Rerefinement of the crystal structure of α-ThBr₄

Single crystals of α-ThBr₄, thorium(IV) tetra­bromide, were obtained as a side product from the reaction of CuBr with β-ThBr₄ at 753 K. In the crystal structure, the Th atom (site symmetry ..) is surrounded by eight Br atoms in the form of a tetra­gonal-disphenoidal coordination polyhedron. The connectivity of these polyhedra is ³_∞[ThBr_4/2Br_4/2]. In comparison with the previous crystal structure refinement [Mason et al. (1974). J. Less-Common Met. 35, 331–338], the current rerefinement resulted in much higher preciscion of the lattice parameters and the atomic coordinates.

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Structure description

A crystal of ThBr₄ in its α-modification was isolated as a side product from the reaction of β-ThBr₄ with CuBr at 753 K.

The crystal structure of α-ThBr₄ has been described only once, from a single-crystal X-ray diffraction study at room temperature (Mason et al., 1974), where the authors refer to this modification also as the low-temperature polymorph. They reported the transition temperature at 699 ± 5 K and the crystal structure of α-ThBr₄ was assigned to the α-ThCl₄ structure type in the space group I4₁/a (No. 88, tI20). A comparison of the structural parameters of the original crystal structure refinement and of the current rerefinement is given in Table 1.

Table 1
Comparison of structural parameters of α-ThBr₄ resulting from the current and previous crystal structure refinements

	This work	Mason et al. (1974)
a (Å)	6.7068 (2)	6.737 (1)
c (Å)	13.5792 (6)	13.601 (3)
x, y, z Th	0, 1/4, 1/8	0, 1/4, 1/8
x, y, z Br	0.33880 (6), 0.47423 (6), 0.20021 (3)	0.3378 (6), 0.4727 (7), 0.1998 (3)

Fig. 1 shows the crystal structure based on the current X-ray diffraction data. There is one Th atom (multiplicity 4, Wyckoff letter a, site symmetry ..) and one Br atom (16f, site symmetry 1) in the asymmetric unit. The Th atom is surrounded by eight Br atoms to form a tetra­gonal-disphenoidal coordination polyhedron. The Th—Br bond lengths of 4 × 2.9100 (4) Å and 4 × 3.0107 (4) Å are in good agreement with previously reported values of 2.909 and 3.020 Å (no s.u. values or temperature given; Mason et al., 1974), but different compared to those in β-ThBr₄ (space group I4₁/amd), with values of 2.85 and 3.12 Å (no s.u. values or temperature given; Brown et al., 1973). Each Br atom bridges two Th atoms, which results in edge-sharing polyhedra to form the crystal structure. The connection motif of α-ThBr₄ is similar to that in β-ThBr₄. Although the two polymorphs differ considerably with respect to the two pairs of Th—Br distances, the connectivities in both structures can be described with the Niggli formula ³_∞[ThBr_4/2Br_4/2]. The closest Th⋯Th distance of 4.77179 (12) Å in α-ThBr₄ is shorter compared to β-ThBr₄, with a value of 4.8774 Å (Brown et al., 1973). In the crystal structure of α-ThBr₄, each Th atom is surrounded by eight other Th atoms in the shape of an irreg­ular polyhedron, with Th⋯Th distances of 4 × 4.77179 (12) Å and 4 × 6.70680 (19) Å.

Figure 1 Crystal structure of α-ThBr4 in a projection along [010]. Displacement ellipsoids are drawn at the 90% probability level.

Synthesis and crystallization

All work was carried under an argon atmosphere (5.0, Praxair) using a fine-vacuum line and a glove-box (MBraun). Silica ampoules were flame-dried under dynamic fine vacuum (10⁻³ mbar; 1 bar = 10⁵ Pa) at least three times before use. Aluminium bromide (Alfa Aesar, 98%) was sublimed in vacuo before use; β-ThBr₄ was prepared according to a literature protocol (Deubner et al., 2017).

A silica glass ampoule was loaded with β-ThBr₄ (149 mg, 0.27 mmol) and CuBr (78 mg, 54 mmol, 2.01 equiv.), and sealed under vacuum. The ampoule was heated in a furnace to 753 K at a rate of 1 K min⁻¹ and kept at this temperature for 480 h for the reaction to take place. Afterwards, it was cooled to 330 K at a rate of 50 K d⁻¹. Several colourless crystals of α-ThBr₄ were obtained.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2.

Table 2
Experimental details

Crystal data
Chemical formula	ThBr4
Mr	551.68
Crystal system, space group	Tetragonal, I41/a
Temperature (K)	100
a, c (Å)	6.7068 (2), 13.5792 (6)
V (Å^3)	610.81 (5)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	50.43
Crystal size (mm)	0.15 × 0.15 × 0.14
Data collection
Diffractometer	Bruker D8 QUEST
Absorption correction	Numerical (SADABS; Krause et al., 2015)
Tmin, Tmax	0.016, 0.078
No. of measured, independent and observed [I > 2σ(I)] reflections	9305, 463, 463
Rint	0.049
(sin θ/λ)max (Å^−1)	0.715
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.021, 0.053, 1.37
No. of reflections	463
No. of parameters	13
Δρmax, Δρmin (e Å^−3)	1.16, −1.72
Computer programs: APEX3 and SAINT (Bruker, 2019), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b), DIAMOND (Brandenburg, 2022) and publCIF (Westrip, 2010).

Structural data

Computing details

Data collection: APEX3 (Bruker, 2019); cell refinement: SAINT (Bruker, 2019); data reduction: SAINT (Bruker, 2019); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: DIAMOND (Brandenburg, 2022); software used to prepare material for publication: publCIF (Westrip, 2010).

alpha-Thorium(IV) tetrabromide

Crystal data

ThBr4	Melting point: 200 K
Mr = 551.68	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I41/a	Cell parameters from 9713 reflections
a = 6.7068 (2) Å	θ = 3.0–30.6°
c = 13.5792 (6) Å	µ = 50.43 mm^−1
V = 610.81 (5) Å^3	T = 100 K
Z = 4	Block, colorless
F(000) = 920	0.15 × 0.14 × 0.14 mm
Dx = 5.999 Mg m^−3

Data collection

Bruker D8 QUEST diffractometer	463 independent reflections
Radiation source: Incoatec Microfocus	463 reflections with I > 2σ(I)
Multi layered optics monochromator	Rint = 0.049
Detector resolution: 10.42 pixels mm^-1	θmax = 30.5°, θmin = 5.2°
φ and ω scans	h = −9→9
Absorption correction: numerical (SADABS; Krause et al., 2015)	k = −9→9
Tmin = 0.016, Tmax = 0.078	l = −19→19
9305 measured reflections

Refinement

Refinement on F^2	Primary atom site location: dual
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.021	w = 1/[σ^2(Fo^2) + (0.0219P)^2 + 6.5321P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.053	(Δ/σ)max < 0.001
S = 1.37	Δρmax = 1.16 e Å^−3
463 reflections	Δρmin = −1.72 e Å^−3
13 parameters	Extinction correction: SHELXL (Sheldrick, 2015b), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0052 (4)

Special details

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

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

	x	y	z	Uiso*/Ueq
Th1	0.000000	0.250000	0.125000	0.00764 (15)
Br1	0.33880 (6)	0.47423 (6)	0.20021 (3)	0.00953 (16)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Th1	0.00844 (17)	0.00844 (17)	0.00604 (19)	0.000	0.000	0.000
Br1	0.0101 (2)	0.0104 (2)	0.0080 (2)	−0.00131 (13)	−0.00114 (13)	0.00179 (13)

Geometric parameters (Å, º)

Th1—Br1	2.9100 (4)	Th1—Br1^iv	3.0107 (4)
Th1—Br1^i	2.9100 (4)	Th1—Br1^v	3.0107 (4)
Th1—Br1^ii	2.9100 (4)	Th1—Br1^vi	3.0107 (4)
Th1—Br1^iii	2.9100 (4)	Th1—Br1^vii	3.0107 (4)
Br1—Th1—Br1^i	138.907 (16)	Br1—Th1—Br1^vi	72.606 (12)
Br1—Th1—Br1^ii	97.075 (5)	Br1^i—Th1—Br1^vi	75.260 (8)
Br1^i—Th1—Br1^ii	97.075 (5)	Br1^ii—Th1—Br1^vi	72.605 (8)
Br1—Th1—Br1^iii	97.076 (5)	Br1^iii—Th1—Br1^vi	148.466 (14)
Br1^i—Th1—Br1^iii	97.075 (5)	Br1^iv—Th1—Br1^vi	128.427 (10)
Br1^ii—Th1—Br1^iii	138.907 (16)	Br1^v—Th1—Br1^vi	75.934 (16)
Br1—Th1—Br1^iv	148.466 (14)	Br1—Th1—Br1^vii	72.605 (8)
Br1^i—Th1—Br1^iv	72.605 (8)	Br1^i—Th1—Br1^vii	148.466 (14)
Br1^ii—Th1—Br1^iv	72.606 (12)	Br1^ii—Th1—Br1^vii	75.260 (8)
Br1^iii—Th1—Br1^iv	75.260 (8)	Br1^iii—Th1—Br1^vii	72.606 (12)
Br1—Th1—Br1^v	75.260 (8)	Br1^iv—Th1—Br1^vii	75.934 (16)
Br1^i—Th1—Br1^v	72.606 (12)	Br1^v—Th1—Br1^vii	128.427 (10)
Br1^ii—Th1—Br1^v	148.466 (14)	Br1^vi—Th1—Br1^vii	128.427 (10)
Br1^iii—Th1—Br1^v	72.605 (8)	Th1—Br1—Th1^vi	107.394 (12)
Br1^iv—Th1—Br1^v	128.427 (10)

Symmetry codes: (i) −x, −y+1/2, z; (ii) y−1/4, −x+1/4, −z+1/4; (iii) −y+1/4, x+1/4, −z+1/4; (iv) −y+1/4, x−1/4, z−1/4; (v) x−1/2, y, −z+1/2; (vi) −x+1/2, −y+1/2, −z+1/2; (vii) y−1/4, −x+3/4, z−1/4.