Synthesis, structure, and optical properties of CsU2(PO 4)3

Article abstract of DOI:10.1016/j.jssc.2011.10.007

CsU₂(PO₄)₃ was synthesized in highest yield by the reaction in a fused-silica tube of U, P, and Se in a CsCl flux at 1273 K. It crystallizes with four formula units in space group P2₁/n of the monoclinic system in a new structure type. The structure of CsU ₂(PO₄)₃ is composed of U and Cs atoms coordinated by PO₄^3- units in distorted octahedral arrangements. Each U atom corner shares with six PO₄^3- units. Each Cs atom face shares with one, edge shares with two, and corner shares with three PO₄^3- units. The structure shares some features with the sodium zirconium phosphate structure type. X-ray powder diffraction results demonstrate that the present CsU₂(PO ₄)₃ compound crystallizes in a structure different from the previously reported β′- and γ-CsU₂(PO ₄)₃ compounds. CsU₂(PO₄)₃ is highly pleochroic, as demonstrated by single-crystal optical absorption measurements.

Full text of DOI:10.1016/j.jssc.2011.10.007

Journal of Solid State Chemistry 185 (2012) 124–129
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Synthesis, structure, and optical properties of CsU2(PO4)3
George N. Oh, Emilie Ringe, Richard P. Van Duyne, James A. Ibers n
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3113, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
CsU2(PO4)3 was synthesized in highest yield by the reaction in a fused-silica tube of U, P, and Se in a
CsCl flux at 1273 K. It crystallizes with four formula units in space group P21/n of the monoclinic system
in a new structure type. The structure of CsU2(PO4)3 is composed of U and Cs atoms coordinated by
PO34ꢀ units in distorted octahedral arrangements. Each U atom corner shares with six PO34ꢀ units. Each
Cs atom face shares with one, edge shares with two, and corner shares with three PO34ꢀ units. The
structure shares some features with the sodium zirconium phosphate structure type. X-ray powder
diffraction results demonstrate that the present CsU2(PO4)3 compound crystallizes in a structure
Received 6 September 2011
Received in revised form
4 October 2011
Accepted 4 October 2011
Available online 11 November 2011
Keywords:
Cesium diuranium triphosphate
Single-crystal X-ray structure
Optical absorption measurements
Pleochroism
different from the previously reported b0
pleochroic, as demonstrated by single-crystal optical absorption measurements.
& 2011 Elsevier Inc. All rights reserved.
- and g-CsU2(PO4)3 compounds. CsU2(PO4)3 is highly
1. Introduction
Additional phases have been identified from X-ray powder diffraction
experiments [13].
Crystalline phosphate compounds have been studied for their
potential as a mineral-like matrix for immobilizing actinides
found in high-level nuclear waste. Such phosphates include
monazites, thorium phosphates, and apatites [1–6]. Phosphates
in particular are of interest owing to their thermal stability [7]
and geochemical stability [8]. Many naturally-occurring tetrava-
lent uranium compounds occur as phosphates [9]. These com-
pounds crystallize in a variety of structure types [10]. Such
structure types can incorporate early actinides, lanthanides, alkali
metals, and alkaline-earth metals [11]. Mixed-metal phosphates
have also been synthesized, which demonstrates their ability to
incorporate a number of elements simultaneously [12]. A class of
compounds in this family are those with formula AAn2(PO4)3
(A¼alkali metal, An¼actinide) [10,11,13]. Two structure types are
common for this class: the NaTh2(PO4)3 or NTP structure type,
and the kosnarite (KZr2(PO4)3) or NZP structure type [14]. These
Compounds with the NZP structure type have also shown
some ability to immobilize alkali metals via ion exchange. For
instance, HZr2(PO4)3 immobilizes Cs via ion exchange [19]. In
some cases, the cations are not immobilized, especially when they
are small and the cavities are enlarged. For example, in LiZr2
(P0.33Si0.66O4)3 the substitution of Si for P enlarges the cavities
such that Li is mobile [20]. Such compounds are used as ionic
conductors, and in this context are referred to as NASICON-type
compounds [21]. Two phases have been reported for CsU2(PO4)3,
b0 and
g [13]. Powder patterns were reported for both, but neither
of their structures was determined. Here we report the synthesis
and crystal structure of cesium diuranium triphosphate,
CsU2(PO4)3. We examine its structural features and compare
them to the NZP structure type. We also report the optical
absorption spectrum of this highly pleochroic compound.
¯
structures are in space groups C2/c and R3c, respectively, and have
been labeled - and -AAn2(PO4)3 [13]. All of the Th compounds
2. Experimental
a
b
crystallize in the NTP structure type [11]. The following actinide
compounds also crystallize in this structure type: LiU2(PO4)3 and
NaU2(PO4)3 [11], KU2(PO4)3 [15], RbNp2(PO4)3, NaPu2(PO4)3 [16], and
KPu2(PO4)3 [13]. Compounds that crystallize in the NZP structure type
include AAn2(PO4)3 (A¼Na, K, Rb; An¼U, Np, Pu) [17,18] and
CsNp2(PO4)3 [13]. Some of these compounds crystallize in both
structure types, and phase transitions have been observed between
them, with the NZP structure type formed at higher temperatures.
2.1. Synthesis
All reactants except UO2 and Se were handled in an argon-
filled glove box. UP2 powder was synthesized by the stoichio-
metric reaction of U turnings (Oak Ridge National Laboratory)
with red P (Alfa Aesar, 99%). The mixture was placed in a 9 mm
outer-diameter carbon-coated fused-silica tube and flame-sealed
under 10ꢀ4 Torr vacuum. It was then placed in a computer-
controlled furnace and heated to 1273 K in 24 h, held for
96 h, cooled to 773 K in 99 h, to 473 K in 30 h, and to 298 K in
12 h. UO2 was synthesized by following a previously reported
n Corresponding author. Fax: þ1 8474912976.
E-mail address: ibers@chem.northwestern.edu (J.A. Ibers).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.10.007

G.N. Oh et al. / Journal of Solid State Chemistry 185 (2012) 124–129
125
procedure [22]: UO3 (Kerr–McGee Nuclear Corp) was heated to
Table 1
Crystallographic details for CsU2(PO4)3.
1173 K, then cooled to 298 K, all under H2 flow. Se (Cerac,
99.999%), CsCl (MP, Optical grade 99.9%), SiO2 (Aldrich), and
P2O5 (Mallinckrodt, 99.5%) were used as received.
Formula mass
Space group
893.88
C52hꢀP21=n
CsU2(PO4)3 was synthesized by combining UP2 (0.100 mmol),
Se (0.100 mmol), and CsCl (0.891 mmol) in a 6 mm carbon-coated
fused-silica tube. The tube was sealed, and heated to 1273 K in
24 h, held for 192 h, cooled to 1223 K in 24 h, held for 192 h,
cooled to 823 K in 120 h, then to 298 K in 24 h. The ampule
emerged somewhat etched, and washing the contents in water
afforded highly faceted crystals of CsU2(PO4)3 in roughly 20 wt%
yield together with green–black crystals of U3O8 and black
crystals of USe2. The crystals of CsU2(PO4)3 are highly pleochroic,
appearing lavender or teal, depending on the direction from
which they are viewed. The color change can also be observed
under polarized light (Fig. 1). The crystals are air-stable. Qualita-
tive EDS-enabled SEM analysis on single crystals indicated the
presence of Cs, U, P, and O, but not of Se. The oxygen source in this
reaction was the silica tube. Attempts at rational syntheses of
CsU2(PO4)3 using UO2, P2O5, with or without additional oxygen
sources such as SiO2, generated the compound in lower yields.
9.2136(4)
˚
a (A)
8.6548(3)
˚
b (A)
15.0161(6)
˚
c (A)
(deg.)
91.240(2)
1197.13
b
3
˚
V (A )
rc (g cmꢀ3
T (K)
)
4.960
100(2)
4
Z
m
(mmꢀ1
)
30.47 (Mo K
0.0199
0.0525
a)
R(F)a
Rw(F2)b
P
P
RðFÞ ¼
:Fo9ꢀ9Fc:= 9Fo9 for F2o 42
s
ðF2o Þ:
a
P
P
RwðF2o Þ ¼ f wFo2ꢀFc2Þ = wFo4g1=2: For F2o o0,wꢀ1
b
2
2
2
2
¼
s2ðFo2Þ; For Fo2 Z0,wꢀ1
¼
s2ðFo Þþð0:0243Fo Þ :
Table 2
Selected Interatomic distances (A) and angles (deg.) for CsU2(PO4)3.
˚
a
-UP2O7 was the major product in these reactions [23].
U1–O9
U1–O5
U1–O11
U1–O8
U1–O3
U1–O4
U1–O4
U2–O1
U2–O7
U2–O12
U2–O10
U2–O6
U2–O2
2.223(4)
2.224(3)
2.250(4)
2.265(4)
2.318(4)
2.327(4)
2.793(4)
2.198(3)
2.214(4)
2.225(4)
2.241(4)
2.255(4)
2.268(4)
O9–U1–O5
O5–U1–O11
O9–U1–O8
O9–U1–O3
O9–U1–O4
O9–U1–O4
O1–U2–O7
O1–U2–O12
O12–U2–O10
O1–U2–O6
O1–U2–O2
92.2(1)
100.1(1)
88.1(1)
77.3(1)
84.1(1)
110.6(1)
83.8(1)
90.7(1)
98.0(1)
90.2(1)
95.1(1)
2.2. Structure determination
Single-crystal X-ray diffraction data were collected on a Bruker
KAPPA diffractometer with an APEXII CCD detector with the use of
˚
Mo K
ing of
a radiation (l¼0.71073 A). A data collection strategy consist-
f
and o scans was devised using COSMO in APEX2 [24].
The data frames were 0.31 in width and taken at a detector distance
of 60 mm with exposure times of 15 s/frame. Examination of the
data showed that the crystal was twinned by pseudomerohedry,
with the second domain rotated 1801 about the b axis. Initial cell
refinement and data reduction were carried out with SAINT in
APEX2 [24]. The structure was solved with XS and refined with XL
of the SHELX package [25]. The data were reintegrated with a
second domain, related to the first by the twin law ꢀ1 0 0 0 1 0 0 0
ꢀ1, then corrected for absorption with TWINABS [26]. A satisfac-
tory solution was obtained with 19.8(1)% of the crystal in the
second domain. Additional crystallographic details are given in
Table 1 and the Supporting material, and selected metrical details
are given in Table 2.
Scan times were 20 s, with 0.021 steps between scans. The
spectrum was calibrated by an internal Si standard with the use
of JADE8 [27]. A simulated powder pattern was generated from
the single-crystal data with the use of PLATON [28].
2.4. Optical measurements
Single-crystal optical absorption measurements were per-
formed over the range from 3.2 eV (387 nm) to 1.5 eV (827 nm)
at room temperature. A single crystal of CsU2(PO4)3 mounted on a
goniometer head was attached to a custom-made holder fitted to
a Nikon Eclipse Ti2000–U inverted microscope. The crystal was
positioned at the focal plane of the microscope and illuminated
with a polarization-filtered tungsten–halogen lamp. The trans-
2.3. Powder X-ray diffraction measurements
The powder diffraction pattern of the present CsU2(PO4)3
compound was obtained in order to compare it to those
previously reported for b0
crystals were ground and then analyzed with a Rigaku Geigerflex
- and g-CsU2(PO4)3 [13]. Single
mitted light was spatially filtered with a 200 mm aperture,
X-ray powder diffractometer with the use of CuK
a
radiation
¼5–351.
dispersed by a 150 groove/mm grating in an Acton SP2300
imaging spectrometer, and collected on a back-illuminated, liquid
nitrogen-cooled CCD (Spec10:400BR, Princeton Instruments).
Spectra were taken at 101 increments of the polarizer angle.
˚
(
l
¼1.5418 A). The sample was scanned in the range 2
y
3. Results and discussion
3.1. Synthesis
CsU2(PO4)3 was synthesized initially in approximately 20 wt%
yield from the reaction in a fused-silica tube of UP2 with Se in a
CsCl flux. The other products were U3O8 and USe2 in about equal
amounts. Subsequent reactions using P2O5 afforded primarily
purple crystals of UP2O7 and only a few crystals of CsU2(PO4)3.
Reactions utilizing UO2 did not provide any green–black crystals
of U3O8, which is unusual considering the oxophilicity of U.
Fig. 1. A CsU2(PO4)3 crystal viewed through polarized light. The polarizer
was rotated approximately 901 between the views. (For interpretation of the
references to color in this figure, the reader is referred to the web version of
this article.)

126
G.N. Oh et al. / Journal of Solid State Chemistry 185 (2012) 124–129
The role of Se is not well understood, but such spectator elements
frequently aid the synthesis of chalcogenide compounds, such as
U in the synthesis of CsTi5Te8 [29].
of the PO34ꢀ tetrahedra sitting on the triangles of the adjacent
layer (Fig. 4).
This structure is related to the NZP structure type, in which
b-KU2(PO4)3 crystallizes. b-KU2(PO4)3 is composed of a triangular
network of corner-sharing UO6 octahedra and PO34ꢀ tetrahedra, in
this case parallel to (001). Unlike CsU2(PO4)3, the alkali metal is
also six-coordinate, so instead of bands of UO6 octahedra sepa-
rated by Cs atoms, the UO6 octahedra are in an hexagonal
arrangement with the K atoms in the middle of the hexagon
(Fig. 5). The layers are stacked with the K atoms on three corners
of the hexagon above it, in an abcabc stacking arrangement
(Fig. 6), resulting in a framework structure. Cs is not stable in
this small coordination environment. The high temperature of the
present reaction favors the thermodynamically stable product,
which contains Cs atoms in a larger coordination environment.
One factor relevant to the immobilizing power of CsU2(PO4)3 is the
strength of the U–O bonds. Shorter interatomic distances correspond
to greater bond strength. As noted earlier, the interatomic U–O
3.2. Structure
The crystal structure of CsU2(PO4)3 is composed of U and Cs
atoms coordinated by PO34ꢀ units in distorted octahedral arrange-
ments. In the structure there are two crystallographically inde-
pendent U atoms. Atom U1 corner shares with six PO34ꢀ units,
with one additional longer distance to another O atom. Atom U2
corner shares with six PO34ꢀ units. Atom Cs1 face shares with one,
edge shares with two, and corner shares with the remaining three
PO34ꢀ units (Fig. 2). Charge balance may be achieved by assigning
formal cationic oxidation states as Cs (I), U (IV), and P (V).
Interatomic distances are typical for the coordination numbers
and valencies of U and Cs: U1–O distances are in the range of
˚
2.223(4)–2.327(4) A with an additional long distance at
distances in CsU2(PO4)3 and
coordination environments.
b
–KU2(PO4)3 are similar, as are the
˚
˚
2.793(4) A, similar to that found in U(UO2)(PO4)2, 2.158–2.604 A
b
–KU2(PO4)3 effectively immobilizes U;
˚
˚
[30]. U2–O distances range from 2.198(4) A to 2.268(4) A, smaller
than in KU2(PO4)3, 2.269–2.320 A [18]. Cs–O distances range from
this suggests that CsU2(PO4)3 might also be able to do so.
˚
Mobility of alkali metals is more closely linked to the presence
of structural motifs such as channels and layers. There are small
channels in the CsU2(PO4)3 structure (Fig. 3). Ionic mobility in
˚
˚
3.019(4) A to 3.592(4) A. These values are within the range of
˚
2.949–3.733 A found in Cs2MoO4 [31].
The overall structure may be visualized by dividing it into
layers parallel to (010). The triangular faces of the UO6 distorted
octahedra are approximately aligned to the plane of the layer.
These octahedra sit on the corners of a band of double triangles
two UO6 octahedra wide. P1 and P2 tetrahedra sit above and
below the triangles, corner sharing with three adjacent UO6
octahedra. P3 tetrahedra corner share with two UO6 octahedra
at the edge of this band. Each Cs atom lies between these bands in
a zig-zag pattern, corner sharing with one of its symmetry-related
equivalents (Fig. 3). The layers are stacked along [010] such that
the Cs atoms in the next layer are coordinated to the last vertices
channel structures may be estimated from the channel size
D,
where is equal to the shortest open distance between two
D
atoms on opposite sides of the channel measured perpendicular
Fig. 4. Stacking of the layers in CsU2(PO4)3, viewed along [100]. Cs–O bonds are
omitted.
Fig. 2. Coordination environments for atoms U1, U2, and Cs1. U is black, Cs is blue,
O is red, and PO34ꢀ tetrahedra are brown. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. A single layer of the CsU2(PO4)3 structure viewed down [010]. U octahedra
are black, Cs atoms are blue, and PO34ꢀ tetrahedra are striped brown. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
Fig. 5. A single layer of the NZP structure type viewed down [001]. U octahedra are
black, K atoms are blue, and PO34ꢀ tetrahedra are striped brown. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version
of this article.)

G.N. Oh et al. / Journal of Solid State Chemistry 185 (2012) 124–129
127
Fig. 6. abcabc stacking arrangement in NZP, viewed down [001]. U octahedra
are black and
octahedra are striped blue. PO34ꢀ tetrahedra are omitted.
K
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Fig. 7. Simulated powder pattern (black) compared to the experimental powder
pattern of CsU2(PO4)3. Here and in Fig. 8 values of 2y (CuKa) are along the abscissa.
Peak at 2
y
¼28.441 is from the Si internal standard.
Fig. 8. Simulated powder pattern (black) compared to the previously reported
(purple) b0- (top) and
g-CsU2(PO4)3 (bottom) patterns. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
to the channel axis. In the present structure, the shortest intera-
this article.)
˚
tomic distance of 4.39(1) A is between atom O8 and one of its
symmetry equivalents. Subtraction of the crystal radii for both
O2ꢀ atoms from this distance leaves an open distance of
Figs.
-CsU2(PO4)3 [13]. Our experimental powder pattern is in good
agreement with the simulated powder pattern, but those for both
b0 and
phases are in poor agreement, in part because they
display many low-angle peaks that do not appear on the simu-
lated pattern. The presence of impurities would not account for
the overall poor agreement between the simulated and those
experimental powder patterns. From these comparisons we con-
clude that the structure presented here is new, being neither that
7 and 8 - and
with those previously reported for b0
˚
1.97(2) A, significantly smaller than the crystal diameter of ten-
g
˚
coordinate Cs, 3.9 A [32]. Any movement of Cs in CsU2(PO4)3
would require a large deformation of the structure, suggesting U
and Cs might both be immobile in CsU2(PO4)3. Note that the NZP-
g
type CsZr2(PO4)3 has been shown to be an effective immobiliza-
˚
tion matrix for Cs and it has a much larger
D of 3.69(2) A [19].
3.3. Comparison to b0- and
g-CsU2(PO4)3
of b0- nor that of
g-CsU2(PO4)3.
Whereas the reaction temperature for the synthesis of
the present CsU2(PO4)3 is in the range used to synthesize
An experimental powder pattern as well as one simulated
from the single-crystal data of CsU2(PO4)3 is compared in

128
G.N. Oh et al. / Journal of Solid State Chemistry 185 (2012) 124–129
b0-CsU2(PO4)3, the overall synthetic route is completely different.
The previously reported compounds were synthesized by reacting
UO2, CsNO3, and 1 M H3PO4 at 373–573 K, then annealing under
an Ar atmosphere between 1173 K and 1373 K for b0-CsU2(PO4)3,
octahedral coordination environment whereas atom U1 is not and
is probably the source of the pleochroism. The f–f transitions most
affected by this anisotropy are located around 1.95, 2.07, and
2.26 eV, the latter two being negatively correlated with the
first one.
and between 1373 and 1773 K for g-CsU2(PO4)3.
3.4. Optical properties
4. Conclusions
The transmittance spectra of CsU2(PO4)3 are presented in Fig. 9.
The peaks are sharp and fairly weak, typical of f–f transitions [33].
Such transitions are between states split by the crystal field of the
U atoms [34]. Weak pleochroism is a common property of colored
solid-state compounds [35]. CsU2(PO4)3 exhibits strong pleochroism,
which is much rarer but is found in minerals such as tourmaline,
iolite, and andalusite. Structures with orthorhombic or lower sym-
metry may display trichroism. However, measurements on additional
faces of CsU2(PO4)3 produced similar spectra; thus the compound is
dichroic rather than trichroic.
We have synthesized and structurally characterized CsU2(PO4)3,
which crystallizes in a new structure type. An X-ray powder diffrac-
tion pattern of this compound is not related to the patterns for b0
and -CsU2(PO4)3 that were reported earlier. Examination of the
-
g
structural features of the present structure suggests that it might be
able to immobilize both Cs and U, two important components of
nuclear waste. Optical measurements emphasize the importance of
crystalline electric field effects in the electronic properties of actinide
compounds.
Pleochroism is a result of anisotropy in the coordination
environment of optically active atoms. Atom U2 is in a nearly
Supporting material
The crystallographic data for CsU2(PO4)3 has been deposited
with FIZ Karlsruhe as CSD number 423508. These data may be
obtained free of charge by contacting FIZ Karlsruhe at
þ497247808666 (fax) or crysdata@fiz-karlsruhe.de (email).
Acknowledgments
Funding for this research was kindly provided by the US
Department of Energy, Basic Energy Sciences, Chemical Sciences,
Biosciences, and Geosciences Division and Division of Materials
Science and Engineering Grant ER–15522, and the National
Science Foundation (CHE–0911145) and NSF MRSEC (DMR–
0520513) at the Materials Research Center of Northwestern
University. SEM analyses were conducted in the Electron Probe
Instrumentation Center (EPIC) at the Northwestern University
Atomic and Nanoscale Characterization Experimental (NUANCE)
Center, supported by NSF-NSEC, NSF-MRSEC, Keck Foundation,
the State of Illinois, and Northwestern University. Single-crystal
data were collected at the IMSERC X-ray Facility at Northwestern
University, supported by the International Institute of Nanotech-
nology (IIN). Powder X-ray diffraction data were collected at the J.
B. Cohen X-Ray Diffraction Facility supported by the MRSEC
program of the NSF (DMR–0520513) at the Materials Research
Center of Northwestern University.
Appendix A. Supplementary material
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2011.10.007.
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