A new semiconducting quaternary mixed halogenide: penta­thallium dimercury penta­bromide tetra­iodide, Tl₅Hg₂Br₅I₄

A novel quaternary mixed halogenide, Tl₅Hg₂Br₅I₄, was synthesized by fusion of thallium bromide and mercury iodide in a 5:2 molar ratio. The crystal structure of Tl₅Hg₂Br₅I₄ represents a new series of composite structures described by the general formula nTlBr*mTl₂[HgBr₂I₂]; in this case, n = 4 and m = 8. Electronic structure calculations indicate that the title compound is a semiconductor.

S1. Introduction

Radiation detectors are critical for high-energy physics, radiochemistry, biomedical imaging and security applications (Li et al., 2012). Despite that, wide-gap and heavy-density semiconductors provide higher energy resolution than scintillator materials (Milbrath et al., 2008; Owens et al., 2004), only a few materials are currently used. We describe herein a new quaternary phase semiconductor.

S2. Experimental

S2.1. Synthesis and crystallization

Thallium bromide was produced by reaction of commercially available TlNO₃ with an aqueous solution of hydro­bromic acid. The obtained precipitate was purified by 30-fold zone melting and subsequent crystallization in a two-zone Bridgman growth furnace at the rate of 20 mm per day. The obtained single crystal was transparent and yellow. Commercial mercury iodide of a starting purity of 99.9 wt% was additionally purified twice by sublimation in an evacuated quartz ampoule under static conditions. The synthesis was performed in an evacuated quartz ampoule in a shaft-type furnace. The batch mass was 1.5 g. The maximum synthesis temperature was 770 K with a 6 h exposure, followed by slow cooling at the rate of 10 K h^-1 to room temperature. The obtained alloy is a compact ingot of orange–red colour.

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The structure was initally solved in the space group Fmmm by direct methods. Based on the observed additional symmetry elements using PLATON (Spek, 2009) and checkCIF (https://checkcif.iucr.org/), the space group was corrected to I4/mcm.

S3. Results and discussion

Single-crystal X-ray structure analysis shows that the title compound crystallizes in a unique structure. The unit cell and coordination polyhedra of seven crystallographically distinct atoms are shown in Fig. 1. The Tl1 atom (Wyckoff position 8h, site symmetry m.2 m) is surrounded by eight Br atoms, which form a bicapped trigonal prism [Tl1Br₈]. The coordination polyhedra of atom Tl2 (Wyckoff position 8f, site symmetry 4..) is a [Tl2Br₅I₄] monocapped tetra­gonal anti­prism, with a coordination number (CN) of 9. The tetra­gonal anti­prism [Tl3I₈], typical for thallium, occupies Wyckoff position 4a, with site symmetry 422. The Hg atom (Wyckoff position 8g, site symmetry 2.mm) has tetra­hedral coordination, i.e. [HgBr₂I₂]. The Br1 atom (in 16l, site symmetry ..m) is enclosed within a strongly distorted [Br1Tl₅Hg] o­cta­hedron. For the Br2 atom (in 4c, site symmetry 4/m..), the o­cta­hedron is formed only from Tl atoms, i.e. [Br2Tl₆]. The strongly distorted o­cta­hedron around the I atom (in 16l, site symmetry ..m) consists of four Tl and two Hg atoms, i.e. [ITl₄Hg₂]. The packing of the polyhedra formed by the Br and I atoms, surrounded by Tl and Hg atoms, is shown in Fig. 2. The Tl1 polyhedra at z = 0 and z = 1/2 share common vertices with four bicapped trigonal prisms. The Tl2 and Tl3 polyhedra are connected by shared edges. The packing of the [Br2Tl₆] o­cta­hedra and distorted [ITl₄Hg₂] o­cta­hedra is shown in Fig. 3. The packing of [Tl3I₈] tetra­gonal anti­prisms, [HgBr₂I₂] tetra­hedra and [Br2Tl₆] o­cta­hedra are presented in Fig. 4.

The Tl₅Hg₂Br₅I₄ quaternary phase represents a new structure type without analogues among previously studied compounds. In Tl₄HgBX₆ (X = Br, I), the Tl atom is enclosed in a bicapped trigonal prism (Brodersen et al., 1973; Kennedy et al., 1990), similar to the situation in the title phase but exhibiting a different coordination for mercury. The observed tetra­hedral coordination of mercury in the title compound is more similar to that reported for HgIn₂Te₄ (Grushka et al., 1982).

The electronic structure of the title compound was calculated using the tight-binding linear muffin-tin orbital (TB–LMTO) method in the atomic spheres approximation (TB–LMTO–ASA; Andersen, 1975; Andersen & Jepsen, 1984; Andersen et al., 1985, 1986), using the experimental crystallographic data. The exchange and correlation were inter­preted in the local density approximation (von Barth & Hedin, 1972).

The maximum electron localization function (ELF = 0.8) is around each halogen atom (Fig. 5). The minimum electron localization is observed around the Hg atom, indicating the shift of the ELF from the Hg to a halogen atom. This can be inter­preted as an ionic bonding and suggests that the Hg and halogen atoms form [HgBr₂I₂]^2- anions which compensate the positive charge of two [Tl]⁺ cations. The space of the unit cell of the title compound is filled by eight Tl₂[HgBr₂I₂] structural fragments and four TlBr (Tl⁺Br^-) structural fragments which are located at z = 0 and z = 1/2. The packing of these structural fragments is shown in Fig. 6. Thus, the title compound represents a new series of composite structures that can be described by the general formula nAX*mA₂[BX₄], where for the title phase phase, A = Tl, B = Hg and X = Br, I. For the title compound, the formula can be written as nTlBr*mTl₂[HgBr₂I₂], where n = 4 and m = 8:

4TlBr + 8Tl₂[HgBr₂I₂] = Tl₂₀Hg₈Br₂₀I₁₆ = 4Tl₅Hg₂Br₅I₄.

The calculated total and partial density of states (DOS) for Tl₅Hg₂Br₅I₄ (Fig. 7a), in the region below E_F in the conducting band exhibits a significant gap extending from 0 to 1.3 eV, suggesting that the title compound is a semiconductor.

The crystal orbital Hamilton population (COHP) curves (Fig. 7b) indicates that the strongest inter­actions in the [HgBr₂I₂] tetra­hedron are between Hg and Br atoms (δ = 2.7469 Å and -iCOHP = 1.551 eV), and between Hg and I atoms (δ = 2.7114 Å and -iCOHP = 2.048 eV). The inter­actions between Tl and halogen atoms in the tetra­gonal anti­prism are weak (δ = 3.3007 Å and -iCOHP = 0.515 eV) in comparison.

Related literature

For related literature, see: Andersen (1975); Andersen & Jepsen (1984); Andersen et al. (1985, 1986); von Barth & Hedin (1972); Brodersen et al. (1973); Grushka et al. (1982); Kennedy et al. (1990); Li et al. (2012); Milbrath et al. (2008); Owens & Peacock (2004); Spek (2009).

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis CCD (Oxford Diffraction, 2008); data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

Fig. 1. The projection of the unit cell and the coordination polyhedra of the atoms of Tl₅Hg₂Br₅I₄.

Fig. 2. The packing of the polyhedra of thallium (bicapped trigonal prisms and tetragonal antiprisms) and Hg atoms (tetrahedra) of Tl₅Hg₂Br₅I₄.

Fig. 3. The packing of the [Br2Tl₆] octahedra and distorted [ITl₄Hg₂] octahedra of Tl₅Hg₂Br₅I₄.

Fig. 4. The mutual packing of [Tl3I₈] tetragonal antiprisms, [HgBr₂I₂] tetrahedra and [Br2Tl₆] octahedra in Tl₅Hg₂Br₅I₄.

Fig. 5. (a) The electron localization function and (b) isosurface from electronic structure calculations for Tl₅Hg₂Br₅I₄.

Fig. 6. The mode of packing of the TlBr and Tl₂[HgBr₂I₂] structural units in Tl₅Hg₂Br₅I₄.

Fig. 7. (a) The electronic density of states (DOS) and (b)/(c) the crystal orbital Hamilton population for Tl₅Hg₂Br₅I₄.

Pentathallium dimercury pentabromide tetraiodide

Crystal data

Tl5Hg2Br5I4	Dx = 6.986 Mg m^−3
Mr = 2330.18	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I4/mcm	Cell parameters from 804 reflections
Hall symbol: -I 4 2c	θ = 3.4–29.3°
a = 8.5611 (6) Å	µ = 64.61 mm^−1
c = 30.227 (2) Å	T = 293 K
V = 2215.4 (3) Å^3	Plate, red
Z = 4	0.08 × 0.07 × 0.01 mm
F(000) = 3808.0

Data collection

Oxford Diffraction Xcalibur3 CCD diffractometer	804 independent reflections
Radiation source: fine-focus sealed tube	725 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.068
Detector resolution: 0 pixels mm^-1	θmax = 29.3°, θmin = 3.4°
ω scans	h = −10→11
Absorption correction: analytical (CrysAlis RED; Oxford Diffraction, 2008)	k = −10→11
Tmin = 0.101, Tmax = 0.517	l = −38→38
6153 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.027	w = 1/[σ^2(Fo^2) + (0.0392P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.064	(Δ/σ)max = 0.001
S = 0.99	Δρmax = 2.22 e Å^−3
804 reflections	Δρmin = −1.98 e Å^−3
29 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.000074 (13)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Tl1	0.35001 (4)	0.14999 (4)	0.0000	0.02175 (17)
Tl2	0.0000	0.0000	0.109198 (17)	0.02532 (17)
Tl3	0.0000	0.0000	0.2500	0.0300 (2)
Hg1	0.0000	0.5000	0.141681 (16)	0.02206 (17)
I1	−0.18966 (5)	0.31034 (5)	0.18938 (2)	0.02205 (18)
Br1	0.15687 (6)	0.34313 (6)	0.07602 (3)	0.0169 (2)
Br2	0.5000	0.5000	0.0000	0.0171 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Tl1	0.0170 (2)	0.0170 (2)	0.0313 (3)	0.00055 (17)	0.000	0.000
Tl2	0.0241 (2)	0.0241 (2)	0.0278 (3)	0.000	0.000	0.000
Tl3	0.0303 (3)	0.0303 (3)	0.0295 (4)	0.000	0.000	0.000
Hg1	0.0226 (2)	0.0226 (2)	0.0210 (3)	−0.00604 (17)	0.000	0.000
I1	0.0193 (2)	0.0193 (2)	0.0276 (3)	−0.0012 (2)	0.00362 (15)	0.00362 (15)
Br1	0.0145 (3)	0.0145 (3)	0.0216 (5)	0.0001 (3)	0.0015 (2)	−0.0015 (2)
Br2	0.0133 (5)	0.0133 (5)	0.0248 (9)	0.000	0.000	0.000

Geometric parameters (Å, º)

Tl1—Br2	3.2600 (3)	Hg1—I1	2.7113 (6)
Tl1—Br2^i	3.2600 (3)	Hg1—Br1	2.7472 (9)
Tl1—Br1^ii	3.2784 (9)	Hg1—Br1^iv	2.7472 (9)
Tl1—Br1	3.2784 (9)	Br2—Tl1^v	3.2600 (3)
Tl1—Tl1^iii	3.6319 (10)	Br2—Tl1^vi	3.2600 (3)
Hg1—I1^iv	2.7113 (6)	Br2—Tl1^vii	3.2600 (3)
Br2—Tl1—Br2^i	136.393 (15)	I1—Hg1—Br1	112.593 (11)
Br2—Tl1—Br1^ii	74.637 (8)	I1^iv—Hg1—Br1^iv	112.593 (11)
Br2^i—Tl1—Br1^ii	74.637 (8)	I1—Hg1—Br1^iv	112.593 (11)
Br2—Tl1—Br1	74.637 (8)	Br1—Hg1—Br1^iv	87.48 (4)
Br2^i—Tl1—Br1	74.637 (8)	Hg1—Br1—Tl1	178.24 (3)
Br1^ii—Tl1—Br1	88.99 (3)	Tl1—Br2—Tl1^v	180.000 (5)
Br2—Tl1—Tl1^iii	111.804 (8)	Tl1—Br2—Tl1^vi	90.0
Br2^i—Tl1—Tl1^iii	111.804 (8)	Tl1^v—Br2—Tl1^vi	90.0
Br1^ii—Tl1—Tl1^iii	135.503 (16)	Tl1—Br2—Tl1^vii	90.0
Br1—Tl1—Tl1^iii	135.503 (16)	Tl1^v—Br2—Tl1^vii	90.0
I1^iv—Hg1—I1	115.76 (3)	Tl1^vi—Br2—Tl1^vii	180.0
I1^iv—Hg1—Br1	112.593 (11)

Symmetry codes: (i) x−1/2, −y+1/2, −z; (ii) x, y, −z; (iii) −x+1, −y, −z; (iv) −x, −y+1, z; (v) −x+1, −y+1, −z; (vi) y, −x+1, −z; (vii) −y+1, x, z.