Flux crystal growth, structure, magnetic and optical properties of a family of alkali uranium(IV) phosphates

Article abstract of DOI:10.1016/j.jssc.2018.10.033

A family of alkali uranium(IV) phosphates, AU₂(PO₄)₃ (A = Li – Cs), was synthesized as single crystals by the reaction of US₂ and (NH₄)₂HPO₄ in the respective ACl (A = Li – Cs) flux contained in a sealed fused-silica tube at 850 °C, and as phase pure powders from a similar reaction using UF₄ as the uranium source. AU₂(PO₄)₃ (A = Li – Rb) crystallize in the NaTh₂(PO₄)₃ (NTP) structure type with monoclinic space group C2/c and consist of a 3D structure that features a framework composed of edge- and corner-sharing polyhedra. The cesium analogue, CsU₂(PO₄)₃, crystallizes in a different structure with space group P2₁/n that is related to the NaZr₂(PO₄)₃ (NZP) structure type and consists of a framework composed of corner-sharing polyhedra. Two new alkali uranium phosphates, Li₂U(PO₄)₂ and Cs₄U₄(P₂O₇)₅, were also grown as single crystals at 700 °C. Li₂U(PO₄)₂ was isolated in approximately 30% yield based on uranium. Li₂U(PO₄)₂ crystallizes in space group P2₁/c exhibiting a layered structure while Cs₄U₄(P₂O₇)₅, crystallizes in space group P2₁/n in a new structure type featuring a 3D framework. The magnetic susceptibilities and the field dependent magnetizations were measured for AU₂(PO₄)₃ (A = Li, Na, K and Cs); all compounds exhibited negative Weiss temperatures with no obvious antiferromagnetic transition. Optical properties were measured by UV–vis and IR spectroscopy.

Full text of DOI:10.1016/j.jssc.2018.10.033

Author’s Accepted Manuscript
Flux Crystal Growth, Structure, Magnetic and
Optical Properties of a Family of Alkali
Uranium(IV) Phosphates
Mohammad Usman, Gregory Morrison, Vladislav
V. Klepov, Mark D. Smith, Hans-Conrad zur Loye
www.elsevier.com/locate/yjssc
PII:
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https://doi.org/10.1016/j.jssc.2018.10.033
YJSSC20442
DOI:
Reference:
To appear in:
Journal of Solid State Chemistry
Received date: 17 September 2018
Revised date: 18 October 2018
Accepted date: 21 October 2018
Cite this article as: Mohammad Usman, Gregory Morrison, Vladislav V. Klepov,
Mark D. Smith and Hans-Conrad zur Loye, Flux Crystal Growth, Structure,
Magnetic and Optical Properties of a Family of Alkali Uranium(IV) Phosphates,
Journal of Solid State Chemistry, https://doi.org/10.1016/j.jssc.2018.10.033
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Flux Crystal Growth, Structure, Magnetic and Optical Properties of a Family of
Alkali Uranium(IV) Phosphates
Mohammad Usman, Gregory Morrison, Vladislav V. Klepov, Mark D. Smith and Hans-Conrad
zur Loye*
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208
Abstract:
A family of alkali uranium(IV) phosphates, AU2(PO4)3 (A = Li – Cs), was synthesized as single
crystals by the reaction of US2 and (NH4)2HPO4 in the respective ACl (A = Li – Cs) flux
contained in a sealed quartz tube at 850 °C, and as phase pure powders from a similar reaction
using UF4 as the uranium source. AU2(PO4)3 (A = Li – Rb) crystallize in the NaTh2(PO4)3 (NTP)
structure type with monoclinic space group C2/c and consist of a 3D structure that features a
framework composed of edge- and corner-sharing polyhedra. The cesium analog, CsU2(PO4)3,
crystallizes in a different structure with space group P21/n that is related to the NaZr2(PO4)3
(NZP) structure type and consists of a framework composed of corner sharing polyhedra. Two
new alkali uranium phosphates, Li2U(PO4)2 and Cs4U4(P2O7)5, were also grown at 700 °C.
Li2U(PO4)2 was isolated in approximately 30% yield based on uranium. Li2U(PO4)2 crystallizes
in space group P21/c exhibiting a layered structure while Cs4U4(P2O7)5, crystallizes in space
group P21/n in a new structure type featuring a 3D framework. The magnetic susceptibilities and
the field dependent magnetizations were measured for AU2(PO4)3 (A = Li, Na, K and Cs); all
compounds exhibit negative Weiss temperatures with no obvious antiferromagnetic transition.
Optical properties were measured by UV-vis and IR spectroscopy.
Graphical Abstract
1

Alkali uranium (IV) phosphates and their magnetic properties.
Keywords: Uranium phosphates, Flux growth synthesis, Magnetic susceptibility, Optical
properties
Corresponding Author: *E-mail: zurloye@mailbox.sc.edu
1. Introduction
Increasing our understanding of the crystal chemistry of uranium containing materials is
highly desirable due to the existence of high-level nuclear waste generated during the Cold War
era and, more recently, as a product of nuclear power generation; in the near future, all of this
needs to be safely sequestered in waste forms for long term storage.1-4 Promising material
families for immobilizing radionuclides include monazites, apatites, and phosphates due to their
ability to serve as a mineral-like matrix with high thermal stability.5-15 The known uranium (IV)
phosphates crystallize in a wide variety of structure types and, within this structural diversity,
uranium phosphates readily incorporate lanthanides, alkali metals, and alkaline-earth metals into
their structures.16-20
Among the alkali uranium phosphates, at least three structures are observed with the
composition AU2(PO4)3, including polymorphism in the K analogue. ⍺-AU2(PO4)3 (A = Li, Na,
K), henceforth and typically referred to without the alpha designation, crystallize in the
NaTh2(PO4)3 (NTP) structure type21 with space group C2/c.22,23 Single crystals have been
synthesized from the respective alkali phosphate or ammonium phosphate and UO2 in a B2O3
flux at 1200 °C (A = Li - K)23-25 and from an intimate mixture of K2CO3, UO2(NO3)2·6H2O,
(NH4)2HPO4, and UO2 heated to 1100 °C (A = K).22 Although single crystals of NTP-type
2

AU2(PO4)3 (A = Li – K) have been previously prepared, only the structure of KU2(PO4)3 has
been determined via single crystal X-ray diffraction.22 Furthermore, with the exception of the
potassium analogue,22 magnetic and optical properties of these compounds have not been
reported.
A second polymorph, β-AU2(PO4)3 (A = K, Rb), crystallizes in the KZr2(PO4)3
(kosnarite)/NaZr2(PO4)3 (NZP) structure type26,27 with space group R-3c.28 This is a high
temperature polymorph and has been synthesized via a solid state reaction at 1200-1250 °C (A =
K) and 1100 °C (A = Rb).28,29
CsU2(PO4)3 crystallizes in a structure related to the NZP structure type with space group
P21/n, and has been synthesized as single crystals from the reaction of UP2, Se, and CsCl flux
with a carbon coated fused-silica tube at 1100 °C.5 Other polymorphs of AU2(PO4)3 have been
prepared from solid-state reactions and identified via powder X-ray diffraction but their
structures have not been elucidated.30
For many years, our group has employed molten halide fluxes for the preparation of high
quality single crystals of a wide variety of actinide compounds and has used these crystals for
physical property measurements.31-35 We are particularly interested in exploring the use of
inorganic halide salts as media in which to dissolve a variety of uranium precursors and from
which to crystallize the product phases in the form of X-ray diffraction quality single crystals.
We actively pursued the growth of single crystals of the AU2(PO4)3 series after a green colored
‘impurity’ crystallized in a reaction involving US2, P2S5, Na2S and Sb2S3, performed to
crystallize new uranium thiophosphate phases.36 Single crystal X-ray diffraction was used to
confirm the identity of the impurity as NTP-type NaU2(PO4)3. With this information in hand, the
rational synthesis of NaU2(PO4)3 was carried out. Herein, we report on the synthesis of the entire
3

NTP-type AU2(PO4)3 (A = Li – Rb) family including the previously unobserved Rb analogue,
their structure determination using single-crystal X-ray diffraction, and the measurements of
magnetic and optical properties. The synthesis and properties of the previously reported
CsU2(PO4)3 are also reported. Finally, the syntheses and structures of two new alkali uranium
phosphates, Li2U(PO4)2, which is isostructural with Li2Th(AsO4)2,37 and Cs4U4(P2O7)5 which
crystallizes in a new structure type are also discussed.
2. Experimental section
2.1. Reagents
UF4 (International Bio-Analytical Industries, powder, ACS grade), Al2O3 (Alfa Aesar,
powder, 99.98%) and (NH4)2HPO4 (VWR) were used as received without any further
modification. LiCl anhydrous (Acros Organics, powder, 99.0%), NaCl (Fisher Chemical,
powder, 99.0%), KCl (Aldrich, 99.98%), RbCl (STREM Chemicals, 99.0%) and CsCl (Alfa
Aesar, 99.0%) were dried overnight in an oven maintained at 260 °C.
Caution! Although the uranium precursors used contained depleted uranium, standard
safety measures for handling radioactive substances must be followed.
US2 was prepared from a stoichiometric reaction between uranium turnings (International
Bio-Analytical Industries, ACS grade) and sulfur (Fisher Chemical). To remove UO2 from the
surface of the uranium turnings, they were briefly treated with concentrated nitric acid and
washed thoroughly with acetone. The mixture was loaded into a carbon-coated fused silica tube
which was evacuated to a pressure of <10-4 Torr and sealed using an oxygen/methane torch. The
tube was placed in a programmable furnace and heated to 700 °C at a ramp rate of 0.5 °C/min,
dwelled at this temperature for 48h and rapidly cooled to room temperature by shutting the
furnace off. A slow ramp rate (<1 °C/min) is used to prevent the tube from bursting by avoiding
4

a sudden pressure buildup caused by rapid vaporization of sulfur. High-purity US2 can be
produced by this method on a gram-scale.
2.2. Synthesis
Li2U(PO4)2. Single crystals of Li2U(PO4)2 were grown by charging a carbon-coated fused
silica tube with a mixture of 2 mmol of (NH4)2HPO4 and 1.50 g of LiCl. This charge was heated
in air at 450 °C for 20 mins to decompose (NH4)2HPO4 and react it with the flux followed by the
addition of 1 mmol US2, evacuating the tube to a pressure of <10-4 Torr, and sealing it using an
oxygen/methane torch. US2 dissolves readily in alkali halide fluxes and was therefore chosen as
the uranium (IV) source over UO2 for all reactions. UO2 has almost negligible solubility in alkali
halide fluxes over a wide temperature range and was, therefore, not used for preparation of the
titled uranium (IV) phosphates. The tube containing the charge was placed in a programmable
furnace and heated to 700 °C at a ramp rate of 10 °C/min, dwelled at this temperature for 20h,
cooled to 500 °C at 3 °C/h and rapidly cooled to room temperature by turning the furnace off.
Pale green needle-like crystals of Li2U(PO4)2 were obtained in approximately 30% yield, based
on uranium, and isolated by dissolving the solidified flux in water aided by sonication for 20 –
30 mins followed by vacuum filtration.
AU2(PO4)3 (A = Li – Cs). Single crystals of AU2(PO4)3 were grown out of an ACl flux.
Each reaction was started by charging a carbon-coated fused silica tube with a mixture of 1.50 g
of ACl and 3 mmol of (NH4)2HPO4. This charge was heated in air at 450 °C for 20 mins
followed by the addition of 1 mmol US2 and quickly evacuating the tube to a pressure of <10-4
Torr and sealing it using an oxygen/methane torch. The sealed tube was placed in a
programmable furnace and heated to 850 °C at a ramp rate of 10 °C/min, dwelled at this
temperature for 20h, cooled to a temperature 100 °C below the melting point of ACl at a rate of 6
5

°C/h followed by rapid cooling to room temperature by turning off the furnace. Post-reaction
workup involved dissolving the solidified flux in water aided by sonication and separating the
crystals using vacuum filtration. The reaction yielded an almost single-phase product consisting
of irregularly shaped green blocks of AU2(PO4)3. CsU2(PO4)3 crystals appear lavender when seen
with a naked eye and teal when viewed under the microscope due to being highly pleochroic.5
For all AU2(PO4)3 phases, PXRD analysis revealed the presence of a weak reflection at ~ 2θ =
36° which could not be identified but was determined to be magnetically active, vide infra. With
the exception of Rb, polycrystalline powders of AU2(PO4)3 without this magnetic impurity were
obtained by changing the uranium source from US2 to UF4. To prevent the silica tube from
corroding and potentially shattering due to chemical etching by the fluoride flux, the dwell time
was reduced to 2h and the reactions were fast cooled by turning off the furnace.
Cs4U4(P2O7)5. Single crystals of Cs4U4(P2O7)5 were grown in an attempt to crystallize a
new alkali uranium aluminophosphate from a reaction involving US2, Al2O3 and (NH4)2HPO4 in
a 1:1:3 molar ratio and 0.80 g of CsCl flux. Decomposition of (NH4)2HPO4 was not carried out
in this case. The tube loaded with the reaction mixture was evacuated to a pressure of <10-4 Torr
and sealed using an oxygen/methane torch. It was placed in a programmable furnace and heated
to 700 °C at a ramp rate of 10 °C/min, dwelled at this temperature for 2h and cooled to 500 °C at
100 °C/h whereupon the furnace was turned off. Green plates of Cs4U4(P2O7)5 were obtained
after dissolving the solidified flux in water aided by sonication followed by vacuum filtration.
2.3. Single-crystal X-ray diffraction
X-ray intensity data from crystals of Li2U(PO4)2, AU2(PO4)3 (A = Li, Na, Rb) and
Cs4U4(P2O7)5 were collected at 301(2) K using a Bruker D8 QUEST diffractometer equipped
with a PHOTON 100 CMOS area detector and an Incoatec microfocus source (Mo Kα radiation,
6

λ = 0.71073 Å).38 The crystals were mounted on a microloop with immersion oil. The raw area
detector data frames were reduced and corrected for absorption effects using the SAINT+ and
SADABS programs.39,40 Final unit cell parameters were determined by least-squares refinement
of a large array of reflections taken from each data set. Initial structural solutions were obtained
with SHELXT. Subsequent difference Fourier calculations and full-matrix least-squares
refinement against F2 were performed with SHELXL-2018 using the ShelXle interface.41 All
atoms were refined with anisotropic displacement parameters except for the disordered and half
occupied lithium and O(18) atoms in Li2U(PO4)2 and Cs4U4(P2O7)5 respectively, which were
refined isotropically. Crystallographic data for Li2U(PO4)2, AU2(PO4)3 and Cs4U4(P2O7)5 and
selected interatomic distances and bond angles are listed in Tables 1 and 2, respectively.
2.4. Powder X-ray Diffraction
Powder X-ray diffraction data were collected on a Bruker D2 Phaser powder X-ray
diffractometer using Cu Kα radiation. The step-scan covered the angular range 5 – 65° 2θ in
steps of 0.02°. All experimental and calculated PXRD patterns are provided in Fig. S1.
2.5. Energy-Dispersive Spectroscopy
Elemental analysis was performed on all reported compositions using a TESCAN Vega-3
SBU scanning electron microscope (SEM) with EDS capabilities. Small batches of powder were
mounted on carbon tape and analysis was carried out using a 20 kV accelerating voltage and an
accumulation time of 20 s. EDS verified the presence of U, P, O and the respective alkali metal
(except for Li which cannot be detected by the instrument). The absence of extraneous elements
such as silicon from the silica ampoule or fluorine from UF4 was confirmed within the detection
limits of the instrument.
2.6. UV-vis Diffuse Reflectance Spectroscopy
7

Diffuse reflectance spectra of polycrystalline AU2(PO4)3 samples were obtained using a
Perkin-Elmer Lambda 35 UV-vis scanning spectrophotometer equipped with an integrating
sphere over the range of 300–900 nm. Reflectance data were converted to absorbance using the
Kubelka–Munk function.42
2.7. Infrared spectroscopy
Infrared spectroscopy was performed using a Perkin-Elmer spectrum 100 FT-IR
spectrometer with a diamond ATR attachment. IR spectra were recorded in the spectral range of
4000 to 650 cm-1. Final IR spectra consist of 16 total averaged scans.
2.8. Magnetic Susceptibility
Magnetic properties of phase-pure polycrystalline AU2(PO4)3 samples were measured
using a Quantum Design Magnetic Properties Measurement System (QD MPMS 3 SQUID
Magnetometer). Magnetic susceptibility was measured under zero-field cooled (zfc) and field
cooled (fc) conditions from 2 to 375 K at applied fields of 0.1 T. Magnetization as a function of
field (M vs. H) was measured from -5 T to 5 T at 2 K. Data were corrected for sample shape and
radial offset effects as described previously.43
3. Results and Discussion
3.1 Synthesis
Flux crystal growth was used to synthesize the AU2(PO4)3 series as well as the new alkali
uranium phosphates, Li2U(PO4)2 and Cs4U4(P2O7)5. The use of alkali halide fluxes allowed these
reactions to take place at 850 °C or less, significantly lower than the previously reported
syntheses that consisted of dwelling temperatures in excess of 1000 °C and longer reaction
times.5,22,24 High-quality single crystals of AU2(PO4)3 were synthesized using US2, (NH4)2HPO4
and ACl (A = Li – Cs) flux as the uranium, phosphate and alkali cation sources, respectively.
8

Polycrystalline powders of KU2(PO4)3 and CsU2(PO4)3 were obtained using the same approach
but by using faster cooling rates. PXRD performed on these compounds revealed that an almost
phase-pure sample was obtained in each case, except for an unidentified weak reflection
observed at ~2θ = 36° for nearly all analogues. Magnetic measurements on this sample of
KU2(PO4)3 indicated the impurity to be magnetically active as the magnetic behavior, an
antiferromagnetic transition at low temperatures and the observed moment, were inconsistent
with the reported magnetic properties of KU2(PO4)3 by Guesdon et al. and the calculated
magnetic moment for U(IV).22,44,45 A myriad of attempts were made to obtain phase-pure
AU2(PO3)4 samples, including tweaking the reaction conditions, using different alkali halide
fluxes, solid state syntheses employing A2CO3/ANO3 precursors; all, however, were
unsuccessful. Finally, changing the uranium source from US2 to UF4 with a 2h dwelling time and
fast cooling instead of slow cooling yielded polycrystalline samples of the Li, Na, K, and Cs
analogues without this magnetic impurity. However, an appreciable amount of LiF was present
in the sample of LiU2(PO4)3. All attempts to prepare phase pure RbU2(PO4)3 were unsuccessful.
3.2 Structure
Li2U(PO4)2. Li2U(PO4)2 crystallizes in the centrosymmetric monoclinic system, space
group P21/c (Table 1), which was uniquely consistent with the pattern of systematic absences in
the intensity data and was confirmed by the structure solution. The asymmetric unit consists of
one uranium atom, two PO4 groups and three lithium atom sites.
Isostructural with Li2Th(AsO4)2,37 Li2U(PO4)2 is composed of [U(PO4)2]2- 2D layers as
shown in Fig. 1. The monovalent Li cations reside in the interlayer space providing charge
balance and connection of the corrugated sheets into a 3D assembly. All uranium atoms occupy
the same crystallographic position and exhibit a 9-fold coordination environment, surrounded by
9

oxygen. The U–O bond distances lie in the range of 2.287(3) – 2.526(3) Å. Two symmetrically
independent phosphorus atoms form PO4 tetrahedra with P–O bond lengths ranging from
1.507(3) – 1.556(3) Å. These UO9 and PO4 polyhedra edge- and corner-share to form corrugated
sheets parallel to the (100) plane. Unlike Li2Th(AsO4)2 with two independent Li sites,
Li2U(PO4)2 features three independent Li sites all located on positions of general crystallographic
symmetry (site 4e). Two lithium atoms, Li(2) and Li(3) are disordered. Both Li(2) and Li(3) sites
were refined to near half-occupancy and gave acceptable displacement parameters. For the final
refinement cycles, the total occupancy Li(2) + Li(3) was constrained to sum to one. The largest
residual electron density peak and hole in the final difference map are +3.36 and -1.71 e-/Å3,
located 0.80 Å from O7 and 0.61 Å from O8, respectively. These are not chemically significant,
and are most likely artefacts arising from the moderate crystallinity of the available crystals. All
lithium atoms exhibit a 4-fold coordination to oxygen with bond lengths ranging from 1.862(8) –
2.269(8) Å.
AU2(PO4)3. AU2(PO4)3 (A = Li – Rb) crystallize in the monoclinic system. The space
groups C2/c and Cc were consistent with the pattern of systematic absences in the intensity data.
The centrosymmetric C2/c was confirmed by structure solution. The compounds adopt the
NaTh2(PO4)3, or NTP, structure type. The asymmetric unit consists of one uranium atom, two
phosphorus atoms, six oxygen atoms and one lithium atom. All atoms are located on general
crystallographic positions (Wyckoff site 8f) except phosphorus P(2), which is located on a
crystallographic two-fold axis of rotation (Wyckoff site 4e). The crystal structure of LiU2(PO4)3
is shown in Fig. 2. In contrast to the layered structure of Li2U(PO4)2, AU2(PO4)3 (A = Li – Rb)
feature a 3D structure consisting of a [U2(PO4)3]– framework which contains corner- and edge-
∞
sharing UO9 polyhedra and PO4 tetrahedra. The 3D [U2(PO4)3]– framework is comprised of
∞
10

[U2O6(PO4)2]∞ slabs that are connected along crystallographic a-axis through P(1)O4 tetrahedra.
The P(2)O4 tetrahedra are located between the UO9 edge-sharing polyhedra in the slabs along
the crystallographic b-axis. This connectivity between the UO9 polyhedra and PO4 tetrahedra
generates eight-sided channels running down the crystallographic c-axis. These channels are
bounded by four UO9 polyhedra and four PO4 tetrahedra arranged in an alternating fashion and
are occupied by the monovalent alkali cations which are disordered across an inversion center
(site 4a) and which provide charge balance. This is consistent with what is observed in
ATh2(PO4)3 (A = Na, K).22 The U–O distances lie in the range of 2.254(2) – 2.534(1) Å, 2.264(4)
– 2.590(4) Å and 2.298(2) – 2.599(2) Å for the Li, Na and Rb analogues respectively.
CsU2(PO4)3. CsU2(PO4)3 crystallizes in the space group P21/n in a structure related to the
NZP-structure type (space group R-3c). It features corner-sharing polyhedra forming channels
running down the a-axis. Uranium in CsU2(PO4)3 is six-coordinated instead of nine as in the
NTP-type AU2(PO4)3 phases. The crystal structure of CsU2(PO4)3 is shown in Fig. 3 with the
detailed structural description provided elsewhere.5 The lattice parameters and the unit cell
volume at 300 K for CsU2(PO4)3 are provided in Table 1; they are slightly larger than the
previously reported values which were determined at 100(2) K.5
Cs4U4(P2O7)5. Cs4U4(P2O7)5 crystallizes in the space group P21/n. The asymmetric unit
consists of two uranium atoms, five phosphorus atoms, eighteen oxygen atoms and two cesium
atoms. All atoms are located on positions of general crystallographic symmetry (site 4e). The
crystal structure of Cs4U4(P2O7)5 comprises of isolated seven-coordinated U(1) and eight-
coordinated U(2) polyhedra which form a 3D framework through corner- and edge-sharing with
pyrophosphate units. Atom U(1) corner-shares with seven P2O7 units. Atom U(2) corner-shares
with six P2O7 units and edge-shares with one P2O7 unit. All PO4 tetrahedra are bridged via the
11

disordered O(18) atom to form pyrophosphate units. The U-O distances for U(1) polyhedra lie in
the range of 2.246 – 2.366 Å and 2.299 – 2.580 Å for U(2) polyhedra. Fig. 4 illustrates the
crystal structure of Cs4U4(P2O7)5 and the local coordination environments of the two
crystallographically unique uranium atoms in the structure.
3.3 Magnetic susceptibility
The magnetic susceptibilities of all phase pure AU2(PO4)3 compositions were measured in
an applied field of 0.1 T. For all compositions, U(IV) is the sole contributor to the magnetic
susceptibility. No differences were observed between the ZFC and FC data and only the ZFC
data are shown. For AU2(PO4)3 (A = Li – K), the susceptibility follows the Curie–Weiss law
reasonably well at higher temperatures and magnetic moments could be obtained from a fit of the
Curie–Weiss law to the inverse susceptibility using only the highest temperature data (200-375
K). At lower temperatures, it is well known that the U(IV) ion begins to adopt a nonmagnetic
single ground state. The singlet ground state is the reason that U(IV) magnetic moments in the
literature are typically reported for a single temperature, 300 K, using the expression
2.828(χmT)1/2 at 300 K. The magnetic moments for all AU2(PO4)3 are listed in Table 3 and are in
good agreement with the magnetic moment values reported for U(IV). The observed deviations
from the calculated value of 3.58 μB are likely due to crystal field effects. In all compounds, the
magnetic behavior begins to deviates from the Curie–Weiss law below ~ 100 K. Interestingly,
CsU2(PO4)3, does not follow the Curie–Weiss law over the entire temperature range measured, 2-
375 K. The magnetic moment for this phase, therefore, was only determined from the value of
2.828(χmT)1/2 at 300 K. The difference in magnetic behavior is likely due to the structural
differences between CsU2(PO4)3 and other AU2(PO4)3 compounds, where the main difference
lies in the uranium coordination environment. Specifically, uranium is located in an octahedral
12

coordination environment in CsU2(PO4)3, compared to the nine-fold coordination environment in
the other AU2(PO4)3 compounds.5,22 The impact of the uranium coordination environment on the
stability of the singlet ground state has been discussed in the literature.46,47
The temperature dependent magnetic properties for all AU2(PO4)3 compositions are
displayed in Fig. 5. The magnetic field dependent magnetic properties at 2 K are provided in Fig.
6. All compositions exhibited large negative Weiss constants, indicative of antiferromagnetic
interactions, but no antiferromagnetic transition was observed down to 2 K.
3.4 Optical properties
The optical properties of all phase pure AU2(PO4)3 compositions were examined. The
UV-vis absorption spectrum was found to consist of many weak absorption bands, as expected
for the f–f electronic transitions in uranium, as shown in Fig. 7. These are similar to what has
been reported for other U(IV) containing materials which are also green in color.45 Considering
the Na analogue, four intense bands are seen at about 470 nm, 480–520, 530–570, and 580–650
3
nm. They correspond to the multiplets P2, 1I6 and 3P1 and, for the broad and intense band in the
red, to a mixture of several levels, 1G4, 1D2, and 3P0. Some weak absorption lines around 750 nm
3
are assigned as H6. All these absorption lines are in good agreement with what is expected for
U4+ ions in the crystal structure as reported by Bénard et al.48 Absence of any bands below 450
nm which are mainly characteristic of uranyl entities or U(VI) ions supports the sole presence of
U(IV) ion in the compounds. The infrared absorption spectra, which can be seen in Fig. 8,
contain a broad absorption band at 1000 cm-1 characteristic of the P–O stretching modes of the
PO4 units in the crystal structure.
4. Conclusion
13

We have synthesized and characterized a series of quaternary uranium phosphates
containing U4+, a new layered lithium U(IV) phosphate, Li2U(PO4)2, AU2(PO4)3 (A = Li – Cs)
and a new cesium U(IV) phosphate, Cs4U4(P2O5)7. All phosphate compositions were grown as
single crystals out of alkali chloride fluxes. The Li2U(PO4)2 structure features corrugated
[U(PO4)2]2- layers with Li+ occupying the interlayer spaces. The AU2(PO4)3 (A = Li – Rb)
compounds exhibit a three-dimensional structure featuring eight-sided channels running down
the c-axis. Cs4U4(P2O5)7 exhibits a three-dimensional structure containing pyrophosphate, P2O7,
units. Temperature dependent magnetization data were collected in an applied magnetic field of
0.1 T for all AU2(PO4)3. Their Curie–Weiss fit and 300 K magnetic moments were consistent
with moments for other U(IV) phases reported in the literature. Although large negative Weiss
constants were measured for these compounds, no antiferromagnetic transition was observed
down to 2 K. Optical property measurements revealed multiple f–f electronic transition bands
characteristic of the U4+ cation and P–O stretching modes consistent with the crystallographic
measurements.
Acknowledgements
Research supported by the US Department of Energy, Office of Basic Energy Sciences, Division
of Materials Sciences and Engineering under award DE-SC0018739.
Supplementary Information
Crystallographic information files (CIFs) for Li2U(PO4)2, AU2(PO4)3 (A = Li, Na, and Rb),
Cs4U4(P2O7)5, and PXRD patterns for AU2(PO4)3 (A = Li, Na, K and Cs)
Table 1. Crystallographic Data for Li2U(PO4)2, AU2(PO4)3 and Cs4U4(P2O7)5.
formula
Li2U(P
LiU2(PO NaU2(PO KU2(PO RbU2(PO4 CsU2(PO CsU2(PO Cs4U4(P2
14

a
c
b
O4)2
)
4 3
)
4 3
)
4 3
)
)
4 3
)
4 3
O7)5
3
formula
weight
441.85
767.91
783.96
800.07
846.44
893.88
893.88
2353.46
(g mol -
1
)
temperat 300(2)
ure (K)
300(2)
300(2)
293
300(2)
300(2)
100(2)
300(2)
space
group
a (Å)
P21/c
C2/c
C2/c
C2/c
C2/c
P21/n
P21/n
P21/n
8.8391( 17.2644( 17.2982( 17.479(2 17.5887(4 9.2030(6 9.2136(4 6.92040(1
6) 4) 4) 0)
13.4150 6.6744(1 6.7076(2 6.758(1) 6.79740(1 8.7205(6 8.6548(3 19.9338(4
(9) 0)
5.3733( 8.0184(2 8.0284(2 8.019(1) 8.0301(2) 15.0356( 15.0161( 12.4330(2
4) 10) 6)
106.335 101.0296 100.6645 102.01(1 102.6030( 90.551(3 91.240(2 101.7420(
)
)
)
)
b (Å)
c (Å)
β (deg)
V (Å3)
Z
)
)
)
)
)
)
)
)
(3)
(9)
(8)
)
10)
)
)
10)
1679.24(5
)
2
611.43( 906.89(3 915.44(4 926.5(3) 936.93(3) 1206.62( 1197.13
7)
4
)
4
)
4
14)
4
4
4
4
density
4.800
5.624
5.688
6.001
4.921
4.960
4.655
24.10
(g cm−3)
μ (mm−1) 27.074
F(000)
36.266
1312
35.977
1344
35.95
40.268
1448
30.47
768
crystal
0.060 x 0.080 x
0.050 x
0.040 x
0.040
0.030 x
0.020 x
0.02
0.050 x
0.040 x
0.010
size (mm 0.020 x 0.060 x
x mm x
mm)
0.010
35.087
2709
0.040
40.312
2857
θmax
27.496
1059
37.868
2524
30.51
3847
(deg)
unique
reflectio
ns
4190
GoF
1.241
1.024
1.181
1.017
1.139
R
R1 =
R1 =
R1 =
R1 =
R1 =
R1 =
R1 =
indices
(all data) wR2 =
0.0477
largest
diff.
0.0288
0.0189
wR2 =
0.0323
1.477
and -
0.0160
wR2 =
0.0351
1.941
and -
0.038
wR2 =
0.036
0.0220
wR2 =
0.0373
2.289 and
-1.521
0.0199
wR2 =
0.0525
0.0263
wR2 =
0.0558
2.240 and
-1.518
3.363
and -
peak.hol 1.709
1.117
0.854
e (e−
Å−3)
a Please refer to Ref. 22 and b Ref. 5 in the manuscript.
c Lattice parameters based off a fast scan consisting of two 180° phi scans
15

Table 2. Selected interatomic distances (Å) and bond angles (deg) for Li2U(PO4)2, AU2(PO4)3
and Cs4U4(P2O7)5.
Li2U(PO4)2
2.287(3)
2.299(3)
2.415(3)
2.423(3)
2.428(3)
2.456(3)
2.483(3)
2.517(3)
2.526(3)
1.507(3)
1.537(3)
71.47(6)
74.03(10)
69.41(6)
84.48(8)
87.27(7)
LiU2(PO4)3
2.254(2)
2.320(1)
2.393(1)
2.446(1)
2.447(1)
2.449(1)
2.473(2)
2.510(1)
2.534(1)
1.517(1)
1.542(1)
72.98(6)
76.98(7)
67.71(6)
84.40(4)
88.91(4)
NaU2(PO4)3
2.264(4)
2.327(4)
2.381(9)
2.388(3)
2.440(4)
2.441(3)
2.454(3)
2.506(4)
2.590(4)
1.518(4)
1.541(4)
72.1(2)
RbU2(PO4)3
2.298(2)
2.317(2)
2.378(2)
2.430(2)
2.444(2)
2.447(2)
2.483(2)
2.486(2)
2.599(2)
1.517(2)
1.552(2)
74.53(8)
77.14(8)
63.53(7)
86.28(5)
90.50(5)
Cs4U4(P2O7)5
U–O
U–O
U–O
U–O
U–O
U–O
U–O
U–O
U–O
P–O
P–O
O–U–O
O–U–O
O–U–O
O–U–P
O–U–P
2.246(5)1 2.299(5)2
2.260(5)1 2.311(5)2
2.273(5)1 2.312(5)2
2.279(5)1 2.317(5)2
2.281(5)1 2.354(5)2
2.292(5)1 2.409(5)2
2.366(5)1 2.437(7)2
2.580(7)2
1.503(5)
1.572(11)
78.6(2)1 74.9(1)2
89.0(2)1 98.1(2)2
79.5(2)1 97.2(1)2
156.1(3)1 102.0(3)2
142.8(3)1 136.2(3)2
73.7(1)
68.4(1)
85.41(9)
88.30(8)
1 U(1) – O bond lengths and bond angles
2 U(2) – O bond lengths and bond angles
Table 3. Magnetic moments for AU2(PO4)3 (A= Li – K, Cs) compositions calculated by 1) fitting
the data to the Curie–Weiss law and 2) determining the 2.828(χmT)1/2 value at 300 K.
Formula
Observed μeff
μeff from
Calculated μeff
θW (K)
(μB/U4+) from C– 2.828(χmT)1/2 (μB/U4+)
W fit (200 – 375 at 300 K
K)
LiU2(PO4)3
NaU2(PO4)3
KU2(PO4)3
CsU2(PO4)3
3.30
3.82
4.01
3.95
3.23
3.58
- 143
- 130
- 84
–
3.40
3.58
3.58 and 3.25a
3.17
Does not fit
3.58
a Please refer to Ref. 22
16

(b)
(c)
(a)
Fig. 1. The crystal structure of Li2U(PO4)2 is shown in (a) down the c-axis formed by infinite
[U(PO4)2]2- layers one of which is shown in (b). Uranium is depicted as green polyhedra,
phosphorus as steel grey tetrahedral, oxygen as red spheres and lithium as maroon spheres. The
local coordination of uranium shown in (c).
(a)
(b)
(d)
(c)
Fig. 2. Polyhedral representation of LiU2(PO4)3. The uranium, phosphorus and lithium cations
are shown as green polyhedra, steel grey tetrahedra and blue spheres respectively in (a), with the
[U2(PO4)3]– 3D framework and the unit cell shown in (b) and (c) respectively. The local
coordination environment of U4+ is shown in (d).
17

(a)
(b)
(a)
(b)
(c)
Fig. 3. Polyhedral crystal structure representation of CsU2(PO4)3 along the a-axis and c-axis
shown in (a) and (b) respectively. Uranium and phosphorus are illustrated as green polyhedra
and grey tetrahedral respectively with the cesium cations shown as pink spheres. The local
coordination environment of U4+ is illustrated in (c).
(a)
(b)
(c)
Fig. 4. Polyhedral representation of the crystal structure of Cs4U4(P2O7)5 down the a-axis shown
in (a). Uranium is depicted as green polyhedra, phosphorus atoms are shown as grey tetrahedra
and cesium cations are represented as pink spheres. The local coordination environments of U(1)
and U(2) atoms are illustrated in (b) and (c) respectively.
18

Fig. 5. Temperature dependent magnetic properties
of AU2(PO4)3 at H = 0.1 T.
Fig. 6. Field dependent magnetic properties of AU2(PO4)3 from -5 T to 5 T.
Fig. 7. UV−vis diffuse absorbance spectroscopy data for AU2(PO4)3. The absorption bands above
400 nm are attributed to the f−f transitions in the U4+ cation.
19

Fig. 8. Infrared spectroscopy data for AU2(PO4)3). The broad absorption band at 1000 cm-1 can
be seen for all phases characteristic of the P–O stretching modes.
20

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22

Highlights
 A family of alkali uranium (IV) phosphates was synthesized using flux crystal growth.
 The crystal structures were solved using single crystal X-ray diffraction.
 Magnetic properties were measured with no evident AF ordering in any phase.
 Optical properties were measured using IR and UV-vis spectroscopy.
23