Polymorphism in alkali metal uranyl nitrates: Synthesis and crystal structure of γ-K(UO2)(NO3)3

Article abstract of DOI:10.1002/zaac.201100200

Single crystals of γ-K(UO₂)(NO₃)₃ were prepared from aqueous solutions by evaporation. The crystal structure [orthorhombic, Pbca (61), a = 9.2559(3) A, b = 12.1753(3) A, c = 15.8076(5) A, V = 1781.41(9) A³, Z = 8] was determined by direct methods and refined to R₁ = 0.0267 on the basis of 3657 unique observed reflections. The structure is composed of isolated anionic uranyl trinitrate units, [(UO₂)(NO₃)₃] ^-, that are linked through eleven-coordinated K⁺ cations. Both known polymorphs of K(UO₂)(NO₃)₃ (α-and γ-phases) can be considered as based upon sheets of isolated complex [(UO₂)(NO₃)₃]^- ions separated by K⁺ cations. The existence of polymorphism in the two K[UO₂(NO₃)₃] polymorphs is due to the different packing modes of uranyl trinitrate clusters that adopt the same two-dimensional but different three-dimensional arrangements. Copyright

Full text of DOI:10.1002/zaac.201100200

ARTICLE
DOI: 10.1002/zaac.201100200
Polymorphism in Alkali Metal Uranyl Nitrates: Synthesis and Crystal
Structure of γ-K(UO2)(NO3)3
Laurent J. Jouffret,*[a] Sergey V. Krivovichev,[b] and Peter C. Burns[a]
Keywords: Uranyl nitrate; Polymorphism; Crystal structure; Hydrothermal synthesis; Potassium; Packing modes
Abstract. Single crystals of γ-K(UO2)(NO3)3 were prepared from Both known polymorphs of K(UO2)(NO3)3 (α- and γ-phases) can be
aqueous solutions by evaporation. The crystal structure [orthorhombic, considered as based upon sheets of isolated complex [(UO2)(NO3)3]–
Pbca (61), a = 9.2559(3) Å, b = 12.1753(3) Å, c = 15.8076(5) Å, V = ions separated by K+ cations. The existence of polymorphism in the
1781.41(9) Å3, Z = 8] was determined by direct methods and refined two K[UO2(NO3)3] polymorphs is due to the different packing modes
to R1 = 0.0267 on the basis of 3657 unique observed reflections. The of uranyl trinitrate clusters that adopt the same two-dimensional but
structure is composed of isolated anionic uranyl trinitrate units, different three-dimensional arrangements.
[(UO2)(NO3)3]–, that are linked through eleven-coordinated K+ cations.
Kapshukov et al.[5] investigated the γ-modification using X-ray
Introduction
diffraction and reported it as orthorhombic, space group Pbca
Uranyl nitrate hexahydrate is one of the main reagents used
(61), a = 9.34(5), b = 12.46(5), c = 15.95(5) Å, Z = 8. They
in the preparation of new uranium compounds in actinide
noted that the a:1:c ratio is very similar to that given by
Groth,[3] if a, b, and c parameters are exchanged, and c and b
chemistry, and it is also the main product resulting from the
dissolution of nuclear fuel rods in nitric acid in the PUREX
parameters are subdivided by 2: c/2:a:b/2 = 0.854:1:0.667.
process.[1,2] During this process, one of the species formed in
This ratio is very close to that reported for the γ-phase (see
solution is the uranyl trinitrate anion [(UO2)(NO3)3]–. Crystal-
lization of this species with alkali cations are known since the
19th century,[3] and, since then, several crystalline alkali metal
uranyl nitrates were reported containing sodium,[4]
potassium,[5,6] rubidium,[6,7] and cesium.[6,8] Spectroscopic and
luminescent properties of these compounds were studied in a
number of works.[9–14] Some alkali metal uranyl trinitrates such
as the trigonal Rb(UO2)(NO3)3 have interesting optical proper-
ties such as strong molecular dichroism in visible light, owing
to the orientation of the linear uranyl ion, [O=U=O]2+, parallel
to the threefold axis. Templeton and Templeton[15] observed the
same effect for X-ray dichrosim and polarized anomalous scat-
tering of the uranyl ion in crystals of Rb(UO2)(NO3)3.
Potassium uranyl trinitrate, K(UO2)(NO3)3, has been known
to crystallize in two polymorphic modifications, α and γ, as
reported by Paul Groth[3] on the basis of goniometric measure-
ments of crystal morphologies (crystals of the α modification
were first studied by Sachs in 1904[16]). Groth determined that
both the α- and γ polymorphs are orthorhombic with a:1:c ra-
tios of 0.7015:1:1.1560 and 0.8541:1:0.6792, respectively.
above). Kapshukov et al.[5] outlined the basic features of the
structure, but never reported the full results of a structure deter-
mination. Krivovichev and Burns[6] prepared crystals of an-
other K(UO2)(NO3)3 polymorph with the monoclinic space
group C2/c, and unit-cell parameters different from those of
the γ modification.
Recently, we prepared single crystals of γ-K[UO2(NO3)3].
Herein we report the results of its crystal structure determina-
tion and structural relationships between different polymorphs
of the title compound.
Experimental Section
Synthesis of γ-K[UO2(NO3)3]: Crystals of γ-K[UO2(NO3)3] were pre-
pared from aqueous solutions as a by-product in a synthesis of potas-
sium uranyl vanadates. All the solution for the starting materials were
aqueous solutions and the molarity and quantities were as follows:
(UO2)(NO3)26H2O (International Bio-Analytical Industries Inc. 2.1 m,
2 mL), V2O5 (Sigma–Aldrich, 0.044 m, 2 mL), HCl (BDH Aristar
37 %, 0.4 m, 3 mL), and KOH (Fisher, 4 m, 1 mL) The solutions were
mixed in a 23 ml Teflon-lined PARR reaction vessel and heated to
220 °C in 1 h and kept at 220 °C for 48 h before cooling down to
ambient temperature in 48 h. The resulting solution was filtered to
remove fine-grained precipitate. The solution was allowed to stand in
a 50 mL beaker and left to evaporate at room temperature for one
month. Large crystals of K[UO2(NO3)3] formed at the bottom of the
beaker. Due to their average size of ca. 0.5 mm, one was broken and
a plate-like shard was used for the crystal structure determination.
* Dr. L. J. Jouffret
E-Mail: ljouffre@nd.edu
[a] Dept. of Civil Engineering and Geological Sciences
University of Notre Dame
Notre Dame, IN, 46556, USA
[b] Department of Crystallography,
St. Petersburg State University,
University Emb. 7/8
St. Petersburg 199034, Russia
Z. Anorg. Allg. Chem. 2011, 637, 1475–1480
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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L. J. Jouffret, S. V. Krivovichev, P. C. Burns
ARTICLE
Table 2. Atomic coordinates and displacement parameters /Å2 for γ-
Crystal Structure Determination: A good shard of a single crystal
of K[UO2(NO3)3] was selected using a polarized-light microscope for
X-ray diffraction experiments. Diffraction data were collected at 110 K
using a Bruker three-circle X-ray diffractometer equipped with an
APEX CCD detector. Monochromatic Mo-Kα radiation was used for
collection with a frame width of 0.3° in ω and a counting time per
frame of 10 s. Unit-cell parameters were refined by least-squares tech-
niques using the Bruker SMART software[17]. The unit cell parameters
are a = 9.2559(3) Å, b = 12.1753(3) Å, c = 15.8076(5) Å, V =
1781.41(9) Å3. These parameters correspond to those previously re-
ported by Kapshukov et al. in 1969.[5] The BRUKER program
SAINT[18] was used for intensity data integration and correction for
Lorentz polarization and background effects.
K(UO2)(NO3)3.
Atoms
x
y
z
Ueq
U
0.73868(1) 0.984210(8) 0.130924(7) 0.00679(3)
K
0.24284(6) 0.69877(6)
0.12227(4)
0.1190(1)
0.1955(1)
0.0475(1)
0.0334(1)
0.2295(1)
0.0948(1)
0.1533(1)
0.2087(1)
0.1596(1)
0.0928(1)
0.0472(1)
0.1115(1)
0.2163(1)
0.0067(1)
0.0123(2)
0.0100(6)
0.0095(6)
0.0111(6)
0.0107(5)
0.0122(6)
0.0128(5)
0.0115(5)
0.0100(5)
0.0122(5)
0.0133(5)
0.0131(6)
0.0158(6)
0.0145(6)
0.0170(6)
N1
N2
N3
O1
O2
O3
O4
O5
O6
O7
O8
O9
O10
O11
0.0508(2)
0.5671(2)
0.5962(2)
0.7303(2)
0.7452(2)
0.9880(2)
0.9680(2)
0.7029(2)
0.5086(2)
0.5275(2)
0.7326(2)
0.1795(2)
0.4986(2)
0.5349(2)
0.9569(2)
0.8033(2)
0.1671(2)
0.9148(2)
0.0528(2)
0.0448(1)
0.8847(2)
0.8108(1)
0.8865(2)
0.0975(1)
0.1569(2)
0.9429(2)
0.7234(1)
0.2370(1)
After data processing, a correction for absorption was performed using
a semi-empirical method based on data redundancy with the SADABS
program.[19] The crystal structure was solved in the orthorhombic space
group Pbca (61). Details of the data collection and refinement are
given in Table 1.
Table 3. Selected bond lengths /Å and valence angles /° for γ-
Table 1. Crystallographic data and refinement parameters for γ-
K(UO2)(NO3)3.
K(UO2)(NO3)3.
U–O1
U–O2
U–O3
U–O4
U–O5
U–O6
U–O7
U–O8
O1–U–O2
1.760(2)
N3–O7
N3–O8
N3–O11
1.279(3)
1.268(3)
1.209(3)
γ-K(UO2)(NO3)3
1.770(2)
Formula sum
Formula weight /g·mol–1
Crystal system
Space group
Unit-cell dimensions
a /Å
U1K1N3O11
495.1
2.4891(18)
2.4695(20)
2.4655(13)
2.4811(20)
2.4671(17)
2.4851(22)
179.21(9)
orthorhombic
Pbca
K–O1i
2.825(2)
2.942(2)
3.148(2)
3.023(2)
2.828(2)
3.212(2)
3.034(2)
3.202(2)
2.811(2)
3.000(2)
3.188(2)
K–O2ii
K–O3iii
K–O5iv
K–O7v
K–O8vi
K–O9vii
K–O9iii
K–O10
K–O11vi
K–O11v
9.2559(3)
12.1753(3)
15.8076(5)
1781.41(9)
8
3.691
1760
b /Å
c /Å
Cell volume /Å3
Z
N1–O3
N1–O4
N1–O9
1.277(3)
1.287(3)
1.209(3)
Density, calculated /g·cm–1
F(000)
Wavelength /Å
θ range /°
Data collected
0.71073 (Mo-Kα)
2.58–36.34
–15 ≤ h ≤ 15
–19 ≤ k ≤ 20
–23 ≤ l ≤ 26
36777
N2–O5
N2–O6
N2–O10
1.277(3)
1.281(3)
1.206(3)
Symmetry codes : (i) –0.5+x, 1.5–y, –z; (ii) 1–x, –0.5+y, 0.5–z; (iii)
1.5–x, –0.5+y, z; (iv) –0.5+x, y, 0.5–z; (v) 0.5–x, –0.5+y, z; (vi) 1–x,
2–y, –z; (vii) –1+x, y, z.
Measured reflections
Redundancy
8.537
3657
Unique reflections
Intensity threshold
μ(Mo-Kα) /mm–1
R(F2)int
[I > 3σ(I)]
18.756
0.054
Further details of the crystal structure investigations may be obtained
from the Fachinformationszentrum Karlsruhe, FIZ, 76344 Eggenstein-
Leopoldshafen, Germany (Fax: +49-7247-808-666; E-Mail: crys-
data@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request for depos-
ited data.html) on quoting the depository number CSD-422978 or also
available from the author on request.
No. of parameters
Weighing scheme
R(F) obs / all
145
1/σ2
0.0267 / 0.0323
0.0338 / 0.0348
5.43 / –5.02
wR(F) obs / all
Max / min Δρ /e·Å–3
Results and Discussion
Structure Analysis
The uranium atom positions were established by direct methods using
the SIR2002 program.[20] The nitrogen, potassium, and oxygen atoms
were localized from difference-Fourier maps. The last cycles of refine-
ment included atomic positions and anisotropic displacement parame-
ters for all atoms. Full-matrix least-squares structure refinements
against F were carried out using the JANA2006 program.[21] Atomic
positions and displacement parameters are given in Table 2, selected
bond lengths and valence angles are listed in Table 3.
The structure of K[UO2(NO3)3] is composed of isolated com-
plex ions of composition [UO2(NO3)3]– (Figure 1). These ions
are built around a nearly linear [UO2]2+ uranyl ion, displaying
U–Our distances of 1.760(2) and 1.770(2) Å, with a O1=U=O2
angle of 179.21(9)° (Table 4). These distances are in the range
of average U–Our distances found within the uranyl cation,[22]
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Z. Anorg. Allg. Chem. 2011, 1475–1480

Polymorphism in Alkali Metal Uranyl Nitrates γ-K(UO2)(NO3)3
Table 4. . Crystallographic parameters for alkali metal uranyl trinitrates.
Formula
Space group.
a /Å
b /Å
c /Å
β /°
Reference
Na[UO2(NO3)3]
α-K[UO2(NO3)3]
γ-K[UO2(NO3)3]
Rb[UO2(NO3)3]
Cs[UO2(NO3)3]
P213
C2/c
Pbca
10.6324
13.4877
9.2559
9.384
10.6324
9.5843
12.1753
9.384
10.6324
7.9564
15.8076
18.899
19.509
–
[4]
116.124
[5,6]
–
–
–
This work
[6,7]
[6,8]
¯
R3c
¯
R3c
9.64
9.64
and compare well with the distances found for the other alkali gen atom adopt longer bonds to the nitrogen atom than the
metal uranyl trinitrates such as sodium, potassium, rubidium, equatorial oxygen atoms, 1.255(11) Å, 1.280(11) Å, and
and cesium compounds with U–Our bond lengths of 1.778, 1.294(14) Å, respectively. The one crystallographically inde-
1.795, 1.746, and 1.749 Å, respectively.[4–8]
pendent potassium cation in the structure of γ-K[UO2(NO3)3]
is coordinated by eleven oxygen atoms with an average K–O
bond length of 3.019(2) Å. The K+ cations are located between
the complex anions, thus providing the three-dimensional link-
age of the structure.
Polymorphism and Structure Comparisons
We shall refer to the monoclinic[6] and orthorhombic phases
of K[UO2(NO3)3] as the α and γ phases, according to the nota-
tion proposed by Groth.[3] Projections of the structures of the
two modifications onto the (010) and (100) planes, respec-
tively, are shown in Figure 2a and b. Both polymorphs can
be considered as based upon sheets of isolated complex ions
[UO2(NO3)3]– separated by K+ cations. The sheets are parallel
¯
to (101) in the α phase and to (001) in the γ phase. Linear
dimensions of the sheets (am, bm) are quite similar, which is
clear from the comparison of the parameters as shown in Fig-
ure 2c and d [amα = 12.277 Å, bmα = 9.584 Å and amγ
12.175 Å, bmγ = 9.256 Å].
=
Whereas the linear uranyl groups of the [UO2(NO3)3]– ions
¯
are parallel to the [101] direction in the α phase, they display
two orientations in the γ phase, where they are parallel to either
Figure 1. Isolated complex ion [UO2(NO3)3]– in the structure of γ-
¯
the [011] or [011] directions. This difference in orientation of
K(UO2)(NO3)3 in ball-and-stick (a) and polyhedral (b) representations.
For (a), displacement ellipsoids are drawn at the 50 % probability the uranyl ions and, as a consequence, of uranyl trinitrate units,
level.
creates a difference in the stacking sequence of sheets in the
two structures. In the α phase, all sheets are translationally
The coordination of the uranium atoms is completed by six equivalent to each other so that all uranyl cations are oriented
equatorial oxygen atoms belonging to three crystallographi- in the same direction (as shown by arrows in Figure 2a). In
cally distinct nitrate groups. The average U–Oeq bond length contrast, in the γ polymorph, the sheets are arranged in pairs
is 2.476(1) Å, comparable to the average distance of 2.460 Å with uranyl cations in the adjacent double sheets pointing in
reported for 32 structures,[22] and to the similar distances found mutually perpendicular directions. Thus the existence of poly-
in sodium, potassium, rubidium, and cesium uranyl trinitrates morphism in the two K[UO2(NO3)3] forms is due to the differ-
(2.462, 2.472, 2.473, and 2.516 Å, respectively[4–8]). The three ent packing modes of uranyl trinitrate units that form the same
nitrate groups display longer distances for the equatorial oxy- two-dimensional but different three-dimensional arrangements.
gen atoms than for the terminal (Ot) oxygen atom, not linked
to the uranium atom, the N–Oeq distance is 1.278(3) Å and N– nitrates are listed in Table 4. Na[UO2(NO3)3] has cubic sym-
Ot 1.208(3) Å (Table 3). metry, whereas Rb[UO2(NO3)3] and Cs[UO2(NO3)3] are iso-
The crystallographic characteristics of alkali metal uranyl tri-
This kind of distortion is typical for alkali metal uranyl typic and have trigonal (rhombohedral) symmetry. The relative
trinitrates[5–8] except the sodium compound,[4] and was also position of the [UO2(NO3)3]– units and Rb+ and Cs+ cations in
observed in uranyl carbonates.[23] The distortion can be ex- the rubidium and cesium compounds is identical to that in the
plained by the environment of the cation. In the sodium α-K[UO2(NO3)3] structure. The difference reveals itself in the
compound,[4] the cation environment is exclusively composed packing arrangement shown in Figure 2.
of oxygen atoms belonging to the equatorial coordination of
In the rubidium and cesium compounds, the linear uranyl
the uranium. Thus creating a distortion where the terminal oxy- groups of the [UO2(NO3)3]– ions are nearly parallel to [001]
Z. Anorg. Allg. Chem. 2011, 1475–1480
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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1477

L. J. Jouffret, S. V. Krivovichev, P. C. Burns
ARTICLE
Figure 2. Projections of the structures of α- (a) and γ-K(UO2)(NO3)3 (b) parallel to the b and a axes, respectively (grey color highlights layers
of uranyl trinitrate clusters; arrows indicate orientation of uranyl ions in the clusters), and projections of the layers of uranyl trinitrate clusters
and K+ cations in the structures of α- (c) and γ- (d) phases.
and packed into layers parallel to (001). Every layer is packed
in a staggered position from its nearest neighbor, which results
in 6-layer repeat along the c axis. Due to the cubic symmetry,
the structure of the sodium compound cannot be interpreted
in terms of layer stacking. The linear uranyl groups of the
–
¯ ¯ ¯
[UO2(NO3)3] ions are parallel to either the [111] or [111] di-
rections (Figure 3).
Discussion
In this work, we have reported the synthesis and structure of
γ-K(UO2)(NO3)3, and its comparison to the structure of the
previously reported α polymorph.
However, it is not clear whether the modification of
K(UO2)(NO3)3 reported in literature[6] is identical to the α
polymorph identified by Sachs[16] and Groth.[3] The monoclinic
unit cell of the α phase (Table 4) can be transformed into a
Figure 3. Packing of the [UO2(NO3)3]– units in the structures of
M[UO2(NO3)3] (M = Rb, Cs). Arrows indicate orientation of the uranyl
pseudo-orthorhombic unit cell by application of the matrix [0.5 ions.
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Z. Anorg. Allg. Chem. 2011, 1475–1480

Polymorphism in Alkali Metal Uranyl Nitrates γ-K(UO2)(NO3)3
1.5 1 / 0.5 –0.5 1 / 1 0 –1], which results in the cell parameters sional topologies (as observed in α- and β-
a = 16.377 Å, b = 9.192 Å, c = 18.432 Å, α = 88.64°, β = Cs2[(UO2)2(MoO4)3][24] and α- and β-K[(UO2)(P3O9)][25]);
89.24°, γ = 92.80°. The b:a:c ratio for this cell is equal to
(ii) Topological isomerism (as in α- and β-
0.561:1:1.125, which is quite different from the ratio Mg2[(UO2)3(SeO4)5](H2O)16[26]);
0.7015:1:1.1560 reported by Sachs.[16] In addition, Sachs[16]
(iii) Geometrical orientational stereoisomerism (as in α- and
reported the α phase to be orthorhombic on the basis of gonio- β-uranophane, Ca[(UO2)(SiO3OH)]2(H2O)5[27,28]);
metric measurements, whereas we have obtained strong devia-
tions from orthorhombic symmetry.
The γ-K(UO2)(NO3)3 polymorph is clearly different with a
true orthorhombic symmetry. The complexation of uranyl ions
(iv) Distortions of local coordination environment (as in α-
and β-K6(UO2)5(VO4)2O5[29]);
(v) Polytypism (as in α- and β-Ag2[(UO2)W2O8][30]), etc.
However, more research is needed to understand thermody-
seem to be derived from a possible higher pressure, giving rise namic and chemical factors controlling formation of a certain
to a slightly more densely packed uranyl hexagonal bipyramid polymorph in the given system.
network. This is confirmed by the higher calculated density for
the γ phase than for the α phase, 3.691(1) compared to 3.820(1)
g·cm–1. In that case, the γ phase and the α phase should not
have crystallized in the same synthesis. Indeed, this is con-
firmed by the powder pattern of the bulk powder and crystals
shown in Figure 4. Profile matching of the powder diffracto-
gram, calculated with the Fullprof program[31] matches only
the γ phase. It highlights a second phase that is not readily
Acknowledgement
This material is based upon work supported as part of the Materials
Science of Actinides Center, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Science, Office of
Basic Energy Sciences under Award Number DE-SC0001089. SVK
was supported in this work through the St. Petersburg State University
identifiable, yet the position of the leftover peaks is clearly not grant # 3.37.84.2011.
the α phase. The cation size does not seem to have a direct
influence on the packing of the structure. With a similar pack-
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Conclusions
This work demonstrates the existence of yet another mecha-
nism of formation of polymorphic modifications in uranium
solid-state chemistry, which can be identified as polymorphism
induced by different packing arrangements of single clusters.
This contributes to the known mechanisms of polymorphic di-
versity in uranyl compounds, which include:
(i) Combinatorial polymorphism, where identical low-di-
mensional structural units are linked into distinct high-dimen-
Z. Anorg. Allg. Chem. 2011, 1475–1480
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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L. J. Jouffret, S. V. Krivovichev, P. C. Burns
ARTICLE
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Received: April 28, 2011
Published Online: July 20, 2011
1480
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© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2011, 1475–1480