Reinvestigation of the crystal structure of UF6 by single-crystal X-ray diffraction

Koch, B.-N.; Kraus, F.

doi:10.1107/S2414314626008898

inorganic compounds

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ISSN: 2414-3146

Volume 11| Part 8| August 2026| x260889

https://doi.org/10.1107/S2414314626008898

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Reinvestigation of the crystal structure of UF₆ by single-crystal X-ray diffraction

Björn-Niclas Koch ^a and Florian Kraus ^a ^*

^aAnorganische Chemie, Fluorchemie, Universität Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany
^*Correspondence e-mail: [email protected]

Edited by M. Weil, Vienna University of Technology, Austria (Received 17 August 2026; accepted 26 August 2026; online 28 August 2026)

Single-crystals of hexafluoridouranium(VI), UF₆, were obtained by sublimation. A re-refinement of the crystal structure using low-temperature (100 K) single-crystal X-ray data was carried out considering all U^ij terms of the displacement parameters of the U and F atoms, leading overall to a significantly improved structural model. UF₆ crystallizes in the orthorhombic space group Pnma with the F atoms forming a double hexagonal closest packing of ABAC stacking with U atoms in one-sixth of the voids. The UF₆ molecules deviate only slightly from octahedral point-group symmetry.

Keywords: crystal structure; uranium hexafluoride; double hexagonal packing; actinoid fluoride; redetermination.

CCDC reference: 2580701

Structure description

Uranium hexafluoride is likely the most important material in the nuclear energy sector for isotope enrichment of the fissile isotope ²³⁵U. UF₆ was probably first synthesized by Henri Moissan (1900 ) by reacting fluorine gas with uranium metal. However, at that time he was unable to further characterize the resulting white smoke. Later, Otto Ruff and coworkers succeeded in obtaining UF₆ and describing its properties (Ruff, 1909 ; Ruff & Heinzelmann, 1911 ). Ruff reported shiny, colorless, crystals with monoclinic symmetry without giving further details (Ruff, 1909). Reports on the crystal structure of UF₆ appeared well after the Manhattan Project. The first one, a single-crystal X-ray diffraction study of a crystal mounted in a glass capillary (Hoard & Stroupe, 1958 ) led to lattice parameters a = 9.900 (2), b = 8.962 (2), c = 5.207 (2) Å, V = 462.0 Å³ at T = 298 K in space group Pnma (no R values reported). In 1973, the space group and structure model were confirmed by powder neutron diffraction at 294 K using the previously reported lattice parameters (Taylor et al., 1973 ) without giving R values. In 1975, another structural study of UF₆ at 193 and 293 K by powder neutron diffraction was carried out (Taylor & Wilson, 1975 ), confirming that it has orthorhombic symmetry at both temperatures [a = 9.843 (11), b = 8.920 (10), c = 5.173 (6) Å, V = 454.2 (8) Å³ at T = 193 K, R = Σ(|I_o – I_c|)/ΣI_o = 0.081; a = 9.924 (10), b = 8.954 (9), c = 5.198 (5) Å, V = 461.9 (8) Å³ at T = 293 K, R = Σ(|I_o – I_c|)/ΣI_o = 0.133]. Levy et al. (1976 ) reported a single-crystal neutron diffraction study at 293 K with a = 9.92 (5), b = 8.97 (5), c = 5.22 (3) Å (V not reported), and gave an R value of 0.041. The latest report refers to a powder neutron diffraction study at 77 K, which led to lattice parameters a = 9.654 (3), b = 8.776 (4), c = 5.084 (3) Å, V = 430.7 (3) Å³ and an R = Σ(|y_o – y_c|)/Σy_o value (y_o and y_c are the background-corrected pattern intensities) of 0.069 (Levy et al., 1983 ).

As all of these structure refinements, with the exception of the single-crystal neutron diffraction study by Levy et al. (1976), used isotropic displacement parameters for all atoms, we reinvestigated the crystal structure of UF₆ based on single-crystal X-ray diffraction data at 100 K. Refinement of all U^ij terms of the displacement parameters of the U and the four F atoms allowed for a more precise structural model.

The U atom (multiplicity 4, Wyckoff letter c, site symmetry .m.) is surrounded by six F atoms (F1 to F4) in a slightly distorted octahedral arrangement (Fig. 1, Table 1). The F1 and the F2 atoms likewise reside on a mirror plane (4 c, .m.), while the F3 and F4 atoms occupy a general position (8 d, 1). In the previous studies, the U—F bond lengths were reported to range from 1.88 (2) to 2.28 (4) Å with F—U—F angles from 88.0 (17) to 92.9 (17)° (Taylor et al., 1973), from 1.95 (1) to 2.03 (2) Å at 193 K with angles from 86.4 (7) to 92.2 (6)°, from 1.86 (3) to 1.99 (2) Å with angles from 83.7 (9) to 95.1 (8)° (Taylor & Wilson, 1975), from 1.992 (3) to 2.004 (4) Å with angles from 89.42 (17) to 90.20 (11)° (Levy et al., 1976), and as mean bond lengths of 2.023 (6) Å (77 K), 1.983 (6) Å (193 K), and 1.995 (2) Å (293 K) without reporting the bond angles (Levy et al., 1983). It is interesting to note that the longest bond lengths were observed at the lowest temperature, underlining the need for a redetermination of the crystal structure. The U—F bond lengths determined in the present work are much more uniform, and the F—U—F bond angles are closer to the ideal values of 90 and 180° than determined in previous studies (Table 1). The deviations of the UF₆ molecule from O_h symmetry are thus small; its crystallographically imposed point symmetry is C_s (m).

Table 1
Selected geometric parameters (Å, °)

U1—F1	1.983 (4)	U1—F3ⁱ	1.984 (3)
U1—F2	1.982 (4)	U1—F4	1.975 (3)
U1—F3	1.984 (3)	U1—F4ⁱ	1.975 (3)

F1—U1—F3	90.12 (11)	F4ⁱ—U1—F2	89.92 (11)
F2—U1—F1	179.95 (14)	F4—U1—F2	89.92 (11)
F2—U1—F3	89.91 (11)	F4—U1—F3	90.20 (13)
F4ⁱ—U1—F1	90.05 (11)	F4—U1—F3ⁱ	179.56 (11)
Symmetry code: (i) $[x, -y+{\script{1\over 2}}, z]$ .

Figure 1
UF₆ molecule in the solid state. Atoms are shown with anisotropic displacement ellipsoids at the 70% probability level; the symmetry code refers to Table 1

The unit-cell parameters determined in this study (Table 2) are in good agreement with those given above, especially with those determined at 77 K (Levy et al., 1983) which are a little smaller, as expected.

Table 2
Experimental details

Crystal data
Chemical formula	UF₆
M_r	352.02
Crystal system, space group	Orthorhombic, Pnma
Temperature (K)	100
a, b, c (Å)	9.6612 (8), 8.7786 (7), 5.0854 (4)
V (Å³)	431.30 (6)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	37.66
Crystal size (mm)	0.3 × 0.2 × 0.1

Data collection
Diffractometer	Stoe IPDS II
Absorption correction	Multi-scan (LANA; Koziskova et al., 2016 )
T_min, T_max	0.0002, 0.001
No. of measured, independent and observed [I > 2σ(I)] reflections	6472, 784, 681
R_int	0.042
(sin θ/λ)_max (Å⁻¹)	0.743

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.022, 0.054, 1.03
No. of reflections	784
No. of parameters	38
Δρ_max, Δρ_min (e Å⁻³)	2.37, −1.68
Computer programs: X-AREA (Folkers-Karlsson et al., 2026 ), SHELXT (Sheldrick, 2015a ), SHELXL (Sheldrick, 2015b ), DIAMOND (Brandenburg, 2022 ), publCIF (Westrip, 2010 ) and OLEX2 (Dolomanov et al., 2009 ).

In the crystal structure, a single UF₆ molecule is surrounded by twelve others in the shape of a distorted anticuboctahedron if U⋯U distances from 5.0854 (4) to 5.1402 (4) Å are considered. Thus, the U atoms adopt a distorted arrangement similar to the Mg atoms in the Mg structure type. In a more detailed description, the U atoms occupy one-sixth of the octahedral voids of hexagonally close-packed layers of F atoms. The layers are parallel to the bc plane, with a stacking sequence along the a-axis direction of ABAC, representing a double hexagonal packing, hc. The close-packing of F atoms is only slightly distorted, as evidenced by the F⋯F distances. Those within a UF₆ molecule range from 2.791 (6) to 2.808 (4) Å, while the intermolecular distances are only slightly larger in the range from 2.900 (6) to 3.119 (6) Å. The crystal structure of UF₆ is shown in Fig. 2.

Figure 2
Crystal structure of UF₆ viewed along the c axis. Atoms are drawn as spheres with arbitrary radii. The z coordinates of the U atoms are indicated.

Synthesis and crystallization

UF₆ of natural isotope ratio was synthesized according to a literature procedure (Chemnitz et al., 2021 ), and single crystals were obtained by slow sublimation at room temperature.

Refinement

Details of the data collection and structure refinement are given in Table 2.

Structural data

CCDC reference: 2580701

Crystal structure: contains datablock I. DOI: https://doi.org/10.1107/S2414314626008898/wm4259sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2414314626008898/wm4259Isup2.hkl

checkCIF report

Computing details top

Hexafluoridouranium(VI) top

Crystal data top

UF₆	D_x = 5.421 Mg m⁻³
M_r = 352.02	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pnma	Cell parameters from 9312 reflections
a = 9.6612 (8) Å	θ = 4.0–32.3°
b = 8.7786 (7) Å	µ = 37.66 mm⁻¹
c = 5.0854 (4) Å	T = 100 K
V = 431.30 (6) Å³	Plate, clear colourless
Z = 4	0.3 × 0.2 × 0.1 mm
F(000) = 584

Data collection top

Stoe IPDS II diffractometer	784 independent reflections
Radiation source: sealed X-ray tube, 12 x 0.4 mm long-fine focus, X-ray tube	681 reflections with I > 2σ(I)
Planar graphite monochromator	R_int = 0.042
Detector resolution: 6.67 pixels mm^-1	θ_max = 31.9°, θ_min = 4.2°
rotation method, ω scans	h = −14→13
Absorption correction: multi-scan (LANA; Koziskova et al., 2016)	k = −13→13
T_min = 0.0002, T_max = 0.001	l = −7→7
6472 measured reflections

Refinement top

Refinement on F²	Primary atom site location: dual
Least-squares matrix: full	w = 1/[σ²(F_o²) + (0.0372P)²] where P = (F_o² + 2F_c²)/3
R[F² > 2σ(F²)] = 0.022	(Δ/σ)_max < 0.001
wR(F²) = 0.054	Δρ_max = 2.37 e Å⁻³
S = 1.03	Δρ_min = −1.68 e Å⁻³
784 reflections	Extinction correction: SHELXL-2019/2 (Sheldrick, 2015b), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
38 parameters	Extinction coefficient: 0.0038 (4)
0 restraints

Special details top

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 (Å²) top

	x	y	z	U_iso*/U_eq
U1	0.12887 (2)	0.250000	0.42275 (4)	0.01189 (11)
F1	0.0110 (4)	0.250000	0.7420 (8)	0.0199 (9)
F2	0.2469 (4)	0.250000	0.1039 (7)	0.0167 (8)
F3	0.0096 (2)	0.0910 (3)	0.2627 (6)	0.0174 (5)
F4	0.2471 (3)	0.0906 (4)	0.5800 (5)	0.0180 (6)

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
U1	0.01144 (14)	0.01073 (13)	0.01350 (14)	0.000	−0.00002 (8)	0.000
F1	0.0201 (17)	0.022 (2)	0.018 (2)	0.000	0.0037 (16)	0.000
F2	0.0174 (16)	0.016 (2)	0.0164 (18)	0.000	0.0033 (14)	0.000
F3	0.0180 (11)	0.0131 (14)	0.0212 (13)	−0.0024 (11)	−0.0012 (11)	−0.0027 (11)
F4	0.0177 (11)	0.0158 (15)	0.0206 (14)	0.0025 (11)	−0.0004 (10)	−0.0010 (12)

Geometric parameters (Å, º) top

U1—U1ⁱ	5.1402 (4)	U1—F3	1.984 (3)
U1—U1ⁱⁱ	5.1072 (4)	U1—F3^iv	1.984 (3)
U1—U1ⁱⁱⁱ	5.0854 (4)	U1—F4	1.975 (3)
U1—F1	1.983 (4)	U1—F4^iv	1.975 (3)
U1—F2	1.982 (4)

F1—U1—F3^iv	90.12 (11)	F4^iv—U1—F2	89.92 (11)
F1—U1—F3	90.12 (11)	F4—U1—F2	89.92 (11)
F2—U1—F1	179.95 (14)	F4—U1—F3	90.20 (13)
F2—U1—F3	89.91 (11)	F4^iv—U1—F3^iv	90.20 (13)
F2—U1—F3^iv	89.91 (11)	F4^iv—U1—F3	179.56 (11)
F3^iv—U1—F3	89.39 (17)	F4—U1—F3^iv	179.56 (11)
F4^iv—U1—F1	90.05 (11)	F4^iv—U1—F4	90.21 (18)
F4—U1—F1	90.05 (11)

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

Acknowledgements

We thank Dr Sergei Ivlev, Philipps-Universität Marburg, for his support with the X-ray diffraction measurements.

Funding information

Funding for this research was provided by: Deutsche Forschungsgemeinschaft (grant No. KR3595/10-1).

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