Journal of the European Ceramic Society 41 (2021) 6000–6009
Contents lists available at ScienceDirect 
Journal of the European Ceramic Society 
journal homepage: www.elsevier.com/locate/jeurceramsoc 
An investigation of LnUO4 (Ln = Dy and Ho): Structures, microstructures, 
uranium valences and magnetic properties 
Kimbal T. Lu a,b, Yingjie Zhang a,*, Tao Wei a, Zhaoming Zhang a, Maxim Avdeev a, 
Rongkun Zheng b 
a Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, New South Wales, 2232, Australia 
b School of Physics, The University of Sydney, Camperdown, New South Wales, 2006, Australia   
A R T I C L E  I N F O   A B S T R A C T   
Keywords: The phase formation, structures, microstructures, uranium valences and magnetic properties of LnUO4 (Ln = Dy 
Dysprosium and Ho) were investigated. Although sintering of the precursors in argon at 1450 ◦C for seven days and 1400 ◦C 
Holmium for six hours both resulted in the desired phase, sintering at higher temperature for longer duration led to the 
Uranium formations of well crystalized lanthanide monouranates with much better homogeneity. Cubic fluorite structures 
Monouranate 
Magnetism were determined using X-ray diffraction data, which was confirmed with transmission electron microscopy and 
Raman spectroscopy. The nature of pentavalent uranium was verified with a combination of diffuse reflectance 
and X-ray photoelectron spectroscopies. The magnetic suseptability measurements revealed that they are para-
magnetic with no long-range magnetic orders, likely due to the extensive short-range oxygen defects. Overall the 
improved structural and spectroscopic understandings of LnUO4 have implications in nuclear materials especially 
for potential accident tolerance fuels and spent fuel management.   
1. Introduction and Cd) [14–20] require the oxidation state of U to be hexavalent U6+
for charge balance, the pentavalent U5+ ions could possibly be stabilized 
Metal monouranates in the form of MUO4 have attracted a growing in MUO4 with the presence of trivalent metal ions, e.g., M3+ = Cr, Fe, Bi, 
interest due to their importance in uranium crystal chemistry and im- Sc and Y [19–25], yet the exact U valences are less certain in many of 
plications in nuclear industry, e.g., their close relationships to UO2 based those cases. 
nuclear fuels such as accident tolerance fuels and the potential role in It is understood that cubic-structured phases are dominating in the 
the management of spent nuclear fuels [1–3]. From the crystal-chemical system UO2–(Y/Ln)2O3 for the entire compositional range [26–33]. The 
point of view, hexavalent uranium species in the form of uranyl (UO2+2 ) observed non-linear variation of the lattice parameters was initially 
ion, including many uranyl minerals [7–9], dominate uranium aqueous explained based on the fact that the substitution increases the amount of 
and crystal chemistry [4–6] in regards to environmental and geological anionic vacancies with increasing (Y/Ln)2O3 content in UO2, assuming 
aspects, largely due to its greater solubility and mobility in the envi- that uranium remains in tetravalent oxidation state. In addition, the 
ronment, in comparison to much less soluble U4+ species present in UO2 charge compensation mechanisms and U valences with Nd/Gd doped 
and its associated compounds. However, the intermediate pentavalent UO2 have been extensively studied [34–36]. More recently, a compre-
oxidation state of uranium, U5+, which can often be found in aqueous hensive study on the oxidation states of U in the Bi–U–O system 
solutions [10,11] and is anticipated to be present in some metal mon- confirmed that BiUO4 is a pure pentavalent uranium phase [23]. It is 
ouranates, is less common and has not been well studied [12,13]. anticipated that the formation of pentavalent U in UO2–(Y/Ln)2O3 sys-
Although observed in aqueous solutions under particular redox condi- tem could account for the non-linear variation of the lattice parameters 
tions, U5+ present in the UO+2 molecular ion is strongly subjected to upon (Y/Ln)2O3 incorporations in UO2. With equal molar of Ln and U, 
disproportionation, which reduces drastically its life-time in these LnUO4 can be formed by sintering in neutral and slightly reducing at-
media, compared to U4+ in reducing and U6+ in oxidative conditions. mospheres [26–33]. Since the magnetic susceptibility measurements for 
While MUO with divalent cations M2+4 (M = Mg, Ca, Sr, Ba, Ni, Co, Mn BiUO4, ScUO4 and YUO4 confirmed that they are metal monouranates 
* Corresponding author. 
E-mail address: yzx@ansto.gov.au (Y. Zhang).  
https://doi.org/10.1016/j.jeurceramsoc.2021.05.040 
Received 19 February 2021; Received in revised form 13 May 2021; Accepted 19 May 2021   
Available online 21 May 2021
0955-2219/Crown Copyright © 2021 Published by Elsevier Ltd. All rights reserved.
K.T. Lu et al.                                                                                                                                                                        J o  u  r n  a l  o  f  t h  e   E u  r o  p  e a  n   C  e  r a  m  i c   S o  c  i e t y   4  1 (2021) 6000–6009
with pure pentavalent uranium [24,25], showing the same maximum at and then carbon coated for SEM analysis. The EDS multiple point ana-
4− 7 K with a similar shape in the magnetic susceptibility vs. tempera- lyses were carried out on polished sample surfaces with Cu standard for 
ture curves, it was reasonable to assume that LnUO4 might also contain calibration. 
pure pentavalent uranium based on the linear trend of lattice parameters 
against ionic radius [23]. If the presence of pentavalent U5+ ions is 2.4. Transmission electron microscopy 
proven in lanthanide monouranates, LnUO4 phases with cubic structures 
and simple stoichiometry would allow some in-depth structural and Transmission electron microscopy (TEM) experiments were carried 
spectroscopic studies to improve our fundamental understandings on out using a JEOL 2200FS (JEOL Ltd, Japan) operated at 200 keV, fitted 
these mixed lanthanide and uranium oxides. with an Oxford X-Max silicon drift detector for Energy Dispersive X-ray 
Despite the increasing attention to LnUO4 compounds and their Spectroscopy. TEM specimens consist of crushed grains dispersed in 
importance in the uranium based nuclear fuel cycle, only a few struc- ethanol and then dispensed onto a holey carbon film supported on a 
tural characterizations are currently available [26–36] and very limited TEM copper mesh grid. 
spectroscopic measurements have been used to probe the local struc-
tures [23,34–36]. The primary aim of the current work was to gain a 2.5. Diffuse reflectance spectroscopy 
better understanding of LnUO4 materials in regards to their crystal 
chemistry and properties, in specific the phase formation, structures and Diffuse reflectance spectra (DRS) in both UV–vis and NIR regions 
microstructures, uranium valences and magnetic properties of LnUO4 were recorded on an Agilent Cary 5000 spectrophotometer equipped 
(Ln = Dy and Ho). Both Dy and Ho were used as neutron poisons in the with a Labsphere Biconical Accessory. 
nuclear fuels due to their relatively large cross sections for neutrons. As 
such their incorporations in UO2 and phase stabilization deserve further 2.6. Raman spectroscopy 
research effort. In addition, DyUO4 was not reported before. Conse-
quently, Dy and Ho were chosen for this study. Raman spectra were collected on a Renishaw inVia spectrometer 
equipped with a 785 nm excitation Ar laser with a spectral resolution of 
2. Experimental ~1.7 cm− 1. The spectrometer was calibrated by measuring a mono-
crystalline Si with the T − 12g band set at 520.5 cm . The laser beam was 
2.1. Sample preparation focused on the sample through a long focal distance (1 cm) objective 
(Numerical Aperture = 0.5) with 50x magnification. The laser spot size 
Both DyUO4 and HoUO4 precursors were prepared by mixing equal on the sample was ~1.5 μm2. The excitation power (5 mW) was opti-
molar of uranyl nitrate hexahydrate and dysprosium/holmium nitrate mized to prevent possible sample oxidation and damage. 
hexahydrate (from Sigma-Aldrich) in deionized water, drying in an oven 
at 110 ◦C followed by calcining at 750 ◦C in argon for 6 h. The precursors 2.7. X-ray photoelectron spectroscopy (XPS) 
were pelletized (~1 g) and then sintered in argon at 1400 ◦C for 6 h 
(DyUO4) and 1450 ◦C for 7 days (DyUO4 and HoUO4). The sintering The HoUO4 sample was examined in ultra-high vacuum with a 
conditions were chosen largely based on the available literature. A Thermo Scientific ESCALAB 250Xi XPS system employing a mono-
preliminary TGA of the DyUO4 precursor showed a gradual weight loss chromatic Al Kα (1486.6 eV) X-ray source. The X-ray gun was operated 
from 1000 to 1200 ◦C, indicating that the reduction of U6+ to U5+ re- at 120 W, and the spectrometer pass energy was set at 20 eV for regional 
quires higher sintering temperature. In addition, the observed in- scans. The diameter of the analysis area was approximately 500 μm. The 
homogeneity in the initial DyUO ◦4 sample sintered at 1400 C for 6 h thickness of the probed surface layer was approximately 5 nm. A low 
could be due to both the thermodynamic (temperature) and kinetic energy electron flood gun was used for the neutralization of surface 
(holding time) factors. Subsequently, higher sintering temperature charge build up due to low electrical conductivity of the HoUO4 sample. 
(1450 ◦C) and longer holding time (7 days) were chosen to address this The binding energies were calibrated by fixing the C 1s peak (due to 
in order to achieve the satisfactory results. adventitious carbon) at 285.0 eV. Peak fitting of the U 4f7/2 region, after 
the subtraction of a Shirley-type background, was performed using the 
2.2. X-ray diffraction CasaXPS software package [39]. 
The X-ray diffraction (XRD) data were collected on a Bruker D8 2.8. Magnetic susceptibility 
Focus diffractometer equipped with Cu–Kα (λ =1.5418 Å) radiation, in 
the range 10◦ < 2θ < 120◦, with a step size of 0.02◦ (2θ) and an Magnetic susceptibility data were collected using a PPMS9 magne-
acquisition time of 4 s per step. Unit cell parameters were determined tometer (Quantum Design) calibrated against a standard palladium 
with the Le Bail method [37] using the RIETICA program (version 2.1) sample. Zero-field cooled (ZFC) and field cooled (FC) DC susceptibility 
[38]. The peak profiles were approximated by the Pseudo-Voigt func- was measured under the field of 1000 Oe in the temperature range of 
tion, and the background was estimated by a 6 term polynomial func- 2.5–300 K. The data were corrected for diamagnetism using the Pascal’s 
tion. The lattice parameters were refined together with the peak profile constants. 
parameters until the R factors ceased to change. 
3. Results and discussion 
2.3. Scanning electron microscopy 
3.1. Phase formation and structures 
Scanning electron microscopy (SEM) was used to analyze the 
morphology and energy dispersive spectroscopy (EDS) to determine the Earlier works [26–33] had established that lanthanide oxides can 
Ln to U ratios. Samples were mounted in resin, carbon coated and form solid solutions with UO2 in cubic fluorite structures. With equal 
examined in a Zeiss Ultra Plus scanning electron microscope (Carl Zeiss molar of Ln and U, LnUO4 phases can be formed by sintering under 
NTS GmbH, Oberkochen, Germany) operating at 15 kV equipped with either neutral or reducing atmospheres. In this work, the DyUO4 pellets 
an Oxford Instruments X-Max 80 mm2 SDD X-ray microanalysis system. were sintered in argon at either 1400 ◦C for 6 h or 1450 ◦C for 7 days and 
The finely crushed samples were dusted onto conducting carbon sticky the HoUO4 pellet was sintered in argon at 1450 ◦C for 7 days to inves-
tapes and then carbon coated before SEM analysis. The sintered samples tigate the phase formation and structures. The XRD patterns (Fig. 1) 
were mounted in resin, polished to 1 μm finish using a diamond paste revealed that cubic structured DyUO4 and HoUO4 were formed by 
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8-fold coordination of 1.027 Å for Dy3+ and 1.015 Å for Ho3+ ions. 
In general, the phase assemblage will be dependent on two control 
factors: chemistry and sintering redox conditions. While chemistry 
control is dominant in some cases such as the formation of NpPO4 by 
sintering in air, sintering redox conditions can significantly influence the 
phase formation of mixed uranium oxides. Sintering in air often results 
in U partially oxidized to U6+ leading to the formation of U3O8 as a 
secondary phase while sintering in Ar-H2 can stabilize both U5+ and U4+
in M-U-O systems. 
3.2. Microstructures 
Apart from the limited structural studies, there is little information 
on the microstructures of LnUO4 compounds available in the literature. 
Consequently, the microstructures for sintered DyUO4 and HoUO4 were 
investigated with both SEM and TEM. The backscattered SEM images 
(Fig. 3) revealed that the crushed DyUO4 pellet sintered at 1400 ◦C for 6 
h showed particles in sub-micrometers (Fig. 3a) while the crushed 
DyUO4 pellet sintered at 1450 ◦C for 7 days showed much larger par-
ticles in tens of micrometers (Fig. 3b). The EDS multipoint analysis 
revealed that some areas have higher or lower Dy than U in the sample 
sintered at 1400 ◦C for 6 h (Fig. S1), indicating inhomogeneous nature. 
◦
Fig. 1. XRD patterns of sample DyUO4 sintered in argon at either 1400 ◦C for 6 In contrast, the sample sintered at 1450 C for 7 days has a more uniform 
h (a), or at 1450 ◦C for 7 days (b) and sample HoUO sintered in argon at 1450 composition with the Dy to U ratio close to unity (Fig. S2). The back-4 
◦C for 7 days (c), together with that of SmUO4 (d) as a reference. scattered SEM images (Fig. 4) for pellets of both DyUO4 and HoUO4 
sintered at 1450 ◦C for 7 days showed porous nature with the apparent 
sintering at either 1400 ◦C for 6 h or 1450 ◦C for 7 days, in good porosity estimated using the ImageJ software [40] to be ~24 % for 
agreement with the XRD patterns of UO (JCPDS card No. 01-075-0421) DyUO4 (an example is shown in Fig. S3) and ~10 % for HoUO . The EDS 2 4
and Sm U O (28727-ICSD) in cubic space group Fm 3 m, consistent analysis results also confirmed that both samples have nearly ideal 2 2 8.34 
with earlier observations on the formation of LnUO sintered at stoichiometry as designed (Figs. S4 and S5; Tables S1 and S2). The 4 ◦
1200− 1400 ◦C under neutral or reducing atmospheres [26–33]. How- observed lack of homogeneity in the DyUO4 sintered at 1400 C for 6 h 
ever, sintering at higher temperature for longer time (1450 ◦C for 7 may arise from both thermodynamic (temperature) and kinetic (holding 
days) can produce better crystalline and homogeneous DyUO in com- time) factors. Subsequently, higher sintering temperature (1450 
◦C) and 
4 
parison to sintering at 1400 ◦C for 6 h, refer to the section for micro- longer holding time (7 days) were chosen to address this in order to 
structures. The refinements of XRD data for both DyUO (Fig. 2a) and achieve the successful outcome within a limited time span, which was 4 
HoUO4 (Fig. 2b) sintered at 1450 ◦C for 7 days gave good χ2 values (2.15 confirmed by multiple points EDS analyses. The high porosity (~24 %) 
for DyUO and 2.30 for HoUO ) with the refined cell parameters, a observed for the DyUO4 may be related to unsuccessful removal of ni-4 4 =
5.36749(3) Å and V 154.637(2) Å3 for DyUO , and 5.34976(2) Å and trate from the precursor in the calcination step. = 4
V = 153.110(1) Å3 for HoUO4. The slight decrease of cell parameters 
Subsequently, the three samples were further characterized with 
◦
from a = 5.36749(3) Å for DyUO4 to a = 5.34976(2) Å for HoUO is 
TEM. For DyUO4 sintered at 1400 C for 6 h, the bright field TEM image 
4 
consistent with the lanthanide ion contraction with an ionic radius in (Fig. 5a) showed fine particles of DyUO4 (~5 nm). TEM-EDS analysis 
results confirmed that the phase contains U, Dy and O with a slightly 
Fig. 2. XRD patterns of samples DyUO4 (a) and HoUO4 (b) sintered at 1450 ◦C in argon with fluorite structures (in cubic space group Fm 3 m). Black crosses represent 
the observed data, and the red line is the fit obtained by the Le Bail method. Blue vertical markers show the peak positions expected in the cubic fluorite structure. 
The green line underneath records the difference between the observed and the calculated patterns. 
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Fig. 3. Backscattered SEM images of crushed sample DyUO4 either sintered in argon at 1400 ◦C for 6 h (a) or 1450 ◦C for 7 days (b).  
Fig. 4. Backscattered SEM images of sample DyUO4 [(a) and (b)] and sample HoUO4 [(c) and (d)] both sintered in argon at 1450 ◦C for 7 days.  
varying atomic ratio of U : Dy, suggesting not quite uniform composi- (Fig. 6c) showed typical fluorite lattice fringes with a FFT image in the 
tions across the grains (Fig. S6) from the microstructural point of view. inset. A SAED pattern in the [011] zone axis (Fig. 6d) was indexed to the 
The selected area electron diffraction (SAED) pattern in the [051] zone cubic fluorite DyUO4. The measured d (111) spacing value from the 
axis as an inset in Fig. 5a was indexed to the cubic fluorite structure. A HRTEM image (Fig. 6c) is consistent with the cell parameter [a =
high resolution TEM (HRTEM) image in the [011] zone axis (Fig. 5b) 5.36749(3) Å] refined using the XRD data. 
showed fluorite lattice fringes with a Fast Fourier Transform (FFT) For HoUO4 sintered at 1450 ◦C for 7 days, the bright field TEM image 
image in the inset. The measured d (111) spacing value is 0.312 nm with (Fig. 7a) showed a large thin section of a crystal. A SAED pattern in the 
the corresponding cell parameter a =5.40 Å. In the case of DyUO4 sin- [011] zone axis (Fig. 7b) was indexed to the cubic fluorite HoUO4 while 
tered at 1450 ◦C for 7 days, both bright field and dark field TEM images the HRTEM image in the [011] zone axis (Fig. 7c) with a FFT image as an 
(Fig. 6a and Fig. 6b) showed much larger particles. The HRTEM image inset showed the lattice fringes for cubic HoUO4. The TEM measured 
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Fig. 5. TEM of the sample DyUO4 sintered in argon at 1400 ◦C for 6 h: a bright field image with an inserted SAED pattern indexed in the [051] zone axis (a), a 
HRTEM image (b) with an inserted FFT image in the [011] zone axis. 
Fig. 6. TEM of the sample DyUO4 sintered in argon at 1450 ◦C for 7 days: a bright field (a) and the corresponding dark field (b) TEM images of a crystal, a HRTEM 
image in the [011] zone axis (c) with an inserted FFT image and an indexed SAED pattern in the [011] zone axis (d). 
d (111) spacing value of 0.307 nm is consistent with that determined space group Fm 3 m exhibits only one Raman mode as predicted by 
from the powder XRD. group theory. This mode corresponds to the symmetric T2g vibration at 
445 cm− 1. With lanthanide ion incorporations in UO2, additional Raman 
3.3. Local structures modes at higher wavenumbers, 530–630 cm− 1, may be observed similar 
to the situation for α-irradiated UO2 [50], reflecting the formation of 
Raman spectroscopy has been widely used to probe local structural oxygen defects [51,52]. The Raman spectra (Fig. 8) for both DyUO4 and 
information of various oxides [41–49]. The UO2 with a cubic structure in HoUO4 sintered at 1450 
◦C for 7 days showed the typical vibration bands 
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Fig. 7. TEM of the sample HoUO4 sintered in argon at 1450 ◦C for 7 days: a bright field TEM image (a); a SAED pattern indexed in the [011] zone axis (b) and a 
HRTEM image with an inserted FFT image in the [011] zone axis (c). 
3.4. Uranium valence 
Several spectroscopic techniques, e.g., diffuse reflectance spectros-
copy (DRS), X-ray photoelectron spectroscopy (XPS) and X-ray absorp-
tion near-edge structure (XANES), have been widely used to probe the U 
valence in oxides [55–59]. The possible U valences in oxides under the 
experimental conditions are 4+ (5f2 electronic configuration), 5+ (5f1) 
and 6+ (5f◦) states. In DRS, the U4+ ion gives sharp (zero-phonon line) 
and broad (vibronic) absorptions across the visible and infrared spectral 
range [55–58] while the U5+ ion is confined to the near infrared as it 
derives only from the crystal-field splitting of 2F 25/2– F7/2 components 
(split by spin-orbit coupling) of the 2F electronic state, with the distin-
guished electronic transitions observable from the splitting of 2F7/2 at 
1538‒833 nm (6500–12000 cm-1) range [55–58]. In contrast, the U6+
ion has no f electrons and the only electronic transitions observable are 
broad charge-transfer bands in the blue and near-ultraviolet spectral 
regions. 
The DRS spectra of DyUO4 and HoUO4 sintered at 1450 ◦C are shown 
in Fig. 9. The absorption bands at 1538‒833 nm (6500‒12,000 cm− 1) 
correspond to a typical 5f1 configuration for the U5+ ion in an octahedral 
coordination environment [55] while the responsible absorption bands 
will shift to higher wavelengths for U5+ ion in an 8-fold coordination 
Fig. 8. Raman spectra of samples DyUO (a, blue) and HoUO (b, red) sintered environment in a cubic fluorite structure. It is apparent that both DyUO4 4 4 
and HoUO contain U5+in argon at 1450 ◦C for 7 days. 4 ions evidenced by the presence of weak ab-
sorption bands at ~900 nm/11111 cm-1 (Fig. 9a) and ~1615 nm/6192 
-1 
expected for the cubic fluorite structure: the T mode around 445 cm− 1. cm (Fig. 9b). The absorption bands at ~1615 nm also suggest that U
5+
2g 
However, the observed T bands broadened and their intensities ions are on the 8-fold coordination site instead of the 6-fold coordination 2g 5+
reduced in comparison to that for pure UO [23], mainly due to the site such as the U ion in Dy0.5U0.5Ti2O6 brannerite [60], which has the 2 
increase of the local cation disorders at the short range distances as a absorption band at ~1445 nm/6920 cm
-1, relatively lower wavelengths. 
result of the ionic radius difference. In addition, strong new vibration DRS spectra at the near infrared region (Fig. S7) do not show the sharp 4+
bands appeared at ~530 cm− 1 and ~615 cm− 1, corresponding to the absorption bands corresponding to U ion, clearly indicating the 4+
formation of extensive oxygen defects, e.g., oxygen vacancies. Such absence of U ions in both compounds. To further confirm these find-
strong vibration bands for oxygen defects are uncommon in metal ings, XPS was also used to study the U valence in HoUO4. 
monouranates, reflecting extensive oxygen defects at the short range Because XPS is an extremely surface sensitive technique, it is crucial 
distances. This may have a profound impact on the physical properties of to first remove the oxidized surface layer so that the results are repre-
LnUO materials. In addition, the lack of the U-O– stretching mode at sentative of the bulk. This was achieved by examining a freshly polished 4 
~717 cm− 1 infers no formation of UO species with non-bonding oxy- HoUO4 sample. Note that the DyUO4 sample was not analyzed using XPS 8 
gens similar to uranyl ions [53,54]. due to its higher porosity. The U 4f XPS spectrum of HoUO4 is shown in 
Fig. 10, displaying two main photoemission lines (U4f7/2 and U4f5/2) 
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Fig. 9. Diffuse reflectance spectra in 850–1050 nm region (a) and 1500-1700 nm region (b) of samples DyUO4 (blue) and HoUO4 (red) sintered in argon at 1450 ◦C 
for 7 days. 
to the sample being exposed to air briefly [5562]. In addition to moni-
toring the BE of the U 4f peaks, the satellite structure provides even more 
definitive signature of the U valence, because not only ΔEs-p is sensitive 
to the U oxidation state but also the number of satellite peaks and their 
relative intensity to the main peak [64]. Pentavalent uranium in ternary 
uranium oxides is known to have a single satellite peak with ΔEs-p ~ 
7.8–8.3 eV, while hexavalent and tetravalent uranium oxides display 
two satellite peaks at about 4 and 10 eV (U6+) and one satellite peak at ~ 
6.9 eV (U4+), respectively [61,64]. Fig. 10 clearly shows a single satellite 
peak at 8.3 eV above the U U4f5/2 peak, confirming that the HoUO4 
sample contains U5+ only (note that the position of the U4f7/2 satellite 
cannot be determined accurately due to the overlap with the U4f5/2 
peak). Overall, both DRS and XPS investigations confirmed that DyUO4 
and HoUO4 sintered in argon at 1450 ◦C are pure pentavalent 
monouranates. 
3.5. Magnetic properties 
In the literature, limited magnetic property investigations are 
Fig. 10. XPS spectrum of the U 4f region for sample HoUO sintered in argon at available for MUO4 phases with trivalent M ions. The magnetic sus-4 
1450 ◦C for 7 days. ceptibility measurements for BiUO4, ScUO4 and YUO4 confirmed that 
they are pentavalent monouranates [24,25]. They showed the same 
around 10.9 eV apart due to spin-orbit splitting along with their corre- maximum at 4− 7 K with a similar shape in the magnetic susceptibility 
sponding satellite peaks. It is well known that the U 4f binding energy vs. temperature curves, with the effective magnetic moment consistent 
(BE) and the energy difference between the satellite and corresponding with S = ½, implying the U
5+ state. For LnUO4 pentavalent mono-
primary U 4f peak (ΔEs-p) are very sensitive to the U oxidation state [61, uranates, it was anticipated that magnetic interactions may involve 
62]. As shown in Fig. 10, the U4f line was fitted with two mixed coupling of the U
5+ and (Dy/Ho)3+ ions. Surprisingly, both DyUO
7/2 4 
and 
Gaussian/Lorentzian peaks after subtracting a Shirley-type background. HoUO4 showed typical paramagnetic behaviour down to ~2 K with no 
The peak position and width were allowed to vary freely, but the width obvious long range magnetic orders (Fig. 11). This could arise from the 
of the two peaks was constrained to be equal. The strong peak (~99 %) is extensive oxygen defects or cation disorders on a single fluorite metal 
located at 380.6 eV, in excellent agreement with NaU5+O [63]. The site or both. However, the exact reason for the observed result needs 3 
very weak peak (~1 %) at 382.8 eV is attributed to surface oxidation due further experimental evidence and deserve a future investigation, 
ideally a neutron diffraction study, which requires synthesis of a 
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Fig. 12. Cell volumes (Å3) versus ionic radii of cations in 8-fold coordination 
geometry for MIIIUO4 (M = Bi, Sc and Y) and LnUO4 (Table S3) in space group 
Fm 3 m, with the data points obtained in this study marked as red triangles. 
4. Conclusions 
Uranium ternary oxides with lanthanide ions are fundamentally 
important owning to their potential applications as accident tolerance 
fuels and possible waste forms for spent nuclear fuels. The aim of this 
work was to invesrigate LnUO4 (Ln = Dy and Ho), in specific the phase 
formation, structures and microstructures, uranium valence and mag-
netic properties. Although sintering in argon at 1400 ◦C for 6 h can 
produce pure solid solution of LnUO4, microstructural studies uncovered 
that sintering at higher temperature for longer time (1450 ◦C for 7 days) 
favours the formation of better crystallinity with improved homogene-
ity. Both DyUO4 and HoUO4 are cubic strucutured pentavalent uranium 
ternary oxides with their structures refined using X-ray diffraction data 
Fig. 11. Magnetic susceptibility measurements of DyUO in space group Fm 3 m, and supported with both transmission electron 4 (a) and HoUO4 (b) 
sintered in argon at 1450 ◦C for 7 days. Note that the ZFC and FC data overlap miscroscopy and Raman spectroscopy. The presence of pure pentavalent 
with each other. uranium in these two oxides was verified using a combination of diffuse 
reflectance and X-ray photoelectron spectroscopies. Surprisingly, the 
gram-scale sample. magnetic suseptability measurements revealed that both DyUO4 and 
HoUO4 are paramagnetic showing no long-range magnetic orders, 
which is most likely due to the extensive local oxygen defects and Ln/U 
3.6. Further discussion and implications disorder over the single metal site in the fluorite-type structure, sup-
ported by Raman spectroscopy. However, the hypothesis needs to be 
With the incorporation of trivalent cations in UO2, it is generally further investigated and verified with a future potential neutron 
accepted that trivalent cations are located on the uranium site in the diffraction study. 
solid solution. Such an assumption is often supported by the cell 
parameter modelling with a Vegard’s law over a range of compositions, 
if the cubic fluorite structure remains after doping [65]. However, some Declaration of Competing Interest 
metal uranates have different crystal structures. For example, both 
CrUO and FeUO have orthorhombic structure in space group Pbcn The authors report no declarations of interest. 4 4 
[18–20], with Cr3+ and Fe3+ cations in six coordination polyhedra 
instead of cubic ones. Consequently, a collective re-arrangement of Cr3+ Acknowledgements 
cations and oxygen anions was proposed [50,52]. For trivalent cations 
suitable for an eight-fold coordination environment in cubic structures The authors wish to thank Dr. I. Karatchevtseva for Raman mea-
such as (Bi/Sc/Y)3+ and Ln3+ cations [66], their direct substitutions on surements, Dr. O. Muransky for porosity estimations, and Nuclear Sci-
the uranium site are anticipated. ence and Technology Landmark Infrastructure (NSTLI) at ANSTO for 
The evolution of the unit-cell volumes of LnUO4 and related MIIIUO4 synthesis and characterization of materials. The XPS measurements 
(M = Sc, Y and Bi) phases as a function of the ionic radius for the eight- were carried out at the Surface Analysis Laboratory of the University of 
fold coordination geometry (Fig. 12, Table S3) can be described well by New South Wales, Sydney. 
a linear relationship, reflecting the general trend for ionic radius 
contraction along the lanthanide series. As pentavalent oxidation state Appendix A. Supplementary data 
of uranium was proven for MIIIUO4 (MIII = Bi, Sc, and Y) based on 
magnetic susceptibility measurements [24,25], it is anticipated that all Supplementary material related to this article can be found, in the 
the LnUO4 compositions are indeed pure pentavalent monouranates, online version, at doi:https://doi.org/10.1016/j.jeurceramsoc.2021.0 
consistent with the uranium valence investigations in this work. 5.040. 
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