Incorporation of Thorium in the Zircon Structure Type through the Th1-xErx(SiO4)1-x(PO4)x Thorite-Xenotime Solid Solution

Article abstract of DOI:10.1021/acs.inorgchem.6b01862

Pure powdered compounds with a general formula Th_1-xEr_x(SiO₄)_1-x(PO₄)_x belonging to the zircon-xenotime family were successfully synthesized under hydrothermal conditions (250 °C, 7 days) as recently reported for the preparation of coffinite. Therefore, a thorough, combined PXRD, EDX, EXAFS, Raman, and FTIR analysis showed the formation of a solid solution in agreement with Vegard's law. Moreover, the examination of the local structure shows that the Th-O distances remain close to those found in ThSiO₄, whereas the Er-O distances show a significant decrease from 2.38(14) to 2.34(7) ? when increasing the erbium content from x = 0.2 to x = 1. The variation of the local structure also affects the PO₄^3- groups that are surely distorted in the structure.

Full text of DOI:10.1021/acs.inorgchem.6b01862

Article
pubs.acs.org/IC
Incorporation of Thorium in the Zircon Structure Type through the
Th1−xErx(SiO4)1−x(PO4)x Thorite−Xenotime Solid Solution
Adel Mesbah,*,† Nicolas Clavier,† M. Janeth Lozano-Rodriguez,‡ Stephanie Szenknect,†
and Nicolas Dacheux†
†ICSM, UMR 5257 CEA/CNRS/ENSCM/University of Montpellier, Site de MarcouleBat
‡HZDR, Institute of Resource Ecology, The Rossendorf Beamline at ESRF, P.O. Box 220, 38043 Grenoble, France
̂
. 426, BP 17171, 30207
̀
Bagnols-sur-Ceze cedex, France
ABSTRACT: Pure powdered compounds with a general formula Th1−xErx-
(SiO4)1−x(PO4)x belonging to the zircon−xenotime family were successfully
synthesized under hydrothermal conditions (250 °C, 7 days) as recently reported
for the preparation of coffinite. Therefore, a thorough, combined PXRD, EDX,
EXAFS, Raman, and FTIR analysis showed the formation of a solid solution in
agreement with Vegard’s law. Moreover, the examination of the local structure
shows that the Th−O distances remain close to those found in ThSiO4, whereas
the Er−O distances show a significant decrease from 2.38(14) to 2.34(7) Å when
increasing the erbium content from x = 0.2 to x = 1. The variation of the local
3−
structure also affects the PO4 groups that are surely distorted in the structure.
presence of the actinides and associated daughter products (U−
Th−Pb) for the precise dating of the rocks in the field of
geochronology.24−32 Moreover, the distribution of REEs
between the monazite and xenotime minerals, which depends
on the temperature of formation, led these phases to be used as
geothermobarometer and then for the determination of the
history of the related metamorphic rocks.21,33 More impor-
tantly, these minerals have attracted much interest in the
industrial field linked to the nuclear fuel cycle, whether as
uranium and thorium sources (front-end) or as specific
ceramics for the conditioning of highly radioactive nuclear
wastes (back-end).13,34−41 They were also proposed for the
disposal of excess plutonium coming from the dismantling of
nuclear weapons.42 For both purposes, monazite and zircon
were generally associated with remarkable properties, such as
high resistance to aqueous leaching and radiation damage.
Additionally, zircon was frequently cited as a promising inert
matrix for the transmutation of minor actinides (particularly
Np, Am, and Cm) in fast-neutron reactors or accelerator-driven
systems.43
1. INTRODUCTION
The crystal chemistry of lanthanides phosphates (LnIIIPO4) and
actinides silicates (AnIVSiO4) is governed, at high temperature,
by the existence of two distinct structural types. On one hand,
the monazite structure (monoclinic, space group P21/n)1 is
generally the most stable phase for light rare earth element
(REE) phosphates (typically from La to Gd) and was also
reported for ThSiO4 (corresponding to the mineral hutton-
ite)2,3 and for PaSiO4.4 The original MO9 coordination
polyhedron encountered in the monazite confers to the
structure the capability to incorporate a wide range of elements
through direct and/or coupled substitutions, either on the
cationic or the anionic site,5 leading to a tremendous variety of
applications. On the other hand, heavy REE phosphates
(namely from TbPO4 to LuPO4) which relate to the mineral
xenotime, and all tetravalent actinides phosphates (including
ThSiO4 thorite,6 USiO4 coffinite,7,8 and associated solid
solutions9−12), preferentially adopt the zircon-type structure
in the tetragonal system with I41/amd as a space group. Silicate-
based compounds of this family then also include zircon
(ZrSiO4) and hafnon (HfSiO4).
Monazite, xenotime, and zircon are widespread naturally
occurring minerals.13−18 They coexist in many granitic and
metamorphic rocks located in different sites on earth19−23 and
exhibit a very rich structural diversity owing to their ability to
contain a wide variety of rare earth elements (REEs) and
natural actinides (U and Th). Indeed, these three minerals
represent the main sources of the REE, U and Th on earth, with
monazite being considered as the first source of thorium and
coffinite as the second source of tetravalent uranium after
uraninite.13 Many works already demonstrated the utility of the
From a structural point of view, uranium and thorium are
incorporated in both monazite and xenotime structures
following various substitution mechanisms. The most encoun-
tered case consists of the miscibility between the monazite
(REEPO4) and the cheralite (CaAn(PO4)2) through
2REE3+ ↔ Ca2+ + An4+
(1)
Received: August 1, 2016
© XXXX American Chemical Society
A
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as observed in natural minerals by Foster44 and confirmed on
synthetic compounds.39,45 Another possibility for substitution
consists of the formation of solid solutions between the
monazite and isostructural ThSiO4 huttonite. Such samples
were reported to exist in natural phases46 and can also be
prepared as synthetic compounds, either through hydrothermal
wet chemistry methods47 or dry chemistry routes, leading to
refinements, different profile and structure parameters were allowed to
vary such as the zero shift, unit-cell parameters, scale factor, and overall
displacement factor. Moreover, an anisotropic size model was applied
in order to evaluate the peaks broadening due to a microstructural
effect. However, the occupancy of each site was fixed to the calculated
values.
2.3. Extended X-ray Absorption Fine Structure. EXAFS
measurements were performed in transmission mode at the Th L3-
edge (16 300 eV) and the Er L3-edge (8358 eV) of Th1−xErx-
(SiO4)1−x(PO4)x samples mixed with boron nitride (BN). The
measurements were carried out at the Rossendorf Beamline (BM20)
dedicated to actinide-based compounds, located at the European
Synchrotron Radiation Facility (ESRF; Grenoble, France). The storage
ring operated at 6 GeV and 170−200 mA. For the Th L3-edge, a
double Si(111) 30° crystal monochromator was used for energy
selection, and a pair of Rh mirrors were used for vertical beam
collimation and suppression of higher-order harmonics. A gas mixture
of 80% N2 and 20% Ar was used in the 30 cm ionization chamber
measuring the incoming beam (I0), and 100% Ar gas was used to
measure the beam transmitted through the sample (I1) and after the
respective reference foil (I2), all filled at ambient pressure. For Er L3-
edge, a double Si(111) 0° crystal monochromator was used for energy
selection combined with a pair of Si/Rh mirrors. Likewise, a gas
mixture of 70% He and 30% N2 was used in the ionization chamber.
Energy calibration was done by measuring the absorption spectra of Y
(17 038 eV) and Ni (8333 eV) metal foils placed between I1 and I2
parallel to the sample scans. All the samples were measured at room
temperature.
complex compositions such as the Ln0. 7 Th0 . 2 7
0.03(PO4)0.7(SiO4)0.3 reported by McCarthy et al.48
In the case of the presence of Y3+ or heavy REE, xenotime
-
U
became the stable phase. According to literature, thorium and
uranium could then only be incorporated through:
REE3+ + PO 3− ↔ (Th, U)4+ + SiO 4−
(2)
4
4
Although both xenotime and zircon compounds are
crystallizing in the same structure type, there is no accurate
study dedicated to the existence of a solid solution between
both end-members.28,49,50 In the latter works, all the attempts
to synthesize a solid solution between xenotime and zircon
have been carried out by solid-state chemistry (flux method)
and only led to the partial insertion of PO43− and REE3+ in the
ZrSiO4 structure type and vice versa.
In this present paper, we thus report the first synthesis of a
complete solid solution Th1−xErx(SiO4)1−x(PO4)x (with 0 ≤ x
≤ 1, by steps of 0.1) in the thorite−xenotime system. Thorium
was selected as a model element for tetravalent actinides, which
avoided any perturbations due to redox reactions. Also, erbium
was considered as the rare earth element due to its use as a
burnable poison in inert matrix fuels, which controls the
reactivity toward irradiation.51,52 All of the compounds
prepared were thoroughly characterized from a structural
point of view by PXRD and EXAFS. Vibrational spectroscopy
features were also investigated by Raman and FTIR.
Data reduction and extraction of EXAFS oscillation was performed
using the Athena and Artemis package.56 The threshold energy, E0,
was defined as the maximum of the first derivate of the absorption
coefficient. Experimental EXAFS spectra were Fourier transformed
using a Hanning window over the k-space ranges 2−11 Å−1 for
thorium and 2.5−11 Å−1 for erbium. The shell fits were performed in
R-space using k1-, k2-, and k3-weighting for thorium and erbium.
Theoretical phase shifts and backscattering amplitudes were obtained
with the ab initio code FEFF8.257 using the ThSiO4 (thorite)2 and
ErPO4 (xenotime) structures14 for thorium and erbium L3-edge
reported in the literature. The calculation was carried out with a radius
cluster of 6 Å, taking into account all single scattering (SS) and
multiple scattering (MS). During the fitting procedure, Th−O, Th···
Si1 (P2), Th···Si2 (P2), and Th−Th (Er) single scattering paths (SS)
were taken into account. Similarly, Er−O, Er···P1 (Si1), Er···P2 (Si2),
and Er−Er (Th) SS were considered for the fitting. The distance (R)
and Debye−Waller (DW) factor were treated as free parameters, while
the coordination number (N) was constrained at crystallographic
values. A single shift in the threshold energy (ΔE0) for each
composition was varied as a global parameter. The amplitude
2. MATERIALS AND METHODS
Caution! Depleted uranium and 232Th are α-emitters and are considered
as a health risk. Experiments involving actinides require appropriate
facilities and trained persons in handling of radioactive materials.
2.1. Synthesis. For the synthesis of the Th1−xErx(SiO4)1−x(PO4)x
solid solutions, the following reactants were used: Th(NO3)4·4−
5H2O, ErCl3·nH2O, NaOH, NaHCO3, Na2SiO3. All chemicals were of
analytical grade and supplied by Sigma-Aldrich. Precise and direct
weighing of the exact amounts of thorium and erbium salts was
precluded because of their hygroscopic character. Therefore, both salts
were dissolved in acidic media. ErCl3 was dissolved in 1 M HCl while
the thorium chloride concentrated solution was prepared by dissolving
thorium nitrate pentahydrate in concentrated HCl solution. Several
cycles of evaporation and redissolution in a solution of 4 M HCl were
undertaken until traces of nitrates were eliminated.53 Concentrations
of resulting solutions were finally determined by ICP-AES using a
Spectro Arcos EOP device. The calibration of the apparatus was
performed by the means of SPEX standard solutions diluted to
concentrations varying between 0 and 15 mg L−1.
2.2. Powder X-ray Diffraction (PXRD). The synthesized
compounds were analyzed by PXRD with the help of a Bruker D8
advance diffractometer equipped with the Lynxeye detector and Cu
Kα1,2 radiation (λ = 1.54184 Å). The measurements were carried out
in parallel geometry and with adapted sample holders (dome) in order
to prevent any radioactive contamination. Each powder XRD pattern
was collected in the angular range (2θ) between 5° and 100° with a
step of 0.019° and a total counting time of about 3 h per sample. Also,
a PXRD pattern of pure silicon powder was collected in similar
conditions in order to evaluate the instrumental function. In all cases
only the zircon structure type54 (I41/amd space group) was the
detected phase. Therefore, all the collected data were refined by the
Rietveld method using the Fullprof_suite package.55 During the
2
reduction factor S0 was obtained for ThSiO4 and for ErPO4 after
keeping the obtained value constant through the whole composition
range. To ensure transferability of phase and amplitude and to allow
the estimation of systematic errors, all the data were analyzed using the
same experimental phases and amplitude, filtering procedures and
parameters in a similar way.
2.4. Scanning Electron Microscopy. SEM observations were
directly conducted on powdered samples without any preparation such
as metallization, using a Tescan Vega3 electronic microscope, in low-
vacuum conditions (10 Pa) with an accelerating voltage of 15 kV.
Additionally, the precise chemical composition of the series Th1−xErx-
(SiO4)1−x(PO4)x was analyzed by the means of energy dispersive
spectrometry (EDX) coupled with an FEI Quanta 200 SEM device. In
this aim, the powder was first embedded in an epoxy resin. The surface
of the sample was then polished to reach an optical grade and
metalized by carbon coating. Experimental data were finally collected
from about 50 different locations of the sample surface, considering
ThO2, ErPO4, and NaAlSi3O8 as internal standards for the elements
Th, Er, P, and Si, respectively.
2.5. Vibrational Spectroscopy. Raman spectra were recorded in
the 100−1500 cm−1 range by the means of a Horiba−Jobin Yvon
Aramis apparatus equipped with an edge filter and using a He−Ne
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laser (633 nm) delivering 10 mW on the sample surface. The laser
beam was then focused on a small fraction of powdered sample simply
deposited on a glass lamella using an Olympus BX 41 microscope. The
scattered Raman light was collected in a 180° backscattering geometry
and dispersed by a grating of 1800 grooves/mm after having passed a
150 μm entrance slit, resulting in a spectral resolution lower than 1
cm−1. For each spectrum, a dwell time of 30 s was considered with an
average of 10 scans. Before analysis, the apparatus was calibrated with a
silicon wafer, using the first-order Si line at 520.7 cm−1.
FTIR spectra were recorded in the 380−4000 cm−1 range thanks to
a Perkin-Elmer FTIR Spectrum 100 device. Powdered samples were
deposited at the surface of an ATR crystal without any prior
preparation. The spectra collected in such operating conditions
exhibited a resolution lower than 2 cm−1.
3. RESULTS AND DISCUSSION
3.1. Synthesis and SEM/EDX Characterization. First
attempts to prepare Th1−xErx(SiO4)1−x(PO4)x solid solutions
were performed by following several methods previously
reported in the literature for the synthesis of rhabdophane or
xenotime-type compounds.58 The first protocol considered
consisted of the direct coprecipitation of the reactants in
aqueous solution. A starting mixture of thorium and erbium in
the desired stoichiometric ratio was then poured in a solution
containing 5 M H3PO4 and dissolved Na2SiO3 and then
transferred to a Teflon container at 90 °C for 2 weeks.
Nevertheless, the PXRD pattern analysis of the resulting
powder (not presented here) showed the formation of an
amorphous gel whatever the chemical composition considered.
The second method consisted of a heat treatment of the
previous mixtures at 250 °C. However, this chemical route also
failed yielding to a polyphase system composed mainly of
thorite (ThSiO4) and Er-xenotime (ErPO4).
Finally, powdered samples of Th1−xErx(SiO4)1−x(PO4)x solid
solutions were prepared by using a method recently reported in
the literature for the synthesis of coffinite USiO4,59 in
modification of the protocol initially proposed by Hoekstra
and Fuchs.8 The calculated amount of Na2SiO3 was dissolved in
deionized water and mixed with a desired amount of 5 M
H3PO4. In parallel, weighed amounts of Th and Er acidic
solutions were mixed and then poured slowly into the anionic
solution under continuous stirring. Then, the pH of the
solution was raised by adding 8 M NaOH dropwise to the
target value ranging between 11 and 11.5. Afterward, the
solution was buffered to 8.7 0.1 by adding NaHCO3. The
final solution was transferred into a Teflon-lined acid digestion
bomb (Parr) and placed in an oven for 1 week at 250 °C. All of
the reactions were carried out in a fume hood in air. The
precipitate collected after this treatment was separated from the
supernatant by centrifugation at 4500 rpm, washed twice with
deionized water and ethanol, and then finally dried overnight at
90 °C.
Figure 1. SEM micrographs of the Th1−xErx(SiO4)1−x(PO4)x solid
solutions.
Table 1. EDX Compositions Determined for
Th1−xErx(SiO4)1−x(PO4)x Solid Solutions
xcalc
xEr
xTh
xP
xSi
(Er + Th)/(Si + P)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
1
0
1
0.12(1)
0.24(2)
0.36(1)
0.44(2)
0.56(3)
0.57(1)
0.75(3)
0.81(3)
0.89(1)
1
0.88(1)
0.76(2)
0.64(1)
0.56(2)
0.47(3)
0.43(1)
0.25(3)
0.19(3)
0.11(1)
0
0.05(2)
0.16(2)
0.27(1)
0.35(1)
0.45(1)
0.53(1)
0.70(1)
0.74(1)
0.86(1)
1
0.95(2)
0.84(2)
0.73(1)
0.65(1)
0.55(1)
0.47(1)
0.30(1)
0.26(1)
0.14(1)
0
1.1(1)
1.2(1)
1.2(1)
1.2(1)
1.2(1)
1.3(1)
1.2(1)
1.1(1)
1.1(1)
values obtained for phosphorus had a systematic under-
estimation of about 5% of P atomic percentage. This problem
is a recurrent one on all our phosphate-based compounds
which is mainly due to a bias caused by the standardization
method. Moreover, attempts to perform a total dissolution of
the powders in aqua regia were unsuccessful because of the
precipitation of amorphous silica which made difficult the
collection of reliable data.
3.2. Structural Characterization. PXRD. All of the
prepared powdered samples were analyzed by PXRD (Figure
2a). Systematically, only the zircon structure type was observed,
conversely to the case of uranothorite (Th1−xUxSiO4),10,11
which led to the formation of mixed phases with the
simultaneous stabilization of nanometric Th1−yUyO2 and SiO2
by-products. Given those results, the reactions performed under
hydrothermal conditions at 250 °C have led to the formation of
a single pure phase crystallizing in the zircon structure type
which corresponds to the expected Th1−xErx(SiO4)1−x(PO4)x
solid solution. All these compounds crystallize in the tetragonal
system, with a space group of I41/amd (general view of the
For all of the studied samples, the powders were found to
consist of small needle-like crystals. The size of these grains was
typically in the 200−800 nm range, but no clear trend was
established versus the erbium weight loading. Figure 1 shows
selected micrographs of four different compositions. The
analysis of the synthesized powders by EDX confirmed the
formation of samples with the presence of Th, Er, Si, and P
within the structure. The trend of the measured values confirms
the formation of a solid solution as shown in Table 1.
Regarding the reported results, we observed a small and
systematic deviation of the measured values compared to the
calculated ones, still acceptable as experimental errors. The
structure down the c axis presented in Figure 3). In this system,
2,60
both end-members, namely, ThSiO4
and ErPO4,61 are well-
known and isostructural but generally differ in their method of
synthesis.
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Table 2. Refined Unit-Cell Parameters for the
Th1−xErx(SiO4)1−x(PO4)x Solid Solution
xcalc
a (Å)
c (Å)
V (Å3)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
7.1443(2)
7.1177(1)
7.1070(2)
7.0761(3)
7.0539(2)
7.0361(3)
7.0132(2)
6.9943(2)
6.9683(2)
6.9288(3)
6.8837(1)
6.3240(2)
6.2882(1)
6.2602(2)
6.2321(3)
6.1986(3)
6.1627(3)
6.1308(2)
6.1004(2)
6.0671(2)
6.0401(3)
6.0227(1)
322.79(1)
318.57(1)
316.20(2)
312.06(2)
308.43(2)
305.10(2)
301.55(1)
298.44(1)
294.61(1)
289.98(2)
285.39(1)
Figure 2. (a) PXRD patterns of the Th1−xErx(SiO4)1−x(PO4)x solid
solution showing a shift toward low angles when increasing erbium
loading. (b) Observed, calculated, and difference curves obtained
during the Rietveld refinement of the PXRD pattern of the x = 0.5
sample.
Figure 4. Variation of the unit-cell volume (Å3) in the Th1−xErx-
(SiO4)1‑x(PO4)x system.
view, it seems that the formed solid solution exhibits a perfectly
random distribution between the elements in both Th/Er and
Si/P sites inside the structure. In order to obtain more
information and details, EXAFS experiments were performed to
probe the local structure as detailed below.
Local Structure. For clarification of the behavior of erbium
and its influence on the host ThSiO4, the local structure of
thorium and erbium in all the Th1−xErx(SiO4)1−x(PO4)x
samples prepared was systematically investigated by EXAFS
at Th and Er L3-edges. The k3-weighted EXAFS spectra and
their Fourier transforms (FTs) obtained for the 10 different
compositions are shown in Figure 5a,b for the Th and Er L3-
edges, respectively.
The FT obtained for the ThSiO4 end-member (x = 0) at the
Th L3-edge shows a first shell around the thorium atom
consisting of eight oxygen contributions at 2.4 Å. The next
neighbor contributions at 3.20 and 3.95 Å were attributed to
the single scattering contribution of two Si1 and four Si2 atoms
in the thorite structure, respectively. Additionally, a cation−
cation contribution related to the Th−Th was fitted at 3.93 Å.
Besides, the FT determined for ErPO4 (x = 1) at the Er L3-edge
(Figure 5b) also shows the eight oxygen contributions in the
first coordination shell at 2.34 Å. In the same way as that for
ThSiO4, the FT of ErPO4 presents a second and a third peak
related to the single scattering contribution of two P1 at 3.03 Å
and four P2 at 3.88 Å. For the ErPO4 end-member, it was also
possible to fit two Er−Er distances, at 3.76 and 5.69 Å. For the
rest of the compositions, the longest Er contribution was no
longer observable. These values are shown in the Table 3 and
Figure 3. General view of the zircon structure type down the c axis.
The analysis of the patterns reported in Figure 2a evidenced
a shift in the XRD lines toward the low angles (2θ) depending
on the erbium content. The experimental unit-cell parameters,
refined using Rietveld methods, are reported in Table 2 while
Figure 2b shows the observed, calculated, and difference curves
for the compound Th0.5Er0.5(SiO4)0.5(PO4)0.5 refined in the
zircon structure type. Moreover, the linear variation of the unit-
cell volume versus the erbium content, viewed in Figure 4,
appears in agreement with Vegard’s law. At first glance, this
variation appears to be in agreement with crystal ionic radius
reported in the Shannon tables for Th4+ and Er3+ in
coordination eight with r = 1.19 and 1.144 Å, respectively.
Moreover, we should also consider the size of Si4+ (0.4 Å) and
P5+ (0.3 Å) in the structure. From the microscopic point of
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independent of the composition of the solid solution (Pauling’s
concept). In contrast, the second corresponds to full relaxation
in the system, where the virtual crystal approximation (VCA) is
valid and bond lengths follow Vegard’s law.62
The cation−oxygen interatomic distances are presented in
Figure 6, with respective error bars calculated from the
difference of EXAFS values and the distances of the crystal
structure mentioned already. The derived Th−O and Er−O
bond length distances showed that Th−O and Er−O distances
are element-specific due to the difference in their crystal
effective radius (1.144 Å for Er and 1.19 Å for Th). The Er−O
bond length determined by EXAFS increased in the zircon-type
structure with the thorium content. However, the individual
Th−O bond length remained close to the value determined in
pure ThSiO4 (2.39 Å); i.e., there is no relaxation. It suggests
that Th−O bond lengths are preserved in the first coordination
shell as erbium content increases, due to its strong covalent
bond. From these results, the host zircon-type structure
(ErPO4) is modified with the change in the actinide content
(i.e., full relaxation).63 Besides, the average EXAFS distance
plotted in Figure 6 showed a similar trend compared to the
PXRD results (Figure 4). Another observation is that the
Debye−Waller factor (σ2) in the Er−O coordination shell is
increasing with thorium content, going from 0.005 50 to 0.0100
Å2. However, the σ2 values for the Th−O contribution
remained at the same scale, with an average of 0.006 Å2.
Note that this reflects the thermal and static contribution since
measurements were performed at room temperature.
Cation−Cation Environment. The cation−cation distribu-
tion was determined with a single broad peak within the
EXAFS resolution. The variation in the second neighbor
distribution is presented in Figure 7 with the corresponding
error bars. First, the Er−Er (Th) and Th−Th (Er) distances are
approximatively equal to their average distances, as reported
previously in the case of covalent and ionic systems.63,64
Second, with a look at the Er−Er (Th) local environment, a
change from 3.76 to 3.94 Å was observed. Nonetheless, the
Th−Th (Er) local environment suffered a small variation from
3.93 to 3.90 Å. These results again suggest that although the Er
local structure was modified, that of Th remained close to that
of pure thorite (ThSiO4). This kind of behavior was already
reported for stabilized zirconia with actinides.65,66 In addition,
as the thorium content increased, the Er−Er (Th) bond length
distances became similar to the Th−Th ones. Finally, Figure 8
shows the variation in the Er−P and Th−Si interatomic
distances. It is important to mention that P and Si are
undistinguishable by EXAFS because of their close atomic
number. However, from fitting, it was observed that Th−Si and
Er−P distances remained almost constant, meaning that there
is a less significant change in their distances than those obtained
for the cations.
Figure 5. Experimental k3χ(k) EXAFS spectra of Th1−xErx-
(SiO4)1−x(PO4)x solid solutions and their respective Fourier trans-
forms (FTs) at the (a) Th L3-edge and (b) Er L3-edge. Colored lines
are the best fit achieved; black lines are the experimental data. FT peak
positions are not corrected for phase shifts.
were found to be in agreement with the average crystal
diffraction distances reported in the literature.2,14,60
For intermediate solid solutions, the FT measurements at the
Th L3-edge from x = 0 to x = 0.5 (Figure 5a) showed little
changes on the local environment around Th. However,
starting from x = 0.5 to x = 0.8, the amplitude of Th peak
contribution decreased whereas a broadening on the Si2 peaks
was revealed. Such behavior might be related to the shortening
of Th−Th(Er) distances as actinide content decreased and/or
to interferences between the two Er and Th scattering paths
resulting in the attenuation of the cation−cation FT peak. The
EXAFS spectra for the Th L3-edge showed no modification at
lower k-ranges. However, small modifications started at 8 Å−1,
which is probably due to the mixing backscattering between Th
and Si atoms. Conversely, the FTs of Th1−xErx(SiO4)1−x(PO4)x
solid solutions at Er L3-edge (Figure 5b) showed more
significant changes in the local environment around Er than
those of the Th L3-edge at higher Er content (x = 0.9). Such
modifications were also revealed in their EXAFS spectra, where
a bigger change in the atomic structure around the xenotime
ErPO4 was observed as Th content increased.
3.3. Vibrational Spectroscopy. The vibrational features of
XO4 anions within a zircon-type structure have been widely
described, either in the case of phosphate species for the
xenotime,67 or for silicates in actinide-based compounds such as
thorite68 or coffinite.9 The factor-group analysis based on the
irreducible representation of the D4h point group leads to the
optical vibrations as follows:
Cation Oxygen Environment. According to the criteria for
isovalent solid solutions, the bond length could be limited by
two cases, which will reflect their ability to adapt to the host
structure. The first is no relaxation in the system, which assumes
that the atomic radii are approximately conserved and
Γ = 2A1g + 2A2g + 4B1g + B2g + 5Eg + A1u + 3A2u
+ B1u + 2B2u + 4Eu
(3)
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Table 3. Th1−xErx(SiO4)1−x(PO4)x Fit Results for the Main Coordination Shells at the Th and Er L3-Edges
Th−O
Th···Si1
Th···Si2
Th···Th
x
R/Å
σ2/Å2
R/Å
σ2/Å2
R/Å
σ2/Å2
R/Å
σ2/Å2
2
0
2.40
(8)
0.0058
(13)
3.20
(25)
0.0045
(36)
3.95
(36)
0.0077
(51)
3.93
(16)
0.0042
(18)
S0 = 0.81
% R-factor = 2.0
E0 = 10.88
0.2
0.3
0.4
0.5
0.6
0.7
0.8
2.39
(6)
0.0057
(7)
3.18
(22)
3.19
(25)
3.19
(71)
3.16
(26)
3.17
(50)
3.16
(22)
3.16
(27)
0.0046
(29)
3.95
(26)
3.95
(24)
3.92
(39)
3.93
(29)
3.94
(25)
3.95
(23)
3.95
(21)
0.0061
(35)
3.92
(14)
3.93
(17)
3.90
(40)
3.91
(37)
3.90
(43)
3.92
(35)
3.90
(33)
0.0044
(15)
% R-factor = 1.1
E0 = 10.16
2.40
(5)
0.0067
(8)
0.0050
(33)
0.0050
(32)
0.0056
(19)
% R-factor = 1.4
E0 = 10.37
2.38
(7)
0.0049
(11)
0.0102
(12)
0.0045
(53)
0.0078
(50)
% R-factor = 2.8
E0 = 10.37
2.38
(9)
0.0053
(8)
0.0033
(36)
0.0036
(37)
0.0084
(45)
% R-factor = 2.2
E0 = 9.75
2.38
(9)
0.0069
(11)
0.0084
(80)
0.0018
(30)
0.0085
(45)
% R-factor = 2.9
E0 = 9.75
2.39
(10)
2.39
(9)
0.0062
(11)
0.0022
(30)
0.0014
(27)
0.0080
(43)
% R-factor= 2.5
E0 = 10.49
0.0060
(10)
0.0039
(39)
0.0010
(24)
0.0074
(39)
% R-factor = 2.1
E0 = 10.47
Er−O
Er···P1
Er···P2
Er···Er
x
R/Å
σ2/Å2
R/Å
σ2/Å2
R/Å
σ2/Å2
R/Å
σ2/Å2
2
0
S0 = 0.77
0.2
2.38
(14)
2.36
(9)
0.0111
(16)
3.08
0.0012
3.88
0.0101
3.94
(54)
3.98
(16)
3.98
(40)
3.97
(24)
3.96
(42)
3.95
(34)
3.78
(62)
3.77
(53)
3.76
(22)
0.0101
% R-factor = 4.4
E0 = 15.47
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0078
(10)
3.05
(29)
3.05
(25)
3.05
(33)
3.06
(22)
3.06
(22)
3.05
(24)
3.04
(18)
3.03
(16)
0.0073
(43)
3.83
(23)
3.84
(38)
3.83
(30)
3.83
(42)
3.85
(35)
3.89
(63)
3.89
(51)
3.88
(34)
0.0041
(14)
0.0041
(14)
% R-factor = 2.5
E0 = 14.69
2.35
(10)
2.36
(10)
2.35
(10)
2.36
(8)
0.0105
(12)
0.0040
(26)
0.0084
(38)
0.0084
(38)
% R-factor = 4.1
E0 = 14.28
0.0108
(14)
0.0088
(58)
0.0066
(23)
0.0066
(23)
% R-factor= 3.5
E0 = 14.77
0.0086
(11)
0.0047
(35)
0.0102
(45)
0.0102
(45)
% R-factor = 3.2
E0 = 14.40
0.0100
(10)
0.0067
(30)
0.0110
(36)
0.0110
(36)
% R-factor = 2.3
E0 = 15.00
2.35
(7)
0.0074
(9)
0.0070
(38)
0.0100
(35)
0.0100
(35)
% R-factor = 2.2
E0 = 14.74
2.35
(8)
0.0073
(9)
0.0036
(25)
0.0096
(26)
0.0096
(51)
% R-factor = 2.3
E0 = 15.02
2.34
(7)
0.0055
(8)
0.0018
(19)
0.0049
(11)
0.0049
(11)
% R-factor = 2.3
E0 = 14.97
Among these vibration modes, 12 are Raman-active and split
between the 7 internal vibrations of the XO4 tetrahedra (Γint)
and 5 external modes corresponding to the motion of solid
tetrahedral in the unit cell (Γext), such as
bending), 999 cm−1 (ν1: symmetric stretching), and 1021 and
1057 cm−1 (ν3: antisymmetric stretching), along with the
rotational external vibration at 296 cm−1. These results match
the previous works undertaken on Er-based xenotime,67,70
although the position of the vibration bands appears to be
slightly modified, probably due to the presence of interference
bands between 400 and 850 cm−1. This rogue signal
corresponds to the red luminescence emission of Er3+ at 652
Γ
= 2A1g + 2B1g + B2g + 2Eg
int
(4)
Γ
= 2B1g + 3Eg
ext
(5)
nm, from the F9/2 → I15/2 transition.71,72 On the other hand,
only 4 internal vibration modes of the silicate group were
observed in the thorite end-member, at 438 (ν2), 592 (ν4), 887
(ν1), and 914 cm−1 (ν3). Nevertheless, such a discrepancy was
already reported by several authors and correlated to the
existence of interactions between SiO4 tetrahedra and the
metallic cation.68 In this case, silicate units cannot be treated
strictly independently, resulting in fewer internal vibration
modes than predicted by the factor-group analysis.
4
4
Similarly, FTIR-active vibration modes can be described as
Γ
= A1u + 2A2u + 2B2u + 2Eu
= A2u + B1u + 2Eu
(6)
(7)
int
Γ
ext
The Raman spectra recorded for the Th1−xErx-
(SiO4)1−x(PO4)x solid solutions generally appeared in good
agreement with this factor-group analysis (Figure 9). For the
ErPO4 end-member, the 7 vibration modes of the phosphate
tetrahedra (numbered after the nomenclature employed by
Kolesov et al.69) were observed at 334 and 473 cm−1 (ν2:
symmetric bending), 590 and 665 cm−1 (ν4: antisymmetric
Between these two extreme cases, the Raman spectra
recorded for the xenotime−thorite solid solutions exhibit the
vibrational features of both structures, although with notable
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■
Figure 6. Interatomic distances Th−O (upper ) and Er−O (bottom
★) vs composition in Th1−xErx(SiO4)1−x(PO4) solid solutions. The
x
square symbols represents the average value obtained by EXAFS.
Figure 9. Raman spectra of the Th1−xErx(SiO4)1−x(PO4)x solid
solutions in the 100−1500 cm−1 range.
variations in their relative intensities. Indeed, the modes related
to silicate units rapidly vanished when decreasing the thorium
content in the sample, and were almost undetectable for x <
0.7. This phenomenon probably arose from the better response
of phosphate tetrahedra in Raman spectroscopy, but also from
the progressive incorporation of erbium in the sample which
induced an increasing parasite emission in the ν2(SiO4) and
ν4(SiO4) zone. The same applies for the PO4 bending mode in
this region of the spectra. Conversely, it appeared easier to
follow the variation of the phosphate stretching modes which
remained visible whatever the thorium incorporation rate (x).
On this basis, the ν1 vibration band was fitted thanks to the
Jandel Peakfit software, using a pseudo-Voigt function with a
Gaussian−Lorenzian ratio systematically higher than 0.7. By
this means, correlation coefficients r2 greater than 0.992 were
usually obtained, and allowed us to plot the variation of the ν1
Raman shift versus x (Figure 10). This latter was found to
follow a linear trend, with
Figure 7. Variation in the interatomic distances Th−Th(Er) (★) and
●
Er−Er(Th) ( ) vs composition in Th1−xErx(SiO4)1−x(PO4)x solid
solutions.
σ(ν1) = 964 + 33 × x
(8)
Such linear variation of the XO4 stretching mode was already
pointed out in the case of cationic substitutions within zircon-
type silicates, such as Zr1−xHfxSiO4,73 Zr1−xUxSiO4,74 and
Th1−xUxSiO4.9 However, it has never been mentioned, to our
knowledge, in the case of coupled substitutions that involve
modifications on both cationic and anionic sites. Moreover, this
linear behavior appears in good agreement with the formation
of a complete solution between ErPO4 xenotime and ThSiO4
thorite. Indeed, with the progressive substitution of Th4+ by
Er3+, and simultaneously that of silicate units by phosphate
groups, leading to the shrinkage of the quadratic unit cell, one
can expect the P−O bond length to decrease, and then there is
an increase in the vibration frequency. Also, one can note that
the progressive replacement of erbium by thorium, on one
Figure 8. Variation in the interatomic distances Th(Er)−Si(P) vs
composition in Th1−xErx(SiO4)1−x(PO4)x solid solutions.
G
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(SiO4)1−x(PO4)x solid solution led to the apparition of a
crossing point at around 910 cm−1. In this case, vibrational
spectroscopy can thus be used to roughly estimate the chemical
composition of the solid.
4. CONCLUSION
The incorporation of actinides in potential conditioning
matrices such as phosphates or silicates remains a very
important point to examine from a chemical point of view.
Although several protocols are reported in the literature on the
basis of the use of solid-state chemistry methods at high
temperature, these methods, due to a lack in the reactivity of
the powders or insufficient homogenization during the mixing
or grinding steps, can lead to the presence of less durable
secondary phases during leaching tests. On the contrary, the
development of wet chemistry routes is considered as a
promising alternative method to obtain directly the target
matrices at lower temperature and with an improved
homogeneity. For this purpose, pure powdered compounds
Th1−xErx(SiO4)1−x(PO4)x belonging to the zircon−xenotime
structure type were successfully synthesized under hydro-
thermal conditions (T = 250 °C, t = 7 days) in application of
the recent protocol used for the synthesis of the coffinite.59 The
characterization by SEM (EDX) and PXRD confirmed the
formation of a complete solid solution over the whole
composition range in agreement with Vegard’s law. The crystal
structure of these solids exhibited a linear decrease of the unit-
cell volume as a function of the erbium/phosphate content.
However, the analysis of the local structure in particular with
EXAFS measurements shows that the Th−O distances do not
evolve so much during the erbium to thorium substitution and
remain close to those found for the thorite end-member
(ThSiO4). On the contrary, the Er−O−(P,Si) sequence suffers
more than Th−O−(P,Si) during the (Th, SiO4) ⇔ (Er, PO4)
substitution, by showing a large increase of the Er−O distance
when increasing the thorium content in the structure.
Figure 10. Variation of the ν1 vibration position versus x in the Raman
spectra of the Th1−xErx(SiO4)1−x(PO4)x solid solutions.
hand, and of phosphate by silicate, on the other, led the ν1
vibration band to widen progressively. Since the PXRD study
did not provide evidence of any significant influence of the
substitution on the crystallization state of the samples, the
broadening of the signal might reflect a modification of the PO4
polyhedra. In these conditions, the phosphate tetrahedrons not
only shrink, but are also probably distorted, leading to the
apparition of additional vibration modes overlapping with the
initial ν1 vibration.
These results were confirmed by the data obtained by FTIR
spectroscopy (Figure 11), which again exhibited the character-
istic vibration modes usually described for xenotime and thorite
end-members. Moreover, the better sensitivity of the silicate
units to FTIR compared with Raman, as well as the absence of
interfering luminescence phenomena, offered the possibility to
directly compare the modifications in the relative intensities of
the SiO4 and PO4 stretching modes. Hence, the progressive
substitution of the anionic species along the Th1−xErx-
Figure 11. FTIR spectra of the Th1−xErx(SiO4)1−x(PO4)x solid solutions in the 400−4000 cm−1 range, with focus on the 650−1200 cm−1 range
showing the stretching modes of the XO4 tetrahedrons.
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AUTHOR INFORMATION
■
Corresponding Author
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*E-mail: adel.mesbah@cea.fr.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to thank C. Gausse, P. H. Imbert, and J.
M. Sandate Dominguez for their help in the preparation of the
samples and EXAFS measurements. They are also grateful to R.
Podor for supporting EDX/experiments.
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DOI: 10.1021/acs.inorgchem.6b01862
Inorg. Chem. XXXX, XXX, XXX−XXX