Retention of a Paramagnetic Ground State at Low Temperatures in a Family of Structurally Related UIV Phosphates

Article abstract of DOI:10.1021/acs.inorgchem.8b01284

A new uranium fluoride phosphate, UFPO₄, was synthesized via a mild hydrothermal route and characterized optically, thermally, and magnetically. Two thermal transformation products, U₂O(PO₄)₂ and U^IVU^VIO₂(PO₄)₂, were discovered to be structurally related, and were subsequently synthesized for bulk property measurements. All three materials failed to follow Curie-Weiss behavior at low temperatures, attributed to the nearly ubiquitous singlet ground state of U(IV), transitioning into a Curie-Weiss paramagnetic regime at high temperatures. Neutron diffraction experiments were performed on UFPO₄ and U^IVU^VIO₂(PO₄)₂ in order to characterize this unusual magnetic behavior.

Full text of DOI:10.1021/acs.inorgchem.8b01284

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Retention of a Paramagnetic Ground State at Low Temperatures in a
Family of Structurally Related UIV Phosphates
Justin B. Felder,† Stuart Calder,‡ and Hans-Conrad zur Loye*,†
†Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States
‡Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 27831, United States
S
* Supporting Information
ABSTRACT: A new uranium fluoride phosphate, UFPO4,
was synthesized via a mild hydrothermal route and charac-
terized optically, thermally, and magnetically. Two thermal
transformation products, U2O(PO4)2 and UIVUVIO2(PO4)2,
were discovered to be structurally related, and were subse-
quently synthesized for bulk property measurements. All three
materials failed to follow Curie−Weiss behavior at low tem-
peratures, attributed to the nearly ubiquitous singlet ground
state of U(IV), transitioning into a Curie−Weiss paramagnetic
regime at high temperatures. Neutron diffraction experiments
were performed on UFPO4 and UIVUVIO2(PO4)2 in order to
characterize this unusual magnetic behavior.
While uranyl chemistry is a rich field that continues to produce
interesting new materials,15−17 exploring the synthesis of U(IV)
containing materials is of particular interest as the U(IV) f2 ion
exhibits properties noticeably different from those found for the
U(VI) f0 ion. In addition, there are comparatively very few reported
U(IV) containing materials, which makes this an attractive area
to investigate.
Whereas the smaller U(VI) ion is fairly soluble, diamagnetic,
and often fluorescent due to the uranyl ion, U(IV) is large,
insoluble, paramagnetic, and has optical properties dominated
by f−f electronic transitions. Among these properties, the mag-
netism of U(IV) is by far the most intriguing. U(IV) is a 5f2
system whose magnetism arises from a combination of spin−
orbit interactions and crystal field effects.18,19 These spin−orbit
effects are primarily due to low-lying thermally excited f states
that can become depopulated at low temperatures. This thermal
depopulation leads to formation of a diamagnetic singlet ground
state, which is extremely prevalent among reported U(IV)
materials.2,20−22
Since the magnetism of U(IV) materials is also dependent on
crystal field effects, it should be possible to influence the
transition to the singlet state by controlling the structure and
composition, which in turn will determine the crystal electric
field. Chemists are extremely good at controlling composition,
less so, however, at controlling structure and crystal-field effects.
Nonetheless, it is not surprising that when exploring the U(IV)
phase space one should encounter materials that have rather
unique crystal-field effects as evidenced by their magnetic
behavior. Herein we report the preparation and magnetic studies
INTRODUCTION
■
Uranium chemistry has been investigated extensively since the
1940s when the focus was primarily on weapons and nuclear
power applications. Only during the past few decades has there
been time to broaden our understanding of uranium chemistry
in general and to explore diverse classes of uranium containing
materials, including oxides,1 fluorides,2 chalcogenides,3 inter-
metallics,4 and oxo-salt phases, such as phosphates.5−8 The
chemistry of uranium phosphates, in particular, has been exten-
sively explored due to the existence of a multitude of known
phosphate minerals. Synthesizing related phosphate materials in
the laboratory has been achieved when using techniques that
mimic nature’s high-temperature, high-pressure solution envi-
ronments. Specifically, the mild hydrothermal regime has
yielded numerous new compositions, including fluorides9 and
phosphates. Although there is a relative dearth of U(IV) mate-
rials, uranium is well-known to form stable phosphate com-
pounds. Phosphate groups have been shown to stabilize the
tetravalent state of uranium, and the various polyoxo phospho-
rus ions interplay with U(IV) to create various structure types.
In particular, there are known orthophosphates (U2O(PO4)2,
U(UO2)(PO4)2),10,11 pyrophosphates (UP2O7),12 and triphos-
phates (U2(PO4)(P3O10).13
Uranium can exhibit a wide range of oxidation states (from +2
to +6), although only the +4 and +6 (and sometimes +5) oxi-
dation states are readily accessible in solid-state materials.14 The
reduced +2 and +3 states are uncommon and unlikely to form
under typical synthetic conditions. Although both the +4 and +6
oxidation states are found in nature, by far the majority of
laboratory-synthesized uranium compounds found in the
chemistry literature focus on U(VI) chemistry, which is domi-
nated by the structure directing uranyl ([OUO]2+) ion.
Received: May 10, 2018
© XXXX American Chemical Society
A
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of three related uranium(IV) phosphates, UFPO4, U2O(PO4)2,
and UIVUVIO2(PO4)2, that are structurally closely related, share
almost identical coordination environments, exhibit unusual
magnetic behavior, and therefore provide us with an opportunity
to further investigate the influence of coordination environment
on U(IV) magnetism.
Table 1. Crystallographic and Refinement Data for UFPO4
empirical formula
formula weight
temperature
F O4 P U
352.00 g/mol
310.52 K
crystal system
space group
orthorhombic
Pnma
a
b
8.6377(2)
6.9826(2)
EXPERIMENTAL SECTION
■
c
6.3607(2)
Materials and Methods. The following materials were used as
received with no further modification: UO2(CH3COO)2, FeF3, H3PO4,
NH4H2PO4, and U3O8. UO2 was synthesized by reducing the U3O8
under a 4% H2 atmosphere at 650 °C for 18 h. UO2 was confirmed pure
by PXRD.
volume
Z
density
absorption coefficient
F(000)
383.637(19) Å3
4
6.095 g/cm3
42.636 mm−1
592.0
0.078 mm × 0.076 mm × 0.022 mm
0.71073 Å
7.958−72.624°
32820
987
987/0/41
1.492
R1 = 0.0176; wR2 = 0.0428
2.30/−1.98 e−/Å3
Caution! Even though the uranium precursors used in these syntheses are
depleted, standard precautions and PPE for handling radioactive materials
should be followed. All work involving uranium was performed in
laboratories specially designated for work with radioactive materials.
UFPO4 was synthesized via a mild hydrothermal reaction between
uranyl acetate and phosphoric acid, with iron(III) fluoride acting as a
fluoride source. The uranyl acetate and iron(III) fluoride were mixed
with 1 mL of methanol in a 23 mL PTFE liner. Next, 1 mL of con-
centrated phosphoric acid was added dropwise to the mixture, and the
liner was sealed inside a stainless-steel autoclave. The sealed autoclave
was placed into a programmable oven that was heated to 200 °C in 1 h,
then held for 24 h followed by slow cooling at a rate of 0.1°/min to
40 °C, at which point the oven was shut off. After cooling to room
temperature, the autoclave was unsealed and the liner opened, revealing
emerald-green product crystals and a green mother liquor. The crystals
were isolated by vacuum filtration and washed thoroughly with water
and acetone. After isolation, the crystals were all found to be of similar
size, ∼0.1 mm on edge.
crystal size
wavelength
2θ range for data collection
reflections collected
independent reflections
data/restraints/parameters
goodness of fit on F2
final R indices (all data)
largest diff. peak and hole
UIVUVIO2(PO4)2 was synthesized by a flux-assisted solid-state
reaction between P2O5 and UO2. The P2O5 was generated in situ by
decomposing ammonium dihydrogen phosphate at 450 °C. Stoichio-
metric amounts of UO2 and NH4H2PO4 were ground together in an
agate mortar, producing an intimate mixture of powders. The powders
were loaded into a fused silica tube, which was placed in a box furnace.
The furnace was heated to 450 °C in 12 h and held at that temperature
for an additional 24 h. Subsequently, the furnace was slowly heated to
800 °C over a period of 12 h to ensure the slow removal of ammonia gas.
The charge was held at 800 °C for 24 h after which point the furnace
was shut off. The product was removed from the tube, ground, and
heated at 850 °C for 24 h. This was repeated twice to yield pure
UIVUVIO2(PO4)2. Samples of U2O(PO4)2 were synthesized by reducing
UIVUVIO2(PO4)2 in a 4% H2/96% N2 mixture at 650 °C for 18 h.
Single Crystal X-ray Diffraction. A suitable crystal of the green
product was selected from the reaction products and X-ray intensity
data were collected using a Bruker D8 QUEST diffractometer. The
instrument utilizes an incoatec microfocus X-ray source (Mo Kα
radiation, λ = 0.71073 Å) and a PHOTON II CMOS area detector. The
detector was operated in the shutterless mode, and additional fast scans
were collected to account for overtopped pixels during the initial data
collection. The final unit cell parameters were determined by least-
squares refinement of a large set of reflections taken from the raw area
detector frames. The data were reduced and corrected for absorption
effects using the SAINT+ and SADABS programs.23 An initial structural
model was obtained using SHELXS24 by direct methods, and subse-
quent least-squares refinements were performed with SHELXL24
through the OLEX2 25 graphical interface.
Figure 1. Fragment of the infinite uranium oxyfluoride chains that make
up the structure of UFPO4 (left). Each UO6F2 polyhedron (right)
shares two edges of oxygen ligands to form the infinite chains running
along the b direction. Uranium is shown as green polyhedra, oxygen is
shown as red spheres, and fluorine is shown as green spheres.
The observed lattice parameters were found to be very similar to
NpFPO4, which was previously reported by Albrecht-Schmitt.26 The
measured crystal was determined to be isostructural with NpFPO4,
crystallizing in the orthorhombic space group Pnma. The data refined
well to an R1 value of 1.76%. The largest peak and hole are 2.30 and
−1.98 e−/Å3 respectively. Selected crystallographic and refinement
information for UFPO4 can be found in Table 1.
and a D/Tex high speed detector. Patterns were collected in the angular
range of 5−65° 2θ with a step size of 0.02°.
Neutron Powder Diffraction. Neutron powder diffraction (NPD)
patterns of UFPO4 and UIVUVIO2(PO4)2 were collected on beamline
HB-2A (POWDER) at ORNL’s HFIR site. Patterns were collected
Powder X-ray Diffraction. PXRD patterns were collected on a
Rigaku Ultima IV diffractometer using Cu Kα radiation (λ = 1.5418 Å)
B
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Figure 2. Edge-sharing uranium chains decorated by phosphate tetrahedra. The chains are connected along the c direction forming “sheets” of linked
chains. The (apparently) unbonded phosphate oxygens shown here serves to bridge these sheets along the a direction (not shown). Uranium is shown
as green polyhedra, phosphorus is shown as orange tetrahedra, oxygen is shown as red spheres, and fluorine is shown as green spheres.
using a bank of 44 3He detectors. Data were collected in the 20−130°
2θ range with a step size of 0.05°. Magnetic contributions were probed
using neutrons with a wavelength of 2.41 Å to obtain good resolution at
low Q.
Thermogravimetric Analysis. Thermal properties were probed
using TGA under flowing N2 or H2 (4% in 96%N2; 100 mL/min) from
room temperature to 800 or 900 °C. The thermal products were
characterized by PXRD and were used to guide the syntheses of
U2O(PO4)2 and UIVUVIO2(PO4)2.
SQUID Magnetometry. Magnetic susceptibility data were collected
using a Quantum Design MPMS3 SQUID magnetometer using
polycrystalline powders obtained by grinding as-synthesized single
crystals. Magnetic susceptibilities were measured in an applied field of
0.1 T in the 2−400 K temperature range. The data were corrected for
shape and radial offset effects using previously described methods.27
RESULTS AND DISCUSSION
■
Powder X-ray Diffraction. Each of the synthesized com-
pounds was characterized by PXRD to confirm the phase purity of
the target composition. The PXRD patterns of UFPO4, U2O(PO4)2,
and UIVUVIO2(PO4)2 are shown in Figure S1.
Synthesis of UFPO4. Although the mild hydrothermal
method has proven extremely effective for synthesizing new
phosphate tetrahedron. This illustrates how the uranium oxyfluoride
Figure 3. Tetranuclear cluster of uranium polyhedra bridged by a
uranium fluorides, the abundance of aqua ligands available in
these systems frequently results in hydrated materials, which
limit thermal stability and due to the presence of hydrogen make
characterization techniques such as neutron diffraction quite
difficult. In order to avoid this, a solvothermal approach using a
methanol/phosphoric acid solution was employed to limit the
amount of water available to the system. In addition, the solu-
bility of many metal species is lower in methanol than in water;
therefore, we expect to observe different crystalline products
upon switching solvent systems.
chains (running along the b direction) are connected in the a direction
by fluoride bridges, and both in the a and c directions by phosphate
bridges. Uranium is shown as green polyhedra, phosphate is shown as
orange polyhedra, oxygen is shown as red spheres, and fluorine is shown
as green spheres.
The use of concentrated HF (48%), while an excellent fluo-
rine source, would introduce a large quantity of water to the
system, and therefore an alternate source of fluorine was desired.
We explored using alkali and alkaline earth metal fluorides as
C
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Figure 4. Overall structure of UFPO4 as viewed down the b axis. The uranium chains running along the b direction are bridged in the c direction only by
phosphate groups, while they are bridged in the a direction by both phosphate groups and fluoride ligands. Uranium is shown as green polyhedra,
phosphorus is shown as orange polyhedra, oxygen is shown as red spheres, and fluorine is shown as green spheres.
Figure 5. View of UFPO4 down the c axis. This view of the structure accentuates the presence of the infinite uranium oxyfluoride chains running parallel
to the b direction. Uranium polyhedra are shown in green, phosphorus is shown in orange, oxygen is shown as red spheres, and fluorine is shown as
green spheres.
D
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soluble fluorine sources; however, the introduction of these
materials resulted consistently in U3F12·H2O, a previously
reported phase.28 We hypothesize that the reduction of U(VI) to
U(IV) via the organic reducing agent, acetate, happens quickly
and that the thus created U(IV) quickly reacts with the available
F− ions and crashes out as the stable U3F12·H2O phase. By intro-
ducing the less soluble (slow-release) FeF3 as a fluorine source,
we ensure that the U(IV) has time to interact with other anions
in solution (i.e., phosphate) before being fluorinated and crys-
tallizing. This method proved successful, and the anhydrous
UFPO4 resulted from these synthetic trials.
Crystal Structure of UFPO4. UFPO4 is a three-dimensional
structure built of cross-linked uranium oxyfluoride chains.
Phosphate groups serve as an additional linkage between the
chains. The chains are built primarily of UO6/2F2/2 8-coordinate
polyhedra. The local environment of a single UO6F2 polyhedron
as well as of one chain fragment are shown in Figure 1. Each
uranium ion is coordinated by 6 oxygen and 2 fluorine ions. Four
of the oxygens lie in an equatorial plane and connect the
polyhedra into chains via edge sharing. The remaining two
oxygen and two fluorine ligands lie in a perpendicular plane with
the fluorine ions arranged roughly trans to one another. Each
chain is decorated with phosphate tetrahedra such that all
phosphate oxygen atoms are shared between uranium and
phosphorus. Figure 2 depicts the phosphate decorated uranium
chain and shows how two chains are bridged by phosphate
tetrahedra in the c direction.
The tetrahedral nature of the phosphate groups causes the
chains shown in Figure 2 to be connected into corrugated sheets.
These corrugated sheets in turn are connected in the a direction
by a combination of phosphate and fluoride bridging groups/
ligands. Figure 3 shows the connectivity in the a direction.
Figure 4 shows the complete structure viewed down the b direc-
tion to emphasize the connectivity of uranium chains. Figure 5
depicts a different orientation to provide a complete view of the
structure down the c axis.
Thermal Behavior of UFPO4. The thermal properties of
UFPO4 were initially investigated by heating UFPO4 to 800 °C
under nitrogen flow. The TG curve reveals a smooth weight loss
beginning immediately upon heating and leveling off at 800 °C.
A plot of the TGA data is shown in Figure 6. The material
obtained by heating UFPO4 to 800 °C and cooling back down to
room temperature was analyzed by PXRD and found to consist
of a mixture of U2O(PO4)2 and several unidentified phases. The
primary phase, U2O(PO4)2, was found to be structurally closely
related to UFPO4, and attempts were made to prepare and
isolate a pure sample of U2O(PO4)2. This was initially attempted
by heating UFPO4 to 900 °C in the anticipation that the higher
temperatures would drive the transformation to completion;
this, however, instead resulted in another structurally related
phosphate, UIVUVIO2(PO4)2. A plot of the TGA data is shown in
Figure 6. The second attempt involved heating UFPO4 under a
4% hydrogen atmosphere at 800 °C. These TGA results are
shown in Figure 6 and reveal that the primary product was the
desired U2O(PO4)2; however, some small impurity phases
persist. Figure S2 shows the post-TGA XRD patterns for the
three discussed TGA runs. To obtain pure samples of both
U2O(PO4)2 and UIVUVIO2(PO4)2 in order to fully investigate
their physical properties and to compare them to UFPO4, they
were prepared via solid-state reactions.
Figure 6. Top: TG curve of UFPO4 from room temperature to 800 °C
under a N2 gas flow. UFPO4 loses weight continuously until 800 °C at
which point it levels out. Middle: TG curve of UFPO4 from room
temperature to 900 °C under flowing N2, followed by a 12 h isothermal
hold at 900 °C. UFPO4 loses weight until 900 °C, at which point it
begins to gain weight, presumably due to the oxidation of one U atom
from IV to VI and the concomitant uptake of oxygen. Bottom: TG curve
of UFPO4 from room temperature to 800 °C under a 4% H2
atmosphere. UFPO4 loses weight to 800 °C at which point it begins
to stabilize.
stoichiometric amounts of UO2 and NH4H2PO4 are combined
in a fused silica tube. The charge is heated according to the
profile given in the Experimental Section. This profile initially
leads to the formation of P2O5 at 450 °C as the ammonia and
water vapor are driven off the phosphate. Once this process is
complete, the charge is heated to 800 °C, which renders the
Synthesis of UIVUVIO2(PO4)2. In order to synthesize the
uranium uranyl phosphate material, we made use of a previously
reported flux-assisted solid-state synthesis.29 In this method,
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Figure 7. Side by side comparison of UFPO4 (left) with U2O(PO4)2 (right). The primary structural consequences of the transformation are the
lowering of the coordination number of uranium from 8 to 7 and a slight rearrangement of the uranium chains. This rearrangement is primarily due to
the decreased coordination number which removes half of the interchain U−U bridges (through F in UFPO4, O in U2O(PO4)2). Uranium is shown in
green, phosphate is shown in orange, oxygen is shown in red, and fluorine is shown in light green.
P2O5 molten allowing it to react efficiently with the uranium.
This initial reaction yields a mixture of unidentified phases.
To homogenize the material and to obtain phase pure
UIVUVIO2(PO4)2, the sample is ground and heated twice more
at 850 °C with one intermediate grinding, resulting in phase pure
UIVUVIO2(PO4)2. The PXRD pattern confirming phase purity is
shown in Figure S1.
Synthesis of U2O(PO4)2. Since these three uranium phos-
phates are structurally related, we reasoned that interconversion
between these phases should be possible. U2O(PO4)2 contains
only one fewer oxide ion than UIVUVIO2(PO4)2, suggesting it
should be possible to reduce UIVUVIO2(PO4)2 to form the
U2O(PO4)2 phase. To test this, freshly made UIVUVIO2(PO4)2
was heated under a 4% H2 flow at 650 °C for 18 h. This resulted
in a pure sample of U2O(PO4)2 as evidenced by the PXRD pat-
tern shown in Figure S1.
where the uranium atoms are arranged in dimers: alternating
U4+−U4+−U6+−U6+−U4+−U4+ and so on. All of the U atoms
remain in a 7-coordinate environment; however, the polyhedra
change from somewhat irregular UO7 polyhedra in U2O(PO4)2
to fairly regular pentagonal bipyramids (as would be expected
for the uranyl ions) in UIVUVIO2(PO4)2. The assignment of
U(IV) versus U(VI) can easily be made by comparing the
lengths of the uranyl bonds (1.7 Å) to the axial bonds of U(IV)
(2.1 Å) in this structure. The oxidation of one of the uranium
atoms results in the loss of U−O−U interchain bonding, which
is a consequence of the now inert uranyl oxygens that cannot
form the connection. The phosphate tetrahedra are still present
in a more or less unchanged location and still serve to connect
the uranium chains. All three reported structures are shown for
comparison in Figure 8.
Structural Comparison of UFPO4 with UXPO4·2H2O (X =
Cl, Br). It should be noted that in addition to the anhydrous
UFPO4 reported here there exist two additional U(IV) halide
phosphates (UClPO4, and UBrPO4) both dihydrates.31 These
materials exist as sheet-type structures with the U(IV)−
phosphate bridges forming two-dimensional rings. These rings
are bridged together by P−O bonds forming a 3D network.
These two tetragonal structures are distinct from the UFPO4
structure described above. The difference in structure might be
attributed to the greater propensity of Cl and Br to form terminal
bonds compared to F.
Structural Comparison of U2O(PO4)2 and UIVUVIO2(PO4)2
with UFPO4. Although the structures of U2O(PO4)2 and
UIVUVIO2(PO4)2 have been reported elsewhere29,30 (ICSD
numbers 402120 and 75358 respectively), they will be discussed
briefly for the purpose of comparing them to the structure of
UFPO4. The structure of U2O(PO4)2 is nearly identical to that
of UFPO4. Conceptually, doubling the unit cell of UFPO4 in the
c direction results in U2F2(PO4)2). Replacing the two fluoride
ions with one oxide ion, to maintain charge balance, results in
U2O(PO4)2. This transformation effectively reduces the coor-
dination number of uranium from 8 to 7 and results in a slight
rearrangement of the uranium chains from an eclipsed to a
staggered arrangement. Figure 7 shows the comparison between
UFPO4 and U2O(PO4)2.
Interestingly, the local coordination environments of uranium
in these halide phosphates are more closely related to those of
uranium in U2O(PO4)2 and U(UO2)(PO4)2, which were
described earlier. All four of these local uranium environments
are based on the pentagonal bipyramidal geometry; however, the
halogen ion in the UXPO4 phases causes the geometry to be
quite distorted.
UIVUVIO2(PO4)2 is structurally also closely related to UFPO4
and U2O(PO4)2, and the existing differences arise primarily
from the fact that half of the U atoms are U(VI) rather than all
U(IV), as is the case in both UFPO4 and U2O(PO4)2. The simul-
taneous presence of U(IV) and U(VI) results in charge-ordering,
Magnetic Susceptibility of UFPO4, U2O(PO4)2, and
UIVUVIO2(PO4)2. Since U(IV) is a 5f2 magnetic ion, we expect
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Figure 8. Comparison of the structures of UFPO4 (top), U2O(PO4)2 (middle), and U(UO2)(PO4)2 (bottom). In U(UO2)(PO4)2 there are two uranyl
ions that alternate with two U(IV) ions. U(IV) is shown in green, U(VI) is shown in yellow, phosphorus is shown in orange, oxygen is shown in red, and
fluorine is shown in light green.
both crystal-field and spin−orbit effects to contribute to the size
of the magnetic moment. Furthermore, the close similarity of the
three reported structures provides us with a unique opportunity
to probe how small changes in coordination environment affect
the magnetism of U(IV). In UFPO4, there are multiple possible
magnetic exchange pathways: U−O−U interactions through
edge shared members of the uranium chains, U−F−U inter-
actions between chains and U−O−P−O−U super-superexchange
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interactions between chains. In U2O(PO4)2, these interactions
are slightly simplified as half of the U−F−U interactions (now
additional U−O−U) have been removed by the doubling of the
c axis. Finally, UIVUVIO2(PO4)2 removes the U−O−U inter-
chain interaction altogether, as well as removes long-range
U−O−U intrachain interactions, as the chains are now inter-
rupted by diamagnetic uranyl ions. The primary magnetic
interactions in this material are therefore U−O−U dimer
interactions. These dimers can interact via super-superexchange
through phosphate groups along the a axis.
The magnetic susceptibility of all three materials are shown in
Figure 9. All three materials exhibit an increase in their
Figure 10. χm·T vs T plots for UFPO4 (green), U2O(PO4)2 (red), and
U(UO2)(PO4)2 (blue). The moments decrease with decreasing
temperature, consistent with a loss of thermally excited f states;
however, the moments never reach 0, which would be indicative of a
singlet ground state.
ordering were observed. The NPD patterns were fit to structural
models using the Rietveld method, and the structural models
were found to account very well for all of the observed NPD peak
intensity. Figure 11 depicts the Rietveld plots for UFPO4 and
Figure 9. Magnetic susceptibilities of UFPO4 (green), U2O(PO4)2
(red), and U(UO2)(PO4)2 (blue). The susceptibilities have been
normalized to represent the magnetic susceptibility per U(IV) ion to
make the values directly comparable.
susceptibility, to varying degree, at low temperatures. UFPO4
exhibits the most dramatic increase and is reminiscent of a para-
magnet, while the upturns in U2O(PO4)2 and UIVUVIO2(PO4)2
are more subtle. This behavior is not consistent with the singlet
ground state that is common among U(IV) materials, although
the exact nature of the magnetism cannot be determined using
the SQUID data alone. For that reason, powder neutron dif-
fraction was performed to probe the potential presence of mag-
netic order.
U(IV) typically does not obey the Curie−Weiss law at low
temperatures, which makes obtaining an effective moment
somewhat difficult. In order to determine effective high tem-
perature moments, a plot of 2.827(χm·T)1/2 versus T was gener-
ated (Figure 10). This plot yields effective moments of 4.2 μB/U
for UFPO4, 2.9 μB/U for U2O(PO4)2, and 2.7 μB/U(IV) for
UIVUVIO2(PO4)2 at 300 K. U(IV) typically displays moments of
3.5−3.9 μB,12 meaning these values are slightly outside the
reported range (although this is not uncommon). Nonetheless,
these results exemplify how significant structural changes
around the U(IV) ion effect small changes in the crystal fields
and hence the magnetic moment of 5f magnetic systems.
NPD of UFPO4 and UIVUVIO2(PO4)2. UFPO4 exhibits a
moment higher than expected based on the reported range of
moments, and for that reason, the potential for the existence of
magnetic correlations was considered. As no obvious magnetic
transition was observed in the magnetic susceptibility data, NPD
data of UFPO4 and UIVUVIO2(PO4)2 were collected at different
temperatures and analyzed for the presence of any magnetic
contribution. NPD patterns collected at 4 and 300 K were
overlaid and found to be identical except for the expected
thermal lattice expansion. No additional peaks or changes in
peak intensity that would indicate the presence of magnetic
Figure 11. Rietveld refinements of the neutron diffraction data of
UFPO4 and U(UO2)(PO4)2 collected at 4 K. Neutrons with λ = 2.41 Å
were used to probe the presence of any magnetic order.
UIVUVIO2(PO4)2. No significant changes to the initial structural
models were observed. Selected refinement parameters are listed
in Table 2. On the basis of these data, we conclude that no
magnetic order is present in UFPO4 and UIVUVIO2(PO4)2.
CONCLUSIONS
Magnetic susceptibility data indicate that UFPO4 with its high
coordination number and large number of magnetic exchange
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DOI: 10.1021/acs.inorgchem.8b01284
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Article
Table 2. Selected Rietveld Statistics for UFPO4
and U(UO2)(PO4)2
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Informationisavailablefree ofcharge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.8b01284.
■
S
UFPO4, PXRD patterns for all reported materials, and
post-TGA PXRD patterns for the thermal products of all
three reported materials (PDF)
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CCDC 1842529 contains the supplementary crystallographic
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ORCID
Hans-Conrad zur Loye: 0000-0001-7351-9098
Notes
The authors declare no competing financial interest.
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We gratefully acknowledge the United States Department of
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award DE-SC0008664 and DE-SC0018739. The work at
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Office of Basic Energy Sciences.
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