Energetics of a uranothorite (Th1-xUxSiO4) solid solution

Article abstract of DOI:10.1021/acs.chemmater.6b03346

High-temperature oxide melt solution calorimetric measurements were completed to determine the enthalpies of formation of the uranothorite, (USiO₄)_x-(ThSiO₄)_1-x, solid solution. Phase-pure samples with x values of 0, 0.11, 0.21, 0.35, 0.71, and 0.84 were prepared, purified, and characterized by powder X-ray diffraction, electron probe microanalysis, thermogravimetric analysis and differential scanning calorimetry coupled with in situ mass spectrometry, and high-temperature oxide melt solution calorimetry. This work confirms the energetic metastability of coffinite, USiO₄, and U-rich intermediate silicate phases with respect to a mixture of binary oxides. However, variations in unit cell parameters and negative excess volumes of mixing, coupled with strongly exothermic enthalpies of mixing in the solid solution, suggest short-range cation ordering that can stabilize intermediate compositions, especially near x = 0.5.

Full text of DOI:10.1021/acs.chemmater.6b03346

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Article
Energetics of Uranothorite (Th1-xUxSiO4) Solid Solution
Xiaofeng Guo, Stéphanie Szenknect, Adel Mesbah, Nicolas Clavier, Christophe Poinssot,
Di Wu, Hongwu Xu, Nicolas Dacheux, Rodney C. Ewing, and Alexandra Navrotsky
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03346 • Publication Date (Web): 14 Sep 2016
Downloaded from http://pubs.acs.org on September 19, 2016
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Energetics of Uranothorite (Th1-xUxSiO4) Solid Solution
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Xiaofeng Guo1,2, Stéphanie Szenknect3, Adel Mesbah3, Nicolas Clavier3, Christophe
Poinssot4, Di Wu5, Hongwu Xu1, Nicolas Dacheux3, Rodney C. Ewing6, and
Alexandra Navrotsky2,*
1
Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos,
New Mexico 87545, United States
2
Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California Davis,
Davis, California 95616, United States
3
Institut de Chimie Séparative de Marcoule, ICSM ꢀ UMR 5257, CNRS / CEA / University of
Montpellier / ENSCM, Site de Marcoule ꢀ Bât. 426 BP 17171, 30207 Bagnols sur Cèze cédex,
France
4
CEA, Nuclear Energy Division, Radiochemistry & Processes Department, BP 17171, 30207
Bagnols sur Cèze, France
5
The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington
State University, Pullman, Washington 99163, United States
6 Department of Geological Sciences, Stanford University, Stanford, California 94305, United
States
* eꢀmail: anavrotsky@ucdavis.edu
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Abstract
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Highꢀtemperature oxide melt solution calorimetric measurements were completed to determine the
enthalpies of formation of the uranothorite, (USiO4)x ꢀ (ThSiO4)1ꢀx, solid solution. Phase ꢀ pure
samples with x = 0, 0.11, 0.21, 0.35, 0.71, and 0.84 were prepared, purified, and characterized by
powder Xꢀray diffraction, electron probe microanalysis, thermogravimetric analysis and
differential scanning calorimetry coupled with in situ mass spectrometry, and high temperature
oxide melt solution calorimetry. This work confirms the energetic metastability of coffinite,
USiO4, and of Uꢀrich intermediate silicate phases with respect to a mixture of binary oxides.
However, variations in unit cell parameters and negative excess volumes of mixing, coupled with
strongly exothermic enthalpies of mixing in the solid solution, suggest shortꢀrange cation ordering
that can stabilize intermediate compositions, especially near x = 0.5.
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Introduction
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Uranothorite (Th1ꢀxUxSiO4), isomorphic to zircon (I41/amd), can be prepared in a complete
series from thorite (ThSiO4) to coffinite (USiO4)1ꢀ5. Its presence in nature is widespread in
uranium deposits6ꢀ11, reflecting direct substitutions between U and Th6, 9ꢀ11. The end members
coffinite12 and thorite2 are also among the few known naturally occurring actinide orthosilicates11,
13, 14. Despite having the same structure as zircon, they exhibit significant differences in ease of
synthesis, geological conditions of formation and grain size. Natural coffinite usually occurs in
microcrystals accompanied by uraninite, zircon, sulfides or organic matters11, 14ꢀ22. Pure synthetic
coffinite is difficult to prepare and purify3ꢀ5, 23, 24. Coffinite cannot be made by direct high
temperature reaction of UO2 and SiO2 but can be synthesized from aqueous precursors over a
limited pH range in the presence of carbonate buffer25, where the formation of uranium hydroxoꢀ
silicate colloids may play an important role in forming the fineꢀgrained (often nanoscale) coffinite
that might be thermodynamically stabilized by structural water or (OH)ꢀ25, 26. Thorite, on the other
hand, occurs as well crystallized macroscopic primary or an accessory mineral in igneous and
metamorphic rocks.11, 15 Thorite is easily synthesized1, even by direct reaction of ThO2 and
SiO2.27, 28 These differences in the ease of synthesis and typical grainꢀsizes obtained reflect
differences in their thermodynamic stability.
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Recent thermodynamic studies have demonstrated that coffinite is metastable. Guo et al.26,
using oxide melt solution calorimetry, reported the standard enthalpy of formation of coffinite
from elements to be ꢀ1970.0 ± 4.2 kJ/mol and its enthalpy of formation from oxides to be 25.6 ±
3.9 kJ/mol. This latter significantly positive value strongly suggests metastability of coffinite with
respect to the binary oxides.26 Szenknect et al. measured the standard Gibbs free energy of
formation to be ꢀ1872 ± 6 kJ/mol5 or, most recently, ꢀ1867.6 ± 3.2 kJ/mol29, and the solubility
constant of coffinite at 25 °C and 1 bar to be ꢀ5.25 ± 0.0529. These data confirm that coffinite is
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metastable relative to uraninite and quartz. The enthalpy of formation of thorite from oxides was
previously reported by Mazeina et al. to be 19.6 ± 2.0 kJ/mol30, but in this work, see below, it has
been remeasured to be ꢀ6.4 ± 5.7 kJ/mol, suggesting thorite may be thermodynamically stable with
respect to ThO2 and SiO2, consistent with its direct synthesis from thorianite plus quartz.
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Uranothorite, as a bridging composition between thorite to coffinite and may be expected to
show intermediate synthetic, structural and thermodynamic features. Szenknect et al.5 used a
series of uranothorites (x = 0 to ~ 0.5) to extrapolate the formation energetics of coffinite and
confirmed the metastability of USiO4. Costin et al.4 explained the difficulty of coffinite synthesis
by showing the increased difficulty of preparing pure uranothorites with high uranium loadings.
Clavier et al.31 and Labs et al.24 have described the effect of compositional changes on unit cell
dimensions and bondꢀlengths in uranothorite.
Uranothorite may have potential applications as a nuclear waste form or as an alteration
product of UꢀTh nuclear fuels. The zircon structure was proposed as a nuclear waste form1, 28 and
as a principal phase in inert matrix fuels16, 27. Thus the determination of solubility, chemistry and
thermodynamics of actinides, including plutonium, in the zircon structure is of crucial interest14, 32ꢀ
34. Th4+ as a Pu4+ surrogate in uranothorite can provide some insight into the synthesis and
thermodynamics of Pu4+ forming solid solutions with U4+, where the latter is usually incorporated
through coffinitization during spent nuclear fuel alteration in silicaꢀrich fluids14, 16, 17, 35ꢀ41
.
Furthermore, the phase transition of ThSiO4 from thorite to huttonite at ~ 1200 ºC42 raises the
possibility of a similar phase transition when uranothorite is heated. Huttonite has been suggested
to have some advantages over the zircon structure as a nuclear waste form, due to the better
performance of monazite (isostructural to huttonite)43 in terms of radiation tolerance and aqueous
durability44ꢀ46
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Despite the wide interest and potential importance of uranothorite, little has reported on its
thermodynamic properties. Ferriss et al.34 obtained the enthalpy of mixing by simulations and
suggested possible phase separation due to the size difference of U and Th cations. Szenknect et
al.5 obtained the Gibbs free energy of formation up to x = 0.5. In the present investigation, thanks
to the successful preparation of a complete series of uranothorite compositions, Th1ꢀxUxSiO4 (x =
0, 0.11, 0.21, 0.35, 0.71, and 0.84) by hydrothermal reactions, thermodynamic studies can be
directly completed on the different pure phase compositions. Specifically, the enthalpies of
formation of uranothorite from constituent oxides and from elements were obtained for the first
time by high temperature oxideꢀmelt solution calorimetric experiments. The mixing enthalpies of
uranothorite solid solutions from coffinite and thorite end members were determined from these
enthalpies of solution. The results explain the difficulty of synthesizing singleꢀphase uranothorite
with high uranium loadings and demonstrate the metastability of coffinite and Uꢀrich uranothorite
solid solution. At intermediate uranium loadings, the solid solutions are somewhat stabilized by a
negative heat of mixing suggestive of local cation ordering. These thermodynamic measurements
provide for an understanding of U/Th behavior in ore deposits, nuclear fuels, and actinide waste
forms.
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Experimental Methods
Materials Preparation and Purification
The starting reactants were all supplied by SigmaꢀAldrich, except of the depleted uranium
turnings, which were provided by CETAMA, France. U(IV) tetrachloride solution was prepared in
application of the method proposed by Dacheux et al.47, 48 consisting of a dissolution of uranium
metal in hydrochloride acid (6M). The thorium chloride concentrated solution was obtained by
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dissolving thorium nitrate pentahydrate in a 6M HCl solution. Several cycles of evaporation and
reꢀdissolution in a solution of 4M HCl were undertaken until traces of nitrates were eliminated.49
The final concentration of both solutions was determined by ICPꢀAES. To avoid the oxidation of
uranium (IV), all the reactions were performed in an argon filled glove box (less than 1 ppm of
O2). Moreover, deionized water was outgassed by boiling approximatively for one hour and then
cooling under N2 flow.
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Different compositions of the Th1ꢀxUxSiO4 series were synthesized by following the procedure
reported by Mesbah et al.25 and obtained by modification of previous literature methods.3, 50 The
synthesis consisted of slowly pouring a solution containing the calculated amounts of thorium and
uranium into an aqueous solution of Na2SiO3 with a molar excess of Si/(U+Th) of about 10 %.
The mixture was then made more basic by adding droplets of NaOH (8M) to reach pH values of
11ꢀ11.5 and finally buffered by NaHCO3 to stabilize the pH at 8.7 ± 0.1. The final mixture was
transferred into a 23 mL teflon container, placed in a Parrꢀtype acid digestion vessel and then
heated in an oven at 250 °C for 7 days. The resulting precipitate was separated from the solution
by centrifugation at 4000 rpm, washed twice with water and then with ethanol and dried overnight
in air at room temperature.
Despite the optimization of the synthesis procedure, the prepared powders also contained Th1ꢀ
yUyO2 and amorphous SiO2 as impurities, as noted previously.4, 51 To obtain single phase
uranothorite samples, a purification step was performed by following the method reported by
Clavier et al.23 consisting of multiple and successive washing cycles in 1M HNO3, deionized
water and then 10ꢀ2 M KOH. Preliminary Raman and FTIR spectroscopy confirmed the formation
of pure single phase uranothorite solid solutions, attested by the absence of the characteristic
spectral features of the by–products, such as the T2g vibration mode of actinide dioxides.31
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Characterization
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About 5 mg of each synthesis product was ground into a fine powder and loaded onto a zeroꢀ
background quartz slide for XRD pattern collection. XRD patterns were collected from 15 to 82 °
(2θ) with a step size of 0.011° and a collection time of 2 s·stepꢀ1 in a Bruker D8 Advance
diffractometer with CuKα radiation and a solidꢀstate detector. Chemical compositions and sample
homogeneity were determined by electron probe microanalysis (EPMA) using a Cameca SX50
coupled with wavelength dispersive spectrometry (WDS), at an accelerating voltage of 20 keV, a
probe current of 10 nA and a spot size of 1 ꢀm. Quantitative WDS was conducted using a lower
accelerating voltage of 15 keV. UO2, ThO2, and SiO2 were used as analytical standards for U, Th,
and Si, respectively. Standard Cameca software (PeakSight 4.0 using XꢀPHI ZAF matrix
corrections) was used to calculate the compositions.
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Thermal Analysis
Thermogravimetric analysis and differential scanning calorimetry (TGꢀDSC) were performed
simultaneously by heating the sample in a flowing argon atmosphere (40 mL/min) to 800 °C with
a rate of 10 °C/min in a Setaram LabSYS simultaneous thermal analyzer. A mass spectrometer
(MKS Cirrus2) was connected to detect the released gases. The system was calibrated by
decomposing CaC2O4. Acquired data were processed with the Calisto software package from
AKTS. Detailed procedures have been described previously26, 52
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High Temperature Oxide Melt Solution Calorimetry
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High temperature oxide melt solution calorimetry was conducted using a custom built Tianꢀ
Calvet twin microcalorimeter53ꢀ55. Powdered samples were hand pressed into small pellets (∼5
mg) and were dropped from room temperature into molten solvent (30 g of lead borate
(2PbO·B2O3)) in a Pt crucible at 802 °C. The calorimeter was calibrated using the heat content of
~ 5 mg αꢀAl2O3 pellets53, 54. O2 gas was continuously bubbled through the melt at 5 mL/min to
ensure an oxidizing environment and facilitate dissolution.56 Flushing O2 gas at ~ 50 mL/min
through the calorimeter chamber assisted in maintaining a constant gas environment above the
solvent 56. Upon rapid and complete dissolution of the sample, the enthalpy of drop solution, ꢁHds,
was obtained. Dissolution of uranium and thorium oxides and some other uranium containing
compounds as well as silica has been demonstrated in this solvent, and drop solution enthalpy data
were obtained previously26, 52, 57ꢀ60. Finally, using appropriate thermochemical cycles (Table 3),
enthalpies of mixing, ꢁHmix, enthalpies of formation of the samples from constituent oxides,
ꢁHf,ox, and standard enthalpies of formation from elements, ꢁHºf, were derived.
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Results and Discussion
Based on EPMA and XRD, all of the synthesized and purified uranothorite samples are singleꢀ
phase and homogeneous, as determined by back scattered electron (BSE) images (Figure 1). Their
chemical compositions, determined by WDS, are listed in Table 1. Samples ThU1, ThU2, ThU4,
ThU7 and ThU8 have chemical formulae Th0.89U0.11SiO4, Th0.79U0.21SiO4, Th0.65U0.35SiO4,
Th0.29U0.71SiO4 and Th0.16U0.84SiO4, respectively. The refined unit cell parameters and molar
volume of uranothorite are given in Table 2 and Figure 2.
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TG/DSC experiments up to 800 ºC under an Ar atmosphere (Figure 3) showed that all the
samples had good thermal stability, showing no decomposition under these conditions. Powder
XRD on the retrieved samples confirmed their zircon structure as in the original samples (Figure
4). The weight losses during heating were due to adsorbed water loss, as confirmed by mass
spectroscopy.
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In order to avoid any thermal effects from absorbed water during drop solution calorimetric
measurements, all samples were annealed at 500 ºC to remove the adsorbed water. After
annealing, the samples were weighed and immediately dropped into the calorimeter to avoid water
reabsorption. The enthalpies of drop solution (ꢁHds) are summarized in Table 4. ꢁHds in a function
of uranium content x is plotted in Figure 5a. Its variation fitted by the function (1) suggests a
negative deviation from the thermodynamic ideality,
ꢁHds = a + b·x + c·x2.
(1)
where a = 152.0 ± 3.8 kJ/mol, b = ꢀ131.9 ± 17.4 kJ/mol, c = ꢀ118.7 ± 17.7 kJ/mol, and adjusted R2
= 0.9989. This quadratic fit is also supported by the fact that the intercept (152.0 ± 3.8 kJ/mol) is
consistent with ꢁHds(USiO4) (154.4 ± 5.4 kJ/mol).
For a description of the energetics of uranothorite solid solutions relative to their end members
coffinite and thorite, the enthalpies of mixing, ꢁHmix, were derived from the drop solution
enthalpies by using the equation:
ꢁHmix = ꢀ ꢁHds(Th1ꢀxUxSiO4) + x·ꢁHds(USiO4) + (1–x)·ꢁHds(ThSiO4).
(2)
4+
4+
Values of ꢁHmix are plotted in Figure 5b. Because Th (VIIIrTh = 1.05 Å) and U (VIIIrU =1.00 Å)
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are similar in size, and uranothorite is isostructural with coffinite and thorite, one might expect
random substitution of U4+ for Th4+ in the zircon structure and a close to zero heat of mixing. In
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addition, computational results suggest a positive deviation from ideality with a tendency toward
exsolution34. However, the experimentally determined heat of mixing curve shows substantial
curvature in the opposite direction, with intermediate compositions more energetically stable than
a mixture of the two end members (Figure 5b). The heat of mixing can be represented by a
quadratic polynomial,
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ꢁHmix = ꢂ·x(1–x).
(3)
where ꢂ is the regular solution parameter that can be obtained by comparing quadratic terms in
equation (3) with equation (2) substituted by equation (1): ꢂ = c = ꢀ118.7 ± 17.7 kJ/mol. The
surprising strongly negative value of ꢂ reflects that the mixing of U and Th in the structure is very
exothermic, and the formed intermediate phases are energetically more favorable than the
corresponding mechanical mixture of the end members. The relatively small size difference
between U (1.00 Å) and Th (1.05 Å) may be a factor that allows for the negative heat of mixing.62
Such a strongly negative interaction parameter is suggestive of shortꢀrange, cation ordering, which
may be most pronounced near x = 0.5. The extended Xꢀray fine structure spectroscopic study by
Labs et al.24 showed no direct evidence for clustering suggestive of exsolution or for ordering in
this complete solid solution.
There may be structural evidence for ordering from the Xꢀray diffraction refinements.
Though previous reports suggested that the unit cell volume decreases linearly as a function of x
following Vegard’s law4, 24, 63, in this work a careful refinement (Figure 2) indicates significant
deviation from Vegard’s law behavior. The change in unit cell volume (Figure 2b) is dominated
by the change in the aꢀcell parameter. The change of a with composition is 2.3 % while that in c is
only 0.8 %. The aꢀcell parameter shows significant curvature with composition that is reflected in
the volume change, while the change in c is roughly linear (Figure 2a). Qualitatively, these
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changes can be understood in terms of the basic structural features of the zircon structure. The
thorite structure consists of two types of chains: ThO8 polyhedra alternating with SiO4 tetrahedra
that share edges and are parallel to the cꢀaxis, and ThO8 polyhedra that form an edge sharing zigꢀ
zag chain parallel to the aꢀaxis. Along the cꢀaxis, changes that result from the substitution of U are
moderated by the intervening SiO4 tetrahedra that accommodate the change in the ionic radius (U
vs. Th) by simply adjusting the length of the shared edge; hence, the percentage change in the cꢀ
cell edge is small and essentially reflects the weighted average of the ionic radii of U and Th for
each composition. In contrast, along the aꢀaxis the U and Th polyhedra share edges and interact
directly, and the percentage change in the aꢀparameter is much more sensitive to composition. It is
possible that ordering of the U and Th ions can occur such that they alternate in the polyhedra
along the chains parallel to the aꢀaxis. Maximum order could occur at x = 0.5. Since no
superstructure has been seen, it is inferred that the ordering is only short range, but it could
explain the negative heat of mixing. A full pair distribution function (PDF) analysis of high
resolution Xꢀray or neutron diffraction data could provide a means of detecting shortꢀrange order
or ordered nanodomains.
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The unit cell volume Vcell in terms of USiO4 mole fraction x, is
Vcell = a* + b*·x + c*·x2.
(4)
where a* = 323.3 ± 0.3 Å3, b* = ꢀ24.8 ± 1.4 Å3, c* = 7.7 ± 1.4 Å3, and adj. R2 = 0.9978 (see Figure
2b). The excess volume or volume of mixing (Figure 2c), is given by ꢁVmix = (ꢀ7.7 ± 1.4)·x(1ꢀx)
Å3 or (ꢀ1.2 ± 0.2)·x(1ꢀx) cm3/mol, and is strongly negative and parallels the enthalpy of mixing in
essentially quadratic behavior. The negative enthalpy and volume of mixing are strongly
suggestive of local ordering. If such ordering indeed produces a negative volume change, it is
possible that high pressure may enhance ordering, even to the point of stabilizing a long range
ordered new phase at or near x = 0.5. The extent of ordering seen for a given set of synthesis
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and/or annealing conditions may be both kinetically and thermodynamically controlled, and
careful further study is needed.
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Finally, the determined enthalpies of formation from oxides (ꢁHf,ox) and elements (ꢁHºf) at
room temperature are summarized in Table 4. Note that the enthalpy of formation of ThSiO4
obtained from this work, ꢀ6.4 ± 5.7 kJ/mol, agrees well with the computational result34, but is
inconsistent with the previously measured formation enthalpy value 19.6 ± 2.0 kJ/mol30. This
discrepancy may be due to the incomplete dissolution of large coarsely ground thorite single
crystals used by Mazeina et al.30. Those early experiments did not use gas bubbling and the
samples indeed dissolved slowly.
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ꢁHf,ox values plotted in Figure 5c are fitted to a quadratic equation,
ꢁHf,ox = a’ + b’·x + c’·x2.
(5)
where a’ = ꢀ3.9 ± 3.9 kJ/mol, b’ = ꢀ93.6 ± 19.0 kJ/mol, c’ = 120.1 ± 17.7 kJ/mol, and adj. R2 =
0.9467. The fitted enthalpy of formation curve suggests that the uranothorite phases formed
between x = 0 and 0.8 are energetically favorable. This is consistent with the formation of these
phases in synthesis experiments4, 5, and their relatively common geologic occurrence6ꢀ11. Förster11
and Pointer et al.8 have documented that most observed uranothorite minerals have 35 ~ 36 mol %
U, which is near the minimum of the enthalpy of formation curve (Figure 5c). As the U loading
exceeds x = 0.8, the enthalpy of formation from oxides becomes positive, suggesting unfavorable
formation of uranothorite phases, with coffinite the most unstable phase relative to the oxides by
25.6 ± 3.9 kJ/mol26. This is also consistent with syntheses by Costin et al.4 that show that, under
the same preparation conditions, beyond x = 0.8, hardly any U,Th ꢀ containing silicate phases were
recovered. In addition, between x = 0.3 and 0.8, secondary phases such as Th1ꢀyUyO2 were found
to coexist with the synthetic uranothorite phase4, 51, and to become the dominant phases with
higher U loadings4. Similar trends relating coffinite and uraninite have been found in ore
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deposits20, 22. These observations suggest that, with increasing U content, tetravalent uranium
prefers to be precipitated in oxide phases rather than in silicate phases from a thermodynamic
point of view. In other words, there may be a crossover between the free energy curves of
uranothorite and uranothorianite, such that, at intermediate U loading, the former becomes less
thermodynamically favorable but both phases can still be formed. With x > 0.8, uranothorite
formation becomes thermodynamically unfavorable, leaving U ꢀ enriched uranothorianite the only
stable phase. Thus, besides possible kinetic hindering mechanisms4, the increasing metastability of
Th1ꢀxUxSiO4 as a function of uranium content may also explain why the synthesis of high uranium
uranothorite or pure coffinite is generally difficult. The thermodynamic data also imply that high
uranium uranothorite may follow a similar formation route as coffinite through aqueous
dissolution of uraninite and reprecipitation of U in silicate phases26, 29, whereas, synthesis of
uranothorite with compositions near the thorite endꢀmember may be possible via a direct solid
state process from U/Th oxides.
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Conclusion
Direct calorimetric experiments on Th1ꢀxUxSiO4 uranothorite have yielded their standard
enthalpies of formation from the constituent oxides and elements. The uranothorite compositions
are energetically more stable than their endꢀmembers, coffinite and thorite, and show a large
negative heat of mixing suggestive of cation ordering for intermediate compositions, which is
consistent with crystallographic observations. These results provide a basis for understanding why
uranothorite with high Uꢀloadings is metastable relative to a mixture of binary oxides plus quartz.
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Acknowledgements
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Calorimetric measurements at UC Davis and later data analyses were supported by the
Materials Science of Actinides, an Energy Frontier Research Center, funded by the U.S.
Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DEꢀ
SC0001089. X. G. was also supported by a Seaborg postdoctoral fellowship from the Laboratory
Directed Research and Development (LDRD) program, through the G. T. Seaborg Institute, of Los
Alamos National Laboratory (LANL), which is operated by Los Alamos National Security LLC,
under DOE Contract DEꢀAC52ꢀ06NA25396. The experiments associated to the preparation and
characterization of single phase and homogeneous uranothorite solid solutions were supported by
the NEEDS Resources program of the CNRS (French National Center for Scientific Research).
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Table 1 Elemental analysis of the synthesized uranothorite determined by wavelength dispersive
spectroscopy (WDS).
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Sample
Th
U
Si
Experimental
formula
ThSiO4 59.67 ± 1.34 (16.90)*
ꢀꢀ
6.75 ± 0.18 (15.79)
7.02 ± 0.18 (16.06)
ThSiO4
ThU1
ThU2
ThU4
ThU7
ThU8
59.55 ± 1.24 (15.10)
47.07 ± 1.12 (13.21)
37.37 ± 0.96 (10.67)
17.32 ± 0.59 (4.79)
9.71 ± 0.43 (2.72)
6.80 ± 0.37 (1.84)
Th0.89U0.11SiO4
Th0.79U0.21SiO4
Th0.65U0.35SiO4
Th0.29U0.71SiO4
Th0.16U0.84SiO4
13.11 ± 0.50 (3.59) 6.97 ± 0.18 (16.16)
20.79 ± 0.63 (5.79) 6.98 ± 0.18 (16.47)
43.63 ± 1.02 (11.75) 7.34 ± 0.19 (16.74)
51.28 ± 1.14 (14.00) 7.16 ± 0.19 (16.56)
* wt. % with at. % in parenthesis. Uncertainty is two standard deviations of the mean.
Table 2 The refined unit cell parameters and molar volume of uranothorite.
Sample
ThSiO4
ThU1
ThU2
ThU4
ThU7
ThU8
USiO4
Formula
a (Å)
c (Å)
Volume (Å3)
323.46(1)
320.79(1)
318.23(1)
315.29(1)
309.86(1)
308.19(1)
305.94(1)
ThSiO4
7.1568(1)
7.1241(1)
7.1004(1)
7.0697(2)
7.0255(1)
7.0106(1)
6.9904(1)
6.3152(1)
6.3206(1)
6.3121(1)
6.3084(2)
6.2778(1)
6.2706(1)
6.2610(1)
Th0.89U0.11SiO4
Th0.79U0.21SiO4
Th0.65U0.35SiO4
Th0.29U0.71SiO4
Th0.16U0.84SiO4
USiO4
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Table 3 Thermochemical cycles for Th1ꢀxUxSiO4 (x = 0, 0.11, 0.21, 0.35, 0.71, 0.84) (based on
9
drop solution calorimetry in molten 2PbO·B2O3 at 802 ºC).
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Reaction
ꢁH (kJ/mol)
Enthalpies of formation of Th1-xUxSiO4 from the binary oxides (
ꢁ
Hf,ox) at 25 °C
(1) Th1ꢀxUxSiO4(s, 25 °C) + x/2O2(g, 802 °C)
ꢁH1 = ꢁHds
→ xUO3(sln, 802 °C) + SiO2(sln, 802 °C) + (1–x)ThO2(sln, 802 °C)
(2) UO2(s, 25 °C) + 1/2O2(g, 802 °C) → UO3(sln, 802 °C)
(3) ThO2(s, 25 °C) → ThO2(sln, 802 °C)
ꢁH2 = ꢀ125.21* ± 3.41†(5)‡26, 59
ꢁH3 = 98.1 ± 1.7(15)30
ꢁH4 = 49.9 ± 0.8(9)30
ꢁH5 = ꢀ1084.9 ± 1.064
ꢁH6 = ꢀ1226.4 ± 3.564
ꢁH7 = ꢀ910.7 ± 1.064
(4) SiO2(quartz, s, 25 °C) → SiO2(sln, 802 °C)
(5) U(s, 25 °C) + O2(g, 25 °C) → UO2(s, 25 °C)
(6) Th(s, 25 °C) + O2(g, 25 °C) → ThO2(s, 25 °C)
(7) Si(s, 25 °C) + O2(g, 25 °C) → SiO2(quartz, s, 25 °C)
Enthalpy of mixing of Th1-xUxSiO4 from USiO4 and ThSiO4:
ꢁHmix(Th1ꢀxUxSiO4) = ꢀ ꢁH1 + xꢁHds(USiO4)26 + (1ꢀx)ꢁHds(ThSiO4)
Enthalpy of formation of Th1-xUxSiO4 from UO2, ThO2 and SiO4:
ꢁHf,ox(Th1ꢀxUxSiO4) = ꢀ ꢁH1 + xꢁH2 + (1ꢀx)ꢁH3 + ꢁH4
Standard enthalpy of formation of Th1-xUxSiO4:
ꢁHºf(Th1ꢀxUxSiO4) = ꢁHf,ox + xꢁH5 + (1ꢀx)ꢁH6 + ꢁH7
* Average. † Two standard deviations of the average value. ‡ Number of measurements.
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Table 4 Enthalpies of drop solution, and enthalpies of formation of Th1ꢀxUxSiO4 from binary
oxides and elements.
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Sample
Formula
ꢁHds(kJ/mol)
154.4 ± 5.4
137.7 ± 7.0
118.8 ± 1.2
91.7 ± 2.5
ꢁHf,ox(kJ/mol)
ꢀ6.4 ± 5.7
ꢁH°f(kJ/mol)
ꢀ2143.5 ± 6.8
ꢀ2135.8 ± 7.9
ꢀ2125.1 ± 3.6
ꢀ2109.5 ± 4.0
ꢀ2045.5 ± 3.1
ꢀ2023.6 ± 4.6
ꢀ1970.0 ± 4.226
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ThSiO4
ThU1
ThU2
ThU4
ThU7
ThU8
ThSiO4
Th0.89U0.11SiO4
Th0.79U0.21SiO4
Th0.65U0.35SiO4
Th0.29U0.71SiO4
Th0.16U0.84SiO4
USiO4
ꢀ14.2 ± 7.2
ꢀ17.7 ± 2.1
ꢀ21.9 ± 3.1
ꢀ8.9 ± 2.6
ꢀ1.7 ± 0.5
ꢀ34.3 ± 3.2
ꢀ102.0 ± 3.126
ꢀ5.3 ± 4.4
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25.6 ± 3.926
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Figure 1. Electron microprobe elemental mapping of Th1ꢀxUxSiO4 samples: Mapping of Th on the
left, U in the middle, and Si on the right.
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Figure 2. Refined unit cell parameters and molar volume of Th1ꢀxUxSiO4: (a) Lattice parameter a
fitted by a = 7.15 – 0.26x + 0.10x2, adj. R2 = 0.9925, and unit cell parameter c fitted by c = 6.32 –
0.06x, adj. R2 = 0.9551; (b) unit cell volume Vcell fitted by Vcell = 323.3 – 24.8x + 7.7x2, adj. R2 =
0.9978 and molar volume; (c) volume of mixing ꢁVmix = ꢀ7.7·x(1ꢀx). Open circles or triangle at x
= 0.43 are from Costin et al. work51.
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Figure 3. DSCꢀTG curves of Th1ꢀxUxSiO4 samples (DSC traces are solid curves, and TG traces are
dashed curves).
Figure 4. Powder XRD patterns of the Th1ꢀxUxSiO4 samples recovered after DSC/TG to 800 ºC in
Ar.
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Figure 5. (a) Enthalpies of drop solution, fitted by ꢁHds = 152.0 – 131.9·x – 118.7·x2; (b)
Enthalpies of mixing, fitted by ꢁHmix = ꢀ118.7·x(1–x); (c) Enthalpies of formation of Th1ꢀxUxSiO4
from their binary oxides at 25 ºC, fitted by ꢁHf,ox = ꢀ3.9 – 93.6·x + 120.1·x2.
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