α- and β-Hg₃S₂Br₂. In this paper, we report the synthesis, crystal structure, and optical properties as well as electric resistivity on γ-Hg₃S₂Br₂.

  2. EXPERIMENTAL

  2.1 Synthesis

  Compound 1 was obtained by moderate-temperature solid-state reactions of a mixture of HgBr₂ (1.0 mmol, 99.5%) and HgS (2.0 mmol, 99.999%). The starting materials were ground into fine powders in an agate mortar and pressed into a pellet in a glove box, followed by being loaded into quartz tubes that were subsequently evacuated to 1 × 10^–4 Torr and flame-sealed. The tubes were placed into a computer-controlled furnace, heated from room temperature to 200 ℃ at a rate of 50 ℃/h and kept for 48 h. The tubes were then heated to 430 ℃ for 1, kept at this temperature for 120 hours, and then slowly cooled down to 50 ℃ at a rate of 2.5 ℃/h.

  Yellow crystals of the title compound were obtained with the yield of about 80%. Semiquantitative microscope analysis using energy-dispersive X-ray spectroscopy (EDS) was performed on a JSM6700F scanning electron microscope (SEM) on a single crystal, which confirmed the presence of Hg/S/Br in the approximate molar ratios 2.97/2.07/1.96 for 1. The purity of the crystals of 1 was confirmed by powder XRD study (Fig. S1) (Caution! HgBr₂ and HgS are toxic, so extreme care must be exercised, and some toxic gases may be released when the Pyrex tubes are opened. HgBr₂ is water-sensitive, thus it is suggested that weighting reagents, grinding mixture, and pressing into pellets are performed in a dry box).

  2.2 Crystal structure determination

  Data collection was performed on a Rigaku Saturn724 CCD diffractometer equipped with graphite-monochromated MoKα radiation (λ = 0.71073 Å) at 293 K. A yellow single crystal of 1 with suitable dimensions was mounted on a glass fiber for single-crystal X-ray diffraction analysis. The intensity data were collected with an ω-scan mode and reduced with CrystalClear software^[22].

  The structure of 1 was solved by direct methods and refined by full-matrix least-squares techniques on F² with anisotropic thermal parameters for all atoms. The calculations were performed with the Siemens SHELXL version 5 package of crystallographic software^[23]. The final refinements included anisotropic displacement parameters for all atoms and a secondary extinction correction. The formula takes collectively into account the crystallographically refined compositions and requirements of charge neutrality. As the results of structure analysis, the occupation factor of the site Hg(3) (Wyckoff site 8p (..m) with multiplicity 8) is set to 0.5. The explanation is as follows: 1. On full occupation, the formula would be Hg₄S₂Br₂ = Hg₂SBr, which would imply a mixed valent state Hg²⁺/Hg¹⁺. The crystal structure gives no hint of (Hg₂)²⁺ pairs, and monovalent mercury should be ruled out. 2. For the charge neutrality under divalent mercury, at least three Hg positions has been refined to be occupied only by 50% of the full amount of Hg, and this should be position Hg(3). When the occupation factor of the site Hg(3) is 1, the abnormal temperature factor of Hg(3) appeared and in reality it is twice as much as normal. So, this question may be caused by one half of the occupation in the crystal structure. When changing the occupation factor, the whole data are reasonable. In the high temperature solid state experiments, the defect in the crystals is always unavoidable. Therefore the occupation of atoms sometime is needed to be revised. The selected bond lengths are reported in Table 1.

  Table 1

  

  Table 1.  Bond Lengths (Å) and Bond Angles (°) for 1

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  Bond	Dist.		Bond	Dist.
  Hg(1)–S(1)	2.407(5)		Hg(1)–Br(3)	2.9682(17)
  Hg(2)–S(1)	2.381(5)		Hg(3)–Br(3)#6	2.730(2)
  Hg(3)–S(1)	2.3475(9)
  Angle	(°)		Angle	(°)
  S(1)#1–Hg(1)–S(1)	165.9(2)		Hg(3)#6–Br(3)–Hg(1)	117.44(2)
  S(1)–Hg(1)–Br(3)	94.41(7)		Hg(1)#5–Br(3)–Hg(1)	102.36(8)
  Br(3)#2–Hg(1)–Br(3)	102.36(8)		Hg(3)#2–S(1)–Hg(3)	160.2(2)
  S(1)–Hg(2)–S(1)#3	179.8(2)		Hg(3)–S(1)–Hg(2)	94.62(12)
  S(1)#5–Hg(3)–S(1)	160.2(2)		Hg(3)–S(1)–Hg(1)	98.10(13)
  S(1)–Hg(3)–Br(3)#6	99.33(12)		Hg(2)–S(1)–Hg(1)	96.93(17)
  Hg(3)#6–Br(3)–Hg(3)#7	85.37(10)
  Symmetry transformations used to generate the equivalent atoms of 1: #1: –x, y, –z + 1; #2: x, y, z + 1; #3: x, –y, z; #4: x, –y, z + 1; #5: x, y, z – 1; #6: –x + 1/2, –y + 1/2, –z; #7: x – 1/2, –y + 1/2, z

  The powder XRD patterns were collected with a Rigaku DMAX 2500 diffractometer at 40 kV and 100 mA for CuKα radiation (λ = 1.5406 Å) with a scan speed of 5 º/min at room temperature. The simulated patterns were produced using the Mercury program and single-crystal reflection data.

  2.3 Infrared, UV-Vis-NIR diffuse reflectance spectroscopies and I–V characteristics

  The UV-Vis-NIR spectrum of 1 was recorded at room temperature on a computer-controlled PE Lambda 900 UV-Vis-NIR spectrometer equipped with an integrating sphere in the wavelength range of 200~1200 nm. A BaSO₄ plate was used as a reference. The absorption spectrum was calculated from the reflection spectrum by the Kubelka-Munk function^{[24, 25]}: α/S = (1 − R)²/2R, where α is the absorption coefficient, S is the scattering coefficient that is practically wavelength independent when the particle size is larger than 5 μm, and R is the reflectance. The IR spectrum was recorded by using a Nicolet Magana 750 FT-IR spectrophotometer in the range of 4000~450 cm^–1. No FT-IR absorption peaks of 1 are observed in the 4000~450 cm^–1 region (Fig. S2). The I–V characteristics of the single crystal of 1 with the dimensions of around 1.5 × 1.5 × 4.0 mm³ was characterized by using Keithley 4200-SCS semiconductor parameter analyzer. The measurements were made under a biased voltage in the range of –5~5 V. The wire terminal was connected to the bias voltage and the PCB electrical contact pad was connected to the ground. Single crystals were mounted with Ag paste by thermal evaporation method with wires as the current voltage electrodes.

  3. RESULTS AND DISCUSSION

  3.1 Structure determination

  Compound 1 belongs to a new phase of γ-Hg₃S₂Br₂ with orthorhombic space group Cmmm, with a = 9.1923(18), b = 18.2262(5), c = 4.6251(7) Å, V = 774.9(3) ų, Z = 4, D_c = 7.078 g/cm³, μ = 70.030 mm^–1, S = 1.072, (Δρ)_max = 1.963, (Δρ)_min = –1.903 e/ų, the final R = 0.0394 and wR = 0.1431. Its structure features a 3D framework constructed from [Hg₃S₂]²⁺ chains with Br⁻ anions acting as the linkages (Fig. 1).

  Figure 1

  

  Figure 1.  Crystal structure of 1 viewed down the c-axis

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  There are three crystallographically independent mercury atoms with different coordination geometries. As shown in Fig. 2, the Hg(1) atom is coordinated by two S(1) atoms with the S(1)–Hg(1)–S(1) angle of 165.9(2)^o and Hg(1)–S(1) bond length of 2.407(5) Å. Hg(2) is nearly linearly coordinated by two S(1) atoms with the Hg(2)–S(1) bond length of 2.381(5) Å. Two Hg(1), two Hg(2) and four S(1) atoms form a near square Hg₄S₄ with sulfur atoms located at the corners and mercury atoms close to the mid-point of edge. All the Hg₄S₄ squares are linked by Hg(3) atoms nearly linearly coordinated to two S(1) atoms of two parallel Hg₄S₄ squares to form one-dimensional infinite Hg₆S₄ chains along the c direction. It is notable here that the Hg(3) position is half occupied. That is to say, averagely, two in four sulfur atoms in the Hg₄S₄ square are bonded to the Hg(3) atom. Of course, there is possibility of superstructure, in which one half of Hg(3) positions is void and void Hg positions form a relative long-range order to multiply the current lattice. But no superstructure diffractions are found. Thereafter, occupancy of 0.5 is a statistical value. The fact is reasonable considering that S(1), Hg(1) and Hg(2) are almost equivalently located at the four corners and mid-point of edges of Hg₄S₄ squares, and edge lengths (S(1)–S(1) distances), 4.761(7) and 4.776 (7) Å, are very close to each other. No obvious anisotropy along the a and b directions exists, so Hg(3) atoms are prone to be occupancy disordered. No abnormal temperature factor of Hg(3) atoms is found, and thus the disorder should be statistical.

  Figure 2

  

  Figure 2.  Hg₄S₄ squares linked by Hg3 atoms (50% occupancy) to form a 1-D cubane chain extending along the c direction

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  The shortest distance between bromine atoms and Hg₆S₄ chains is 2.730(2) Å of Hg(3)–Br(3) pair. The value is longer than the expected covalent bond length of Hg(3)–Br(3) (2.63 Å), but shorter than the sum (~3.8 Å) of van der Waals radii of mercury and bromine atoms. So, electrostatic interaction between bromine atoms and Hg₆S₄ chains is expected, similar to the cases in other mercury-contained halide host-guest systems^[26]. The shortest Hg⋅⋅⋅Hg distance is 3.5839(13) Å, which falls in the range of the sum of van der Waals radii (3.4～4.0 Å)^[27] and comparable with those of other mercury compounds^{[28, 29]}, indicating the existence of weak Hg⋅⋅⋅Hg interaction, which may solidify the structure of the title compound.

  Two isomers of the title compound (γ phase), α- and β-Hg₃S₂Br₂, have been reported^[15]. The main difference between the structures of α and γ phases is that the occupancy disordered Hg(3) atoms in γ phase are ordered in α phase, in which the Hg positions between four pair sulfurs of neighboring Hg₄S₄ squares are either all fully occupied or all void, forming discrete cubane-like Hg₁₂S₈ units arranged along the b direction with equal intervals. C axis of γ phase is doubled in α phase, and the structure of α phase can be viewed as a type of superstructure of that of the γ phase. Superstructure-structure transformation may happen between α and γ phases under some driving forces.

  The structure of γ phase is significantly different from that of β-Hg₃S₂Br₂, which includes [SHg₃] pyramids, connected by Hg atoms into parallel endless zigzag chains which extend along the ab plane and form a complex cation _∞²[Hg₃S₂]²⁺. The energy barrier between γ and β phases is predicted to be larger than that between γ and α phases because no disorder can be found in the structure of β phase. The structure transformation between γ and β phases can only happen under strong driving forces.

  3.2 Optical properties

  Optical diffuse reflectance spectrum of 1 indicates optical band gap of 2.8 eV (Fig. 3), which agrees well with their yellow color of crystals. IR spectrum of the present compound shows no obvious absorption in the range of 4000~450 cm^–1 (Fig. S2), which supports the idea that the compound may be potentially used as infrared window material for laser delivery media and an infrared transmitter for optical fiber applications in telecommunications^{[27, 28]}.

  Figure 3

  

  Figure 3.  Diffuse reflection spectrum of 1

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  3.3 Thermogravimetric analyses (TGA)

  TGA analyses under a nitrogen atmosphere indicate that compound 1 is stable up to 230 ℃, and continuously loses weight up to about 430 ℃ (Fig. S3). The TGA displays a minor change peak at 368 ℃. The first weight loss of 52.29% can be attributed to the release of 1 molecule of HgBr₂ (calcd. 43.6%) and a small amount of HgS, and its thermal decomposition mid-products are characterized by energy-dispersive X-ray spectroscopy (Fig. S4 and S5). Above 368 ℃, it is further decomposed and continually loses weight up to about 430 ℃.

  3.4 I–V characteristics

  The room-temperature resistance for 1 is 1.11 × 10⁷ Ω (Fig. 4). According to the single-crystal's dimensions of 1.5 × 1.5 × 4.0 mm³, the resistivity of 1 is 6.27 × 10⁵ Ω⋅cm, which indicates that compound 1 is a semiconductor, which is in full agreement with the experimental observations described from diffuse reflection spectrum.

  Figure 4

  

  Figure 4.  I–V characteristics measured on a single crystal of 1

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  4. CONCLUSION

  In the present work, two Hg(1), two Hg(2) and four S(1) atoms form a near square Hg₄S₄ with sulfur atoms located at the corners and mercury atoms close to the mid-point of edges. All the Hg₄S₄ squares are linked by Hg(3) atoms nearly linearly coordinated to two S(1) atoms of two parallel Hg₄S₄ squares to form one-dimensional infinite Hg₆S₄ chains along the c direction. The results of diffuse reflectance spectrum and I–V characteristics indicate that compound 1 is a semiconductor with optical gap of 2.80 eV. Further exploration on similar systems is underway in our laboratory.

  
  
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• 

  Figure 1  Crystal structure of 1 viewed down the c-axis

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  Figure 2  Hg₄S₄ squares linked by Hg3 atoms (50% occupancy) to form a 1-D cubane chain extending along the c direction

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  Figure 3  Diffuse reflection spectrum of 1

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  Figure 4  I–V characteristics measured on a single crystal of 1

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  Table 1.  Bond Lengths (Å) and Bond Angles (°) for 1

  Bond	Dist.		Bond	Dist.
  Hg(1)–S(1)	2.407(5)		Hg(1)–Br(3)	2.9682(17)
  Hg(2)–S(1)	2.381(5)		Hg(3)–Br(3)#6	2.730(2)
  Hg(3)–S(1)	2.3475(9)
  Angle	(°)		Angle	(°)
  S(1)#1–Hg(1)–S(1)	165.9(2)		Hg(3)#6–Br(3)–Hg(1)	117.44(2)
  S(1)–Hg(1)–Br(3)	94.41(7)		Hg(1)#5–Br(3)–Hg(1)	102.36(8)
  Br(3)#2–Hg(1)–Br(3)	102.36(8)		Hg(3)#2–S(1)–Hg(3)	160.2(2)
  S(1)–Hg(2)–S(1)#3	179.8(2)		Hg(3)–S(1)–Hg(2)	94.62(12)
  S(1)#5–Hg(3)–S(1)	160.2(2)		Hg(3)–S(1)–Hg(1)	98.10(13)
  S(1)–Hg(3)–Br(3)#6	99.33(12)		Hg(2)–S(1)–Hg(1)	96.93(17)
  Hg(3)#6–Br(3)–Hg(3)#7	85.37(10)
  Symmetry transformations used to generate the equivalent atoms of 1: #1: –x, y, –z + 1; #2: x, y, z + 1; #3: x, –y, z; #4: x, –y, z + 1; #5: x, y, z – 1; #6: –x + 1/2, –y + 1/2, –z; #7: x – 1/2, –y + 1/2, z

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