UF4 and the High-Pressure Polymorph HP-UF4

Article abstract of DOI:10.1002/chem.201900639

A laboratory-scale synthesis of UF₄ is presented that utilizes the reduction of UF₆ with sulfur in anhydrous hydrogen fluoride. An excess of sulfur can be removed by vacuum sublimation, yielding pure UF₄, as shown by powde

Full text of DOI:10.1002/chem.201900639

A Journal of
Accepted Article
Title: UF4 and the High-Pressure Polymorph HP-UF4
Authors: Benjamin Scheibe, Jörn Bruns, Gunter Heymann, Malte
Sachs, Antti Karttunen, Clemens Pietzonka, Sergei Ivlev,
Hubert Huppertz, and Florian Kraus
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To be cited as: Chem. Eur. J. 10.1002/chem.201900639
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UF
4
and the High-Pressure Polymorph HP-UF
4
[a]
[b]
[c]
[a]
[d]
Benjamin Scheibe , Jörn Bruns , Gunter Heymann , Malte Sachs , Antti J. Karttunen , Clemens
[a]
[a]
[c]
[a]
Pietzonka , Sergei I. Ivlev , Hubert Huppertz , and Florian Kraus*
Abstract: A laboratory-scale synthesis of UF
utilizes the reduction of UF
An excess of sulfur can be removed by vacuum sublimation, yielding
pure UF , as shown by powder X-ray diffraction, micro X-ray
4
is presented that
4
Furthermore, enriched UF can be used in Generation IV reactors,
6
with sulfur in anhydrous hydrogen fluoride. most notably in Molten Salt Reactors (MSR) or in the advanced
Molten Salt Fast Reactors (MSFR).[8,9]
4
Various methods for the synthesis and technical production of UF
4
have been reported and extensively reviewed.[
3,4,10]
Thus, only a
fluorescence analysis, infrared and Raman spectroscopy, as well as
magnetic measurements. Furthermore, a single-crystalline, high-
short overview will be given here. In principle, there are two main
approaches for the synthesis of UF , a wet and a dry chemical
pressure modification of UF
4
was obtained in a multi-anvil press at
was
4
elevated temperatures. The high-pressure polymorph HP-UF
4
process. The first is based on the reduction of a uranyl(VI) salt in
aqueous medium to U(IV), followed by the precipitation of uranium
tetrafluoride hydrates upon addition of hydrofluoric acid. Suitable
reducing agents are for example tin(II) chloride, sodium dithionite,
characterized by means of single-crystal and powder X-ray diffraction,
as well as by magnetic measurements, and presents a novel crystal
structure type. Quantum-chemical calculations show the HP-
modification to be 10 kJ/mol per formula unit higher in energy
Na
obtain neat UF
or in a stream of hydrogen fluoride at temperatures of up to 500 °C.
So, due to hydrolysis (by the H O molecules of crystallization),
2
S
2
O
4
, or sulfur dioxide in the presence of Cu2+ ions.[10–12] To
, the UF hydrates have to be dried either in vacuo
compared to UF
4
.
4
4
2
usually UO
separated from UF .
4
2
is obtained as a side product that cannot be easily
Introduction
[3,10]
4
The most important process for the production of UF is probably
The first synthesis of uranium tetrafluoride was likely published in
(Equation 2).[3,6]
the dry-chemical hydrofluorination of UO
2
1
3 8 4
861, starting from U O and hydrofluoric acid, yielding UF and
UO2
+
4 HF
UF4
+
2 H O
2
(Equation 1).[
1]
(2)
UO
2
F
2
U O
+
8 HF
UF4
+
2 UO2F2
+
4 H O
The reaction is carried out at temperatures between 250 and
00 °C and water released during the reaction is constantly
removed to prevent the hydrolysis of UF . The obtained UF
usually contains a small percentage of UO and UO , the latter
due to UO impurities in the starting material (Equation 3). By
adding a few percent of H to the gas stream, U(VI) as well as
oxidic impurities can be significantly reduced.
3
8
2
(
1)
6
Since the Manhattan Project, uranium tetrafluoride has been a
4
4
2
35
key intermediate in the enrichment of the U isotope, which is
used for the production of nuclear weapons and fuels.[2–5] UF
is
commonly referred to as “green salt”, which is converted by
various fluorination agents to volatile UF for the enrichment
process. The 235U isotope is then enriched, for example, by gas
centrifugation, reconverted to UF and usually further processed
to UO
, which is a common nuclear fuel.[3] Uranium tetrafluoride
2
2 2
F
4
[3]
3
2
6
UO3
+
2 HF
UO2F2
+
H O
2
(
3)
4
2
4 6
The synthesis of UF is also possible through the reduction of UF ,
which, however, has been of little technical interest.[3,13] Suitable
reducing agents are for example, H , HCl, HBr, N H , and NH .
can be reduced with Ca or Mg for the production of uranium
metal.[ Enriched uranium metal, or its alloys with aluminum, are
also used as targets for the production of 99Mo, which decays to
the valuable 99mTc isotope, which is used in medical diagnostics.[7]
4–6]
2
2
4
3
On a small scale H has been used in a technical process for the
2
reduction of depleted UF to UF and HF. The UF is then further
6
4
4
reacted with oxides like B
2
O
3
or SiO
2
3
producing valuable BF or
[
13]
SiF
4
and uranium oxides (Equation 4).
[a]
M. Sc. B. Scheibe, M. Sc. M. Sachs, C. Pietzonka, Dr. (RUS) S. I.
Ivlev, Prof. Dr. F. Kraus
3
UF
4
+ 2 B 3 UO
2
O
3
2
+ 4 BF
3
(4)
Fachbereich Chemie, Philipps-Universität Marburg
Hans-Meerwein-Straße 4, 35032 Marburg (Germany)
E-Mail: f.kraus@uni-marburg.de
Homepage: https://www.uni-marburg.de/fb15/ag-kraus
Dr. J. Bruns
Institut für Anorganische Chemie, Universität zu Köln
Greinstr. 6, 50939 Köln (Germany)
Prof. Dr. G. Heymann, Prof. Dr. H. Huppertz
Institut für Allgemeine, Anorganische und Theoretische Chemie
Universität Innsbruck
Based on a report from 1911 on the reaction of pure UF
sulfur, yielding US , UF and possibly SF
reduction of UF with sulfur in anhydrous HF (aHF), which yields
UF was further reacted in a multi-anvil press
.[14] The obtained UF
under high temperature and pressure to obtain a single-crystalline
high-pressure modification of UF
6
with
2
4
4
, we investigated the
[
b]
c]
6
4
4
[
4
.
Innrain 80-82, 6020 Innsbruck (Austria)
Prof. Dr. A. J. Karttunen
[d]
Department of Chemistry and Materials Science, Aalto University,
Kemistintie 1 FI-02150 Espoo (Finland)
Supporting information for this article is given via a link at the end of
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Results and Discussion
4 8 6
that SF is the main product of the reaction between S and UF
in the presence of aHF. The overall reaction can then be
described by Equation 5.
6 8
The reaction of UF with an excess amount of S in aHF proceeds
quickly at the melting point of aHF (−83.37 °C). Initially, a bluish
supernatant HF solution is formed, indicating the presence of the
[
2 UF
6
+ 1/8 S
8
2 UF
4
+ 1/8 SF
4
(5)
UF
6
]− ion.[15] After a few minutes of reaction time at room
temperature, a brown supernatant HF solution is obtained that is
accompanied by the precipitation of a grayish solid, which
indicates the formation of mixed uranium(IV/V) fluorides, such as
The removal of HF and other volatiles from the reaction mixture
using vacuum distillation yields sulfur-containing UF , due to the
excess of sulfur used. The remaining sulfur was removed in vacuo
at 350 °C. The phase purity of the UF was checked by a Rietveld
4
U
2
F
9
and U
4
F
17 (see Figure S1).[3] After two to three days of
4
reaction time at room temperature, a green solid and a colorless
supernatant HF solution results.
The fluorination process of sulfur in this reaction is probably quite
complex, with the possible intermediate formation of sulfur
refinement of a powder X-ray diffraction (PXRD) pattern recorded
at 293 K (see Figure 1 and Table S1). The following lattice
parameters were refined: a = 12.815(1) Å, b = 10.806(1) Å, c =
3
8.3804(2) Å, β = 126.26(1)°, V = 935.71(3) Å , C2/c (no. 15),
mS60, β-ZrF
4
structure type.[ The determined values are in
21]
difluoride, SF
.[16,17] These low valent sulfur fluorides are either unstable
towards disproportionation in the presence of aHF or unstable at
room temperature (S
).[16,17] Uranium hexafluoride is also
known to oxidize H S at 25 °C in the gas phase or when liquid
S is used both as a solvent and reactant.[18] In both cases UF
SF , and HF are formed. Besides that, it has been reported that
2
, thio-thionyl fluoride, SSF
2
, and disulfur difluoride,
S
2
F
2
good agreement with so far reported data, see Table 1. Based on
X-ray diffraction, UF was obtained as a phase-pure product.
4
2 2
F
2
Table 1. Reported lattice parameters of ZrF
4
-type UF . RT stands for room
4
H
2
4
,
temperature.
4
Radiation
type
no reaction between UF
6
and SF
4
takes place at temperatures up
to 300 °C.[ At a temperature of 500 °C the formation of SF
has
6
T / K
a / Å
b / Å
c / Å
β / °
Reference
19]
[
[
[
22]
23]
24]
Neutron
X-ray
X-ray
X-ray
X-ray
X-ray
300 12.7941(2) 10.7901(2) 8.3687(2) 126.25(2)
been observed. In comparison to that it has been shown that PuF
reacts with SF
at 30 °C.[19] Furthermore, the reaction between
gaseous UF
30 °C, with SF
4
being the fluorination product.[ This reaction
6
RT
RT
RT
RT
12.79(6)
10.72(5)
8.39(5) 126.2(5)
4
12.73(1) 10.753(7) 8.404(8) 126.33(5)
12.760(4) 10.768(2) 8.399(2) 126.32(2)
12.801(1) 10.799(1) 8.374(1) 126.30(1)
6
and sulfur was reported to be slow up to at least
[25]
[26]
20]
1
293 12.815(1) 10.806(1) 8.3804(2) 126.26(1) this work
2
could be accelerated by increasing the pressure with N , but in
both cases several heating cycles had to be performed to obtain
quantitative yields. It is likely from the afore-mentioned reactions
4 4
-type UF
after Rietveld refinement.[21] The observed pattern is shown in blue, the
Figure 1. Observed and calculated powder X-ray diffraction patterns of ZrF
calculated one in red. The calculated reflection positions are indicated by the vertical bars below the patterns. The curve at the bottom represents the difference
p p
between the observed and calculated intensity. Profile R-factors: R = 3.15%, wR = 4.51%, GOF = 2.13. Further details on the structure determination are available
from the Supporting Information.
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35,45]
3
We attempted the detection of a potential sulfur content of our
powder samples by micro X-ray fluorescence analysis (µXRF). No
sulfur-containing impurities (like S or US ) were detected in the
8 2
under-occupancy of U.[
4
A cubic modification of UF with a =
6.34(5) Å and V = 254.8 Å (T = not reported) has been obtained
by explosive compression, utilizing RDX (hexogen) as an
explosive under conditions of 16 to 50 GPa and 400 to 1600 °C.[
However, no further structural data were reported. Furthermore,
36]
sample after the sublimation. Furthermore, infrared and Raman
spectra were recorded (see Figure S2 and Figure S3). In the
–
1
measured IR spectrum, a weak band at 612 cm and a partly
resolved broad band with a maximum at approximately 430 cm–1
can be seen, which are in agreement with previous reports on
4
a Raman spectroscopic study was conducted on UF at a
pressure of 4.7 GPa, assigning a phase transition at 2.2 GPa. The
phase transition was observed by powder X-ray diffraction.[30]
UF
UO
respectively.
The Raman spectra of UF
wavelengths show bands at 473, 219, and 151 cm . The
4
.[27,28] Possible oxygen-containing species like UO
show bands at 450, 700-950, and 1020 cm ,
2 3
, UO , and
–
1
2 2
F
[
3,29]
Such bands are absent.
obtained at different excitation
4
–
1
4
intensities of these bands are relatively low and UF seems to be
a very weak Raman scatterer.[
28,30,31]
In the Raman spectra, no
, which is a strong
Raman scatterer, could be observed.[32] Furthermore, no bands
attributable to oxygen-containing species like UO , UO , and
bands attributable to sulfur species, such as S
8
2
3
2 2
F
could be observed in the Raman spectra.[
3,33,34]
UO
We were furthermore interested in obtaining a crystalline, high-
pressure modification of UF
4
since it had only been scarcely
studied under high-pressure conditions.[
30,35,36]
The only reported
crystal structure for ambient-pressure UF
4
to date is that of the
4
β-ZrF type, which is a structure type observed for several
transition metals, as well as for some lanthanoid and actinoid
4
tetrafluorides MF (M = Hf, Ce, Tb, Th, Pa, Np, Pu, Am, Cm,
Bk).[22,23,37–41] In this structure type, each metal cation is
coordinated by eight F atoms in a square-antiprismatic shape.
The ZrF
D network that can be described with the Niggli formula ∞3 [ZrF8/2].
In contrast to UF , there are several reports of crystallographic
studies on high-temperature and high-pressure modifications of
single-crystalline ZrF . The high-temperature modification α-ZrF
has been obtained by sublimation of β-ZrF under an argon
atmosphere with a temperature gradient of 1020 K → 580 K.[ It
8
coordination polyhedra share corners, which results in a
4
Figure 2. Crystal structure of HP-UF . Displacement ellipsoids are shown at the
3
90% probability level at 183 K.
4
4
Green HP-UF single-crystals, with essentially the same color as
4
4
regular UF
4
, were obtained in this study under a pressure of
4
9.5 GPa and a temperature of 600 °C in a multi-anvil press. HP-
42]
4
UF is metastable at room temperature and all characterizations
crystallizes in space group P4
2
/m (no. 84), tP40, with a =
have been carried out under ambient pressure. This high-
pressure modification of UF , called HP-UF , crystallizes in the
monoclinic space group P2 /m (no. 11) with four formula units per
unit cell, mP20. To the best of our knowledge, HP-UF is a new
3
7
.896(1) Å, c = 7.724(1) Å, V = 481.6 Å , Z = 8 (T = not
4
4
reported).[ Each Zr atom is coordinated by eight fluorine atoms,
42]
1
forming a polyhedron of square-antiprismatic shape. The ZrF
8
4
polyhedra share corners and edges, resulting in a 3D framework,
structure type. See Table 2 for crystallographic details. The crystal
3
which can be described with the Niggli formula ∞[ZrF8/2]. The
structure of HP-UF is shown in Figure 2.
4
high-pressure modification γ-ZrF
β-ZrF at temperatures of 1173 to 1273 K under pressures of 4 to
GPa.[43,44] It crystallizes in the space group P2
/c (no. 14), mP20,
4
has been obtained by heating
4
8
1
with a = 5.554(2) Å, b = 5.639(2) Å, c = 7.973(3) Å, β = 105.98(5)°,
V = 240.1 Å, Z = 4 (T = not reported).[
43,44]
The Zr atom has a
similar coordination environment to α-ZrF
4
, with edge and corner
3
sharing ZrF
8
square-antiprisms, ∞[ZrF8/2].
Up to now, only a few high-pressure studies on UF
published. It is reported that a hexagonal modification of UF
4
have been
has
4
been obtained by shock compression. Based on powder X-ray
diffraction data recorded on a film, the authors assigned this
modification to the Cu
.05 Å and c = 7.27 Å, and a site occupation factor of ¾ for the
uranium position. We note that this modification would be isotypic
to UF and the given composition UF is only generated by an
3 3
P type structure (hP26, P6 mc) with a =
7
3
4
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Table 2. Selected crystallographic data and details of the
single-crystal structure determination of HP-UF
4
.
HP-UF
4
Empiric formula
UF
4
Color and appearance
Crystal size / mm
Molar mass / g·mol−1
green plate
0.02 × 0.03 × 0.03
314.02
3
Crystal system
monoclinic
Space group (No.)
1
P2 /m (no. 11)
a / Å
b / Å
c / Å
β / °
4.6015(2)
6.9786(3)
7.6271(3)
101.013(2)
240.41(2)
4
8.676
0.71073 (Mo-K
183
4
Figure 3. A Section of the crystal structure of HP-UF . Displacement ellipsoids
3
V / Å
are shown at the 90% probability level at 183 K. Symmetry transformations for
the generation of equivalent atoms: F(1): #1 − x, − 1/2 + y, 2 − z; #2 x, 3/2 − y,
z; #3 − x, 2 − y, 2 − z; #4 x, y, − 1 + z; #5 x, 3/2 − y, − 1 + z; F(2): #1 1 − x,
1/2 + y, 1 − z; #2 1 − x, 1 − y, 1 − z; #3 x, 3/2 − y, z; #4 − x, 1 − y, 1 − z; #5 − x,
1/2 + y, 1 − z; F(3): #1 – 1 + x, y, z. F(4): #1 x, 3/2 − y, z; #2 1 − x, 1 − y, 1 − z;
#3 1 − x, 1/2 + y, 1 − z.
Z
ρ
calcd. / g·cm−3
λ / Å
T / K
µ / mm
α
)
−1
67.395
θ
max
39.47
Range of Miller indices
−8 ≤ h ≤ 8
Table 4. Selected interatomic distances d of HP-UF
4
.
−
12 ≤ k ≤ 12
13 ≤ l ≤ 13
−
d / Å
Multiplicity
R
int, R
σ
0.037, 0.015
0.015, 0.019
0.028, 0.029
1.16
1530, 53, 0
2.83, −2.12
R(F) (I ≥ 2σ(I), all data)
wR(F ) (I ≥ 2σ(I), all data)
S (all data)
Data, parameter, restraints
Δρmax, Δρmin / e·Å−
U(1)–F(1)
U(1)–F(1)
U(1)–F(2)
U(1)–F(3)
U(1)–F(3)
U(1)–F(4)
U(1)–F(5)
U(2)–F(1)
U(2)–F(2)
U(2)–F(2)
U(2)–F(4)
U(2)–F(5)
2.349(2)
2.674(2)
2.401(2)
2.335(3)
2.344(2)
2.414(2)
2.359(2)
2.358(2)
2.420(2)
2.434(2)
2.217(2)
2.222(2)
2
2
2
1
1
2
1
2
2
2
2
1
2
3
There are two symmetry-independent uranium atoms U(1) and
U(2) in the structure of HP-UF , both occupy the Wyckoff position
e (m). U(1) is coordinated by eleven fluorine atoms, which form
a polyhedron best described as a distorted mono-capped
pentagonal antiprism, see Figure 3 and Figure 4. U(2) is
coordinated by nine fluorine atoms, forming a polyhedron that can
be described as a prismatoid composed of a pentagon and
quadrangle in parallel orientation. The coordination spheres of the
uranium atoms can also be described by the Niggli formulas
4
2
The fluorine atoms F(3), F(4), and F(5) are µ
the uranium atoms with U–F distances in the range of 2.217(2) to
2.414(2) Å and U–F–U angles from 155.8(1) to 159.0(2)°. The
2
-bridging between
3
1/2–
3
1/2+
{
[UF6/3F5/2
]
}
for U(1) and {[UF6/3F3/2
]
}
for U(2),
3
fluorine atoms F(1) and F(2) are µ -bridging between both
∞
∞
respectively. Atom coordinates and the isotropic displacement
parameters are available from Table 3, anisotropic displacement
parameters from Table S2. Selected atomic distances, d, and
angles, ∠, are given in Table 4.
uranium atoms with increased U–F distances of 2.349(2) to
2.674(2) Å and decreased U–F–U angles of 117.2(1) to 123.1(1)°,
which may be explained by the different coordination numbers.
Such µ
containing fluorido complexes like RbUF
average U–F distance for U(1) is 2.429 Å and for U(2) 2.342 Å.
For comparison, in the structure of ZrF -type UF , both uranium
atoms are coordinated by eight fluorine atoms, forming distorted
square antiprisms.[21] All fluorine atoms in this modification are µ
bridging between the uranium atoms with distances in the range
of 2.230 to 2.354 Å. The average U–F distance for U(1) in ZrF
type UF is 2.290 Å and for U(2) it is 2.266 Å. Comparing the
average U–F distances of both modifications, those in HP-UF are
3
-bridging fluorine atoms are also present in U(IV)
13.[46] The
5
and RbU
3
F
Table 3. Atomic coordinates and equivalent isotropic displacement
parameters, Uiso, for HP-UF
4
at 183 K.
4
4
Wyckoff
Atom
x
y
z
U
eq / Å2
position
2e
2
-
U(1)
U(2)
F(1)
F(2)
F(3)
F(4)
F(5)
0.03554(3)
0.45712(3
0.2048(4)
0.2975(4)
0.5525(6)
0.7764(4)
0.1582(5)
3/4
3/4
0.82749(2)
0.34398(2)
0.0933(2)
0.3961(2)
0.8844(3)
0.2513(2)
0.5410(3)
0.00233(3)
0.00229(3)
0.0064(3)
0.0046(3)
0.0068(4)
0.0068(3)
0.0056(4)
2e
4
-
4f
0.9198(3)
0.4259(2)
3/4
4
4f
4
increased due to the higher coordination numbers, which is
generally observed in high-pressure modifications.[47]
2e
4f
0.5546(3)
3/4
The overall structure of HP-UF
the UF and UF11 polyhedra share corners and edges. The UF
polyhedra are surrounded by two UF and seven UF11 polyhedra;
the UF11 polyhedra are surrounded by seven UF and four UF11
polyhedra. In HP-UF the F∙∙∙F distances are observed in a range
4
is a 3D network structure in which
2e
9
9
9
9
4
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from circa 2.37 to 2.89 Å and are thus shorter compared to ZrF
4
-
refinement fit: a = 4.6080(1) Å, b = 6.9955(1) Å, c = 7.6364(1) Å,
β = 101.07(1)°, V = 241.58(1) Å , T = 293 K. The lattice
parameters obtained at 293 K show a good agreement with the
single-crystal data obtained at 183 K, with a slight difference due
to the different measurement temperatures and X-ray
wavelengths.
3
type UF
4
(2.56 to 3.08 Å). The nearest U–U distances in HP-UF
-type UF
4
are in the range of 4.045(1) to 4.602(1) Å, those in ZrF
4
4
are between 4.468 and 4.584 Å. The average U–U distances for
the nearest U atoms are 4.382 Å for U(1) and 4.284 Å for U(2) in
4 4 4
HP-UF , the ones in ZrF -type UF are 4.507 Å for U(1) and
4.504 Å for U(2). These observations are in line with the
expectations for a high-pressure structure.
The magnetic properties of ZrF -type UF4 and HP-UF4 were
4
investigated by means of temperature-dependent DC-magnetic
measurements in a temperature range between 300 K and 5 K.
The magnetic susceptibilities are shown in Figure 7. At
temperatures between 100 to 300 K, the susceptibilities of both
compounds can be fitted with a Curie-Weiss law (C/(T − Θ)). An
effective moment, µeff, of 3.67(1) µ
unit and a paramagnetic Curie constant Θ of –160.8(7) K and –
47.1(2) K resulted from the fits of ZrF -type UF and HP-UF
respectively.
The effective moments for ambient pressure UF
of similar magnitude. The obtained values resemble the effective
B B
and 3.61(1) µ per formula
1
4
4
4
,
4
and HP-UF
4
are
Figure 4. Coordination environment of the two symmetry-independent uranium
4
atoms in HP-UF . Left: U(1) with eleven neighboring fluorine atoms; Right: U(2)
2
4+
3
B 4
atomic moment of 3.58 µ of a [Rd]5f U cation with a H
ground
with nine neighboring fluorine atoms. Uranium atoms are depicted in green and
fluorine atoms in yellow. Displacement ellipsoids are shown at the 90%
probability level at 183 K.
state indicating that crystal field effects contribute only to a small
extent within this temperature region. This behavior is found for
many U(IV) systems.[46,48–50] The negative values of Θ of both
compounds indicate an antiferromagnetic interaction between the
U(IV) ions, although the course of the susceptibility at low
temperatures does not display an explicit magnetic ordering.
4
The U atoms in HP-UF are approximately hexagonal close
packed with the hexagonal layers perpendicular to the b axis.
However, the F atoms do neither occupy octahedral nor
tetrahedral voids but reside either on the faces (µ
edges (µ -F) of these polyhedra.
The phase purity of the HP-UF obtained was checked by powder
3
-F) or on the
2
4
X-ray diffraction, see Figure 5. No additional reflections resulting
from crystalline impurities, such as the ambient pressure
modification of ZrF
used during the synthesis, were observed. Thus, HP-UF
prepared phase-pure based on powder X-ray diffraction. The
following cell parameters were obtained after Rietveld
4
-type UF
4
or Pt from the container material
4
was
a
4
Figure 5. Observed and calculated powder X-ray diffraction patterns of hp-UF after the Rietveld refinement. The observed pattern is shown in blue, the
calculated one in red. The calculated reflection positions are indicated by the vertical bars below the patterns. The curve at the bottom represents the difference
p p
between the observed and calculated intensity. Profile R factors: R = 1.98%, wR = 2.69%, GOF = 5.31. Further details on the structure determination are
available from the Supporting Information.
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Figure 6. Different plots of the temperature dependence of the molar susceptibility of a) ZrF
4
-type UF
4
and b) hp-UF
4
with an applied field of 0.1 T.
The temperature range used for the Curie-Weiss fits is shown in red.
The low temperature magnetic susceptibilities of ZrF
and HP-UF are given in Figure 6. The susceptibilities of both
compounds exhibit saturation effects. However, the saturation is
more pronounced for HP-UF . A saturation of the susceptibility at
low temperatures is often observed in compounds containing
U(IV) cations.[46,48–50] This effect is discussed in terms of crystal
field effects that should stabilize a singlet ground state, which is
4
-type UF
4
our data, we can reproduce the results of previous studies finding
an energy gap Δ of 10.6(1) cm−1 (see Figure 6). In the case of HP-
4
−
1
UF
the value of ZrF
increase of crystal field effects as the average coordination
number of the uranium atoms increases from eight in ZrF -type
UF to nine and eleven (ten in average) in HP-UF . Due to the
higher energy difference between the ground state and the first
excited state of HP-UF the saturation of the magnetic
4
, we determine an energy gap Δ of 21.9(2) cm , almost twice
4
4
-type UF . This is in line with the expected
4
4
4
4
4 4
populated with decreasing temperature. In case of ZrF -type UF ,
51]
magnetic,[52]
heat
capacity,[
and
spectroscopic
4
measurements[ reveal an energy difference Δ between the
53]
susceptibility is more pronounced as in ZrF
6).
4 4
-type UF (see Figure
−
1
ground state and the first excited state of about 10 cm . Due to
the small energy difference, the saturation of the susceptibility and
therefore the deviation from the Curie-Weiss law occurs only at
low temperatures, e.g. k
B
T ≈ Δ. Leask and coworkers suggested
the following fitting function (Equation 6) for the magnetic
susceptibility of ZrF
spectroscopic measurements, that only the first exited state is
accessible within this temperature range.[52]
4
-type UF
4
below 20 K, assuming, in line with
[
53]
2
B
8
N μ 1 1–exp(–∆/T)
A
χ(T) = p∙
∙
∙
+ μ0
(6)
kB
∆ 1+exp(–∆/T)
In the equation, N
A
is the Avogadro constant, µ
B
the Bohr
magneton, and k
for reduction of the magnetic susceptibility due to
antiferromagnetic coupling, Δ is the energy difference in K
between the ground state and the first excited state and µ is the
temperature-independent susceptibility. Applying Equation 6 to
B
the Boltzmann constant. The factor p accounts
a
0
4 4 4
Figure 7. Section of the molar susceptibility of ZrF -type UF (o) and HP-UF
(Δ) at low temperatures measured with a field of 0.1 T. The data was fitted with
Equation 6.
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Using quantum chemical calculations, we optimized the
Table 5. Energy difference of different magnetic structures of ZrF
(C2/c, mP60) (DFT-PBE0/TZVP). Magnetic ordering:
4
-UF
4
structures of ZrF
theory (DFT) applying the hybrid functional PBE0. The results of
the structural optimization of the magnetic ground state of ZrF
type UF and a comparison with experimental data are collected
in Table S3 and Table S4. The data concerning HP-UF is
4 4 4
-type UF and HP-UF with density functional
AF
(antiferromagnetic), FR (ferrimagnetic), F (ferromagnetic).
4
-
ΔE / meV and
Magnetic
Magnetic
ordering
per
unit
0
formula
Spin arrangement a
4
space group
4
summarized in Table S5 and Table S6. The calculated lattice
parameters of the two compounds agree well with the
experimental results. The difference is in the order of 1%. The
experimental U–F distances (in the following given in brackets)
C
P 2/c (13.74)
AF
FR
AF
–
↓↑↓↑↓↑↓↑↓↑↓↑
↑↑↑↑↑↑↑↑↓↓↓↓
↓↑↓↑↑↓↑↓↓↑↓↑
–
C2’/c’ (15.89)
12
P
C
2/c (13.74)
13
non magnetic
1313
are also well reproduced by the DFT calculations. In case of ZrF
4
-
a
The atomic positions of the corresponding uranium atoms are listed in Table
4
type UF , the calculated U–F distances range from 2.21 Å (2.230
S7 in the SI.
Å) to 2.34 Å (2.354 Å).[ The average distance of U(1) and U(2)
21]
to their coordinating fluorine atoms is calculated as 2.27 Å
Table 5. Energy difference of different magnetic structures of HP-UF4
(
2.290 Å and 2.266 Å, respectively). In case of HP-UF
calculated U–F distances range from 2.18 Å (2.217(2) Å) to
.68 Å (2.414(2) Å). The average U–F distance of U(1) is 2.43 Å
2.429 Å) and of U(2) 2.32 Å (2.342 Å). This demonstrates that
PBE0 calculations can reproduce the experimental structures of
ZrF -type UF and HP-UF reasonably well.
We estimated the magnetic ground states of ZrF
HP-UF performing single-point calculations with different starting
4
, the
(
P2
1
/m, mP20) (PBE0/TZVP). Magnetic ordering: AF (antiferromagnetic),
FR (ferrimagnetic), F (ferromagnetic).
2
(
ΔE / meV and
Magnetic
Magnetic
ordering
Spin arrangement a
per formula
space group
unit
0
4
4
4
P2
P2
P2
1
1
1
’/m (11.54)
’/m (11.52)
’/m (11.54)
AF
AF
F
↑↑↓↓
↓↑↓↑
↑↑↑↑
–
4
4
-type UF and
11
4
21
spin-orientations. Details concerning the calculations are given in
the Experimental Section. The results of the calculations are
summarized in Table 5 and Table 6. We find an antiferromagnetic
ground state for both structures in line with the negative
paramagnetic Curie-temperatures determined from experiment.
non magnetic
–
1358
aThe atomic positions of the corresponding uranium atoms are listed in Table
S8 in the SI.
Comparing the two ground state energies, ZrF
4
-type UF
4
is about
Conclusions
10 kJ/mol per formula unit more stable than HP-UF . This is
4 4
-type UF to be the
4
expected from our experiments finding the ZrF
stable polymorph under normal conditions.
Based on powder X-ray diffraction, micro X-ray fluorescence
analysis, IR and Raman spectroscopy, a route for the synthesis
For both structures, the energy difference of the ground state
magnetic structures to magnetic structures higher in energy is in
the order of 10 meV to 20 meV. These small energy differences
point to only weak antiferromagnetic coupling between the
uranium atoms, reflecting that experimentally no magnetic
ordering is found in the investigated temperature range between
of UF
4
free from oxygen-containing impurities has been
developed. The magnetic behavior was re-examined and shown
to be consistent with previously reported values. A high-pressure
modification HP-UF was synthesized in a multi-anvil press. The
4
compound is metastable under ambient temperature and
pressure and its structure was determined by single-crystal X-ray
diffraction. HP-UF
5
K and 300 K.
4
has a complex 3D network structure that is
. To
As pointed out above, U(IV) systems are often discussed having
a singlet ground state. However, our calculations reveal a triplet
structurally not related to high-pressure modifications of ZrF
4
the best of our knowledge, it presents a novel structure type. The
coordination numbers of the uranium atoms are nine and eleven,
respectively. The magnetic behavior at high temperatures is
similar to that of ambient pressure ZrF -type UF . The magnetic
4 4
susceptibility shows saturation effects at low temperatures that is
often observed for U(IV) systems. Quantum chemical solid-state
ground state with a spin magnetic moment of approximately 2 μ
B
per uranium atom. This discrepancy could stem from neglecting
spin-orbit interactions in our calculations. The typical magnitude
of spin-orbit effects are discussed to be around 300 meV for
actinoid ions.[54] It is thus likely that the degeneracy of the triplet
ground state can be lifted by including spin-orbit interactions in
calculations indicate HP-UF
stable than ZrF -type UF
4
to be 10 kJ/mol per formula unit less
4
our calculations driving UF to the experimentally indicated singlet
4
4
.
ground state.
Experimental Section
General: All operations were performed in either stainless steel (316L) or
Monel metal Schlenk lines, which were passivated with 100% fluorine at
various pressures before use. Preparations were carried out in an
atmosphere of dry and purified argon (5.0, Praxair). Hydrogen fluoride
(
99%, Hoechst) was dried over K
2
NiF
6
prior to its use. The well-educated
6
can be dangerous if not properly
reader is aware that F , HF, and UF
2
handled. Uranium compounds are radioactive and should therefore not be
ingested. Sulfur is safe at ambient conditions.
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Magnetic measurements: DC-magnetic data were collected with the aid
of the VSM option of a physical property measurement system (ppms) of
LOT-Quantum Design. Temperature dependent magnetic data were
recorded in the range from 1.8 K to 300 K with an applied field of 0.1 T.
The collected data were corrected with respect to the diamagnetic moment
of the sample holder, as well as to the diamagnetic contribution (χdia =
Preparation of UF
reaction, sublimed S
ethylene propylene copolymer (FEP) reaction tube and heated in vacuo
several times. Approximately 2 mL of anhydrous HF were added by
vacuum distillation. The suspension was frozen with liquid nitrogen and
4
from the reaction of sulfur with UF
6
: In a typical
8
(455 mg, 14.2 mmol) was placed in a perfluorinated
−7.9∙10−5cm mol ) of the sample derived from Pascal constants taking into
3
−1
UF
6
(3.27 g, 9.29 mmol) was distilled onto it. The reaction mixture was
slowly warmed, and reaction began at the melting point of HF. The
supernatant solution turned bluish for a few seconds, then brown with the
formation of a greyish solid. After two to three days of reaction time at room
temperature, a green solid formed, with a colorless supernatant HF
solution (see Figure S1). The solvent and volatile reaction products were
distilled into a separate FEP reaction tube and the crude product was
transferred into a flame-dried glass Schlenk tube. The tube was attached
account the composition of UF
4
, the result being the net paramagnetic
data.[
68]
Quantum-chemical calculations
We performed periodic density functional theory (DFT) calculations to
calculate the energy difference of the two UF modifications. We further
wanted to estimate the magnetic ground state of these structures. The
calculations were performed with the software package CRYSTAL17 that
uses Gaussian-type atom-centered basis functions.[ We applied the
4
to a vacuum line and residual S
8
was sublimed off in a fine vacuum at
6
is quantitative with respect to UF .
69]
approximately 350 °C. The yield of UF
4
PBE0 hybrid functional and triple-zeta-valence + polarization (TZVP) level
70–72]
Preparation of the high-pressure modification of UF
UF was filled in crucibles, which are made of hexagonal boron nitride
HeBoSint® P100, Henze BNP GmbH, Kempten, Germany) and lined with
4
:
basis sets for the uranium and fluorine atoms.[
The basis sets derived
4
from the molecular Karlsruhe def2 basis sets were taken from our previous
studies.[ We applied a 5x5x5 Monkhorst-Pack-type k-points grid for the
reciprocal space integration. For the evaluation of the Coulomb and
exchange integrals (TOLINTEG) we used tightened tolerance factors of 8,
8, 8, 8, and 16. We performed the structural optimizations of the atomic
positions and lattice constants within the constraints imposed by the
respective space group symmetry and the default optimization
convergence thresholds. We applied spin-unrestricted formalism using an
antiferromagnetic spin orientation. We expect only minor effects of
different spin orientations on the results of the optimization.
73]
(
a platinum foil (thickness 0.027 mm, Ögussa GmbH, Vienna, Austria). For
the heating process the boron nitride crucible was surrounded by two thin
graphite tubes, which are employed as a graphite resistance heater. The
assembly was then placed inside the octahedral pressure medium made
from 5% magnesium chromite doped magnesium oxide (Ceramic
Substrates & Components Ltd., Isle of Wight, UK). In order to transfer the
uniaxial load of the press onto the eight octahedral faces of the magnesium
oxide octahedron, eight tungsten carbide cubes (ha-7%Co, Hawedia,
Marklkofen, Germany) were used. As soon as the compression process
We estimated the magnetic ground states performing single-point
calculations with different starting spin-orientations using the optimized
(300 min) of the Walker-module to a maximum pressure of 9.5 GPa was
finished, the sample was heated to T = 873 K within 10 min. After keeping
this temperature constant for 15 min, the sample was steadily cooled to
room temperature within 40 min. The completed heating process was
followed by a decompression to ambient pressure within 700 minutes.
Under strictly inert conditions, the surrounding crucible material has been
easily removed from the crystalline sample. Further information about the
multi-anvil technique and the construction of the various assemblies can
be obtained from numerous references.[
structure obtained as above as input. For each UF
calculated a non-magnetic, a ferromagnetic, and two antiferromagnetic
starting configurations. In case of ZrF -type UF , we could not find a stable
ferromagnetic solution as the calculations always converged in
4
modification, we
4
4
a
ferrimagnetic state. The antiferromagnetic spin-orientations were derived
from the maximal magnetic subgroups of the space groups of the
paramagnetic phases using a single magnetic propagation vector along
the crystallographic b axis. For that, we used the MAXMAGN-toolkit from
the Bilbao Crystallographic Server.[ The starting magnetizations were
set with the input keyword FDOCCUP that defines initial f-orbital starting
occupations. We found that for both spin orientations an initial occupation
of (0 0 0 0 0 1 1) converged to the lowest energy.
55–59]
74]
Powder X-ray diffraction: Powder X-ray diffraction patterns were
obtained with a StadiMP diffractometer (Stoe) using Cu-Kα radiation (λ =
1.54059 Å), a germanium monochromator and a Mythen1K detector. The
compounds were placed on adhesive tape (Scotch Tape, 3M) and
measured in transmission geometry. The data were handled within the
[
60]
WINXPOW software.
Rietveld refinement was performed with the
[
61]
TOPAS program suite.
Acknowledgements
Single-crystal X-ray diffraction: X-ray structure analysis of a single-
crystal of HP-UF was carried out with a D8 QUEST PHOTON 100
diffractometer (Bruker) with monochromated molybdenum radiation (Mo-
, λ = 0.71073 Å). Evaluation and integration of the diffraction data was
carried out with the Bruker software package. An empirical multi-scan
absorption correction was applied. The structure was solved with Direct
4
F. K. thanks the Deutsche Forschungsgemeinschaft DFG for
funding and Solvay for generous donations of F
for a research stipend (BR5269/1) donated by the DFG. We thank
Prof. Willner, Wuppertal, for UF donations. Dr. Ivlev, Marburg, for
2
. J.B. is thankful
K
α
[
62]
6
Methods (SHELXT 2014/5) in the space group P2
against F (SHELXL 2014/7).
1
/m (No. 11) and refined
All atoms were refined anisotropically.
Representations of the crystal structure were created with the Diamond
performing the Rietveld refinements and Dr. Conrad, Marburg, for
helpful discussions. A.J.K. thanks CSC, the Finnish IT Center for
Science, for computational resources.
2
[63,64]
[
65]
software.
obtained from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge, CB2 1EZ, United Kingdom
https://www.ccdc.cam.ac.uk/structures/) on quoting the depository
number CCDC-1896463.
Further details of the crystal structure investigation can be
Keywords: uranium • fluorine • high-pressure chemistry •
(
magnetic properties • actinoids
Micro X-ray fluorescence: Micro X-ray fluorescence spectra were
obtained with a Tornado M4 µXRF spectrometer (Bruker). A tungsten
X-ray tube was used for the measurements.
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Chemistry - A European Journal
10.1002/chem.201900639
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Uranium under pressure: The reaction
B. Scheibe, J. Bruns, G. Heymann, M.
Sachs, A. J. Karttunen, C. Pietzonka, H.
Huppertz, and F. Kraus*
of UF
oxygen-free conditions yields UF
high purity. Subsequent treatment of
the obtained UF in a multi-anvil press
yields the high-pressure modification
HP-UF in single-crystalline state, with
6
with S
8
in anhydrous HF under
4
of
Page No. – Page No.
4
4
4
UF and the High-Pressure
unusual coordination environments for
the uranium atoms. Both compounds
Polymorph HP-UF
4
are
investigated
by
magnetic
measurements and quantum-chemical
calculations.
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