# Immobilization of radiotoxic elements with Y-stabilized zirconia: The thorium case

Volodymyr Svitlyk, Stephan Weiss, Christoph Hennig

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Volodymyr Svitlyk, Stephan Weiss, Christoph Hennig. Immobilization of radiotoxic elements with Y-stabilized zirconia: The thorium case. Journal of the American Ceramic Society, 2022, 105 (9), pp.5975-5983. 10.1111/jace.18543. hal-05077634

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# Immobilization of radiotoxic elements with Y-stabilized zirconia: The thorium case

**Volodymyr Svitlyk**<fnref n="1" />,<fnref n="2" /> ID |

**Stephan Weiss**<fnref n="1" /> |

**Christoph Hennig**<fnref n="1" />,<fnref n="2" />

## Correspondence

Volodymyr Svitlyk, Helmholtz-Zentrum
Dresden-Rossendorf, Institute of Resource
Ecology, 01314 Dresden, Germany.

Email: svitlyk@esrf.fr

## Funding information

Bundesministerium für Bildung und
Forschung (BMBF), Grant/Award

Number: 02NUK060

## Abstract

Y-stabilized zirconia (YSZ) phases were found to incorporate Th atoms, and the corresponding solubility ranges depend on the relative Y content. For the tetragonal phases with a lower Y concentration of 14 at.%, a maximal possible Th intake on the Zr/Y metal site reached ca. 10.3 at.%. Cubic phases with higher Y content could dissolve 11 at.% Th in an equilibrium state and up to ca. 12.3 at.% Th under nonequilibrium conditions. Larger Th–Zr/Y solubility range for phases with higher Y concentration was found to be related to the associated symmetry increase, as concluded from synchrotron radiation powder diffraction data. Specifically, the introduction of Th into tetragonal YSZ induces evolution toward higher cubic symmetry via flattening of the Zr/YO₈ polyhedra. In addition, a tetragonal YSZ crystal lattice exhibits strongly anisotropic expansion with a concomitant decrease in tetragonality upon the intake of Th. This results in an easier accommodation of bigger Th atoms via a structural stabilization of longer Zr/Y–O bonding distances which yields more symmetrical coordination of central Zr/Y metal ions by surrounding O atoms. Cubic symmetry is, therefore, more favorable to the incorporation of large tetravalent actinide elements.

## KEYWORDS

actinides, high pressure, nuclear waste, structure, symmetry, YSZ, zirconia ceramics

## 1 | INTRODUCTION

Continuous accumulation of nuclear waste represents enduring ecological threat for current and, especially, future distant generations. Currently, the majority of generated radiotoxic elements originate from spent nuclear fuel (SNF). It is composed essentially of tetravalent uranium and plutonium and minor actinides (An) like ²³⁷Np, ²⁴¹Am/²⁴³Am, and ²⁴⁴Cm. The half-life of these elements ranges from a few decades (ca. 18 years for ²⁴⁴Cm)<fnref n="1" /> to

millions of years (ca. 2.1 million years for ²³⁷Np)<fnref n="2" />. Consequently, solutions for secure and long-term (at least 10⁶ years scale) storage of man-made radioactive products are of high societal importance.

During the process of evaluation of a specific phase as a potential host for radiotoxic elements, the first scientific goal is to determine the maximum possible intake of tetravalent An without phase separation. Further, structural stabilities of these phases under various external extreme parameters, for example, temperature (T),

<footnote n="1">¹Helmholtz-Zentrum
Dresden-Rossendorf, Institute of Resource
Ecology, Dresden, Germany</footnote>

<footnote n="2">²Rossendorf Beamline (BM20), European
Synchrotron Radiation Facility, Grenoble,
France</footnote>
<figure><img src="image_3.png" /><figcaption>FIGURE 1 Different structural polymorphs of ZrO₂ as a function of *T* and *P*: (A) monoclinic modification composed of edge- and corner-shared ZrO₇ polyhedra; (B) and (C) tetragonal and cubic structures, respectively, composed of edge-shared ZrO₈ polyhedra; and (D) ortho-II modification composed of face-, edge-, and corner-shared ZrO₉ polyhedra. Ortho-I modification (*Pbca*) is structurally similar to the monoclinic *P*2₁/*c* phase (A) and is also composed of edge- and corner-shared ZrO₇ polyhedra</figcaption></figure>

pressure (*P*), irradiation, and resistance against leaching are essential in order to exclude possible discharge of the incorporated radioactive elements over a long-time scale. In this context, derivatives of zirconium-based ceramics, for instance, zirconia, ZrO₂, or zircon, ZrSiO₄, are promising materials as these phases are known to remain stable in geological cycles for more than 10⁹ years.³⁻⁶

Zirconia is one of the major phases of modern ceramic-based materials.⁷ Due to remarkable physicochemical, electrical, thermal, and mechanical properties, zirconia, together with its derivatives, is widely employed in refractory materials, in high-temperature (HT) solid fuel cells, and, among others, in biocompatible high-strength structural devices materials. In addition, its high radiation hardness makes it one of the reference materials in nuclear applications, notably as an inert fuel matrix in nuclear reactors⁸,⁹ or as a host matrix for radiotoxic elements present in SNF.¹⁰,¹¹

Five different structural modifications of zirconia are known to exist. At ambient pressure undoped ZrO₂ phase exhibits three different polymorphs as a function of temperature—low-temperature monoclinic (sevenfold coordination of Zr atoms, *P*2₁/*c*, Figure 1A) and parent *HT* tetragonal (*T* > 1170°C), and cubic (*T* > 2370°C) forms (eightfold coordination, *P*4₂/*nmc* and *Fm*-3*m*, Figure 1B,C, respectively).¹² Monoclinic-to-tetragonal structural transition is accompanied by a ~4% volume change, and a resulting sample fracture that occurs on cooling during *HT* synthesis of pure zirconia compromises the mechanical performance of this material. As a result, doping with various elements is widely used to stabilize *HT* modifications of zirconia at ambient temperature. Another two structural polymorphs of ZrO₂ are formed upon applications of high pressure. At a pressure of ~4 GPa, the monoclinic modification of zirconia transforms into orthorhombic-I phase (*Pbca*, sevenfold coordination of Zr atoms similar to that observed for the parent monoclinic *P*2₁/*c* phase, Figure 1A).¹³ Upon further compression at

ambient temperature, orthorhombic-I modification transforms into orthorhombic-II phase at *P* > 25 GPa (*Pnma*, ninefold coordination, Figure 1D)¹⁴, and this phase was found to be stable up to at least 100 GPa.¹⁵ Interestingly, Ortho-I–II transition pressure is significantly reduced at higher temperatures, and this transformation takes place already at ~12 GPa at *T* > 600°C.¹⁵,¹⁶

Various properties of ZrO₂ can be efficiently controlled by doping.¹⁷ In particular, the introduction of Y³⁺ ions is known to significantly influence a phase diagram of zirconia and to stabilize desired *HT* modifications. Specifically, tetragonal Y-stabilized zirconia (YSZ) phases, denoted as *t'*, can be obtained at ambient temperature for Y content of ca. 3–15 at.% by quenching from the *HT* region of the YSZ phase diagram where the cubic phase is stable.¹⁸,¹⁹ The cubic YSZ phase itself can be stabilized for Y content ≥15 at.%.¹⁹,²⁰ The YSZ phases are, however, much more poorly studied compared to the parent ZrO₂ compound. In particular, these phases are underexplored in the context of uptake of An⁴⁺ elements.

Here we study the incorporation of Th into Y-stabilized ZrO₂ phases of tetragonal and cubic symmetries, which correspond to lower and higher Y content, respectively. Associated structural changes were tracked in detail using the synchrotron radiation powder diffraction technique, and corresponding correlations with Th intake were revealed.

## 2 | EXPERIMENT

Four series of samples have been synthesized for the current study: (I) Zr₀.86⁻*y*Y₀.14Th*y*O₂⁻*z* (*y* = 0.04, 0.07, 0.10, 0.12), (II) Zr₀.89⁻*y*Y₀.11Th*y*O₂⁻*z* (*y* = 0.01–0.07, ~0.01 step), (III) Zr₀.79⁻*y*Y₀.21Th*y*O₂⁻*z* (*y* = 0–0.16, ~0.03 steps), and (IV) (Zr₀.57Y₀.17)⁻*y*Th*y*O₂⁻*z* (*y* = 0.25–0.90, ~0.10 steps). Samples from the series I were synthesized in order to determine Th solubility limit for tetragonal YSZ phases
(lower Y content). Compositions with finer steps in Th doping (series II) were intended to probe in detail structural evolution induced by the incorporation of large Th⁴⁺ ions on the Zr/Y site of the corresponding tetragonal YSZ phases. Samples from the series III were produced in order to determine maximum Th intake for the cubic YSZ phases (higher Y content). Finally, the series IV was intended to study the phase composition of the YSZ-ThO₂ pseudo-binary system for high concentrations of Th. For these samples, the Y/Zr ratio was kept constant and equal to ca. 1/3 that corresponds to cubic symmetry for the parent YSZ system.¹⁹

Synthesis and elemental analysis of samples were performed at the Institute of Resource Ecology (IRE HZDR), Dresden, Germany, following the procedure of safe handling for Th-based radioactive materials. All samples were obtained via a coprecipitation method using solutions of corresponding metal salts. Initially, thorium(IV) nitrate pentahydrate (Merck, p.a.) and yttrium(III) nitrate hexahydrate (Sigma-Aldrich, >99.8%) were dissolved in distilled water. Obtained solutions were mixed with a zirconium(IV) acetate solution (Sigma-Aldrich, 15%-17% Zr, pH 3.3-3.8) following molar ratios corresponding to targeted compositions. Precipitation of amorphous precursors was induced by dropwise addition of ammonia solution under constant stirring until pH = 7.5. Obtained slurries were separated by ultracentrifugation at 286 000 × g (XPN-80, Beckman Coulter). Solid residues were subsequently dried for 12 h at 250°C. Final calcination was performed in air at 1400°C for 10 min using a simultaneous thermogravimetry-differential scanning calorimetry system (STA 449 F5 Jupiter, Netzsch). This setup allows rapid cooling with a rate of 20°C/min that is crucial to stabilize tetragonal phases with lower yttrium content.¹⁹ Chemical compositions of all samples were controlled using an inductively coupled plasma mass spectrometry (ICP-MS) technique (NexION 350×, Perkin Elmer), and both initial solutions before precipitation and corresponding supernatants after ultracentrifugation were analyzed. From these measurements, yttrium concentration was found to vary typically by ca. 1% compared to the nominal concentration of the corresponding series. Throughout the text, including figures and tables, actual Th concentrations as obtained from the ICP-MS analysis are used.

Ambient temperature synchrotron radiation powder diffraction experiments were performed at the ROBL BM20 beamline²¹ at the European Synchrotron Radiation Facility, Grenoble, France. Diffraction data were collected on high-resolution XRD1 machine equipped with a Dectris Pilatus 100k photon counting detector. For data collection, powders of the samples were packed in glass capillaries of 0.3-mm diameter enclosed in 1-mm Kapton tubes that serve as confinement barriers. The energy of syn-

<figure><img src="image_4.png" /><figcaption>**FIGURE 2** Evolution of the unitcell volume in tetragonal Zr<sub>0.86-y</sub>Y<sub>0.14</sub>Th<sub>y</sub>O<sub>2-z</sub> series upon Th intake indicating a corresponding solubility limit of ca.10.3 at.% Th</figcaption></figure>

chrotron radiation was set at 16 000 eV in order to avoid fluorescence associated with Th atoms with an L3 absorption edge situated at 16 300 eV. Experiments were performed in a transmission mode and corresponding 2D data were reduced using the PyFAI library adapted for diffractometers mounted on a goniometer arm.²² Quantitative phase and full structural analysis was performed with FullProf Suite²³ that is based on the least-squares minimization method.

### 3 | RESULTS AND DISCUSSION

From powder diffraction data, all the YSZ phases from the series I Zr<sub>0.86-y</sub>Y<sub>0.14</sub>Th<sub>y</sub>O<sub>2-z</sub> (y = 0.04, 0.07, 0.10, 0.12) were found to crystallize in tetragonal symmetry. This is in agreement with the experimental ZrO<sub>2</sub>-Y phase diagram¹⁹ and demonstrates that the cooling rate of 20°C/min is sufficient to stabilize *HT* tetragonal (denoted as *t′*) YSZ phases. Upon introduction of Th atoms into the parent zirconia, the phase volume of the YSZ unitcell increases nearly linearly until Th content on the metal Zr/Y site reaches 10% (Figure 2; all data in this manuscript are presented with experimental error bars that may be not visible on figures if smaller than the corresponding symbols; corresponding refined values are listed in Table S1). This behavior is explained by the larger ionic radius of Th⁴⁺ compared to Zr⁴⁺ (1.19 vs. 0.98 Å, respectively, in eightfold coordination²⁴), and this response is in agreement with the Vegard law. The latter was found to be useful in studying solubility ranges of tetravalent An elements, including in Th-based systems.²⁵ Traces of tetragonal ThO₂ have been observed for the sample with 10 at.% Th (Figure S3)
indicating that this composition situates closely to the solubility limit of Th. Indeed, the next point that corresponds to the Th concentration of 12 at.% does no longer follow this linear increase in the unitcell volume and, in addition, a significant precipitation of cubic ThO₂ was observed in this sample. However, as the unitcell volume of the phase with 12 at.% Th is larger than the corresponding volume of the 10 at.% Th phase, the solubility limit of Th situates between 10 and 12 at.%. Using linear extrapolation (Figure 2, red lines), maximum Th intake for the Zr₀.86-y Y₀.14ThyO₂-z system is estimated to be equal to ca. 10.3 at.%.

The incorporation of Th was found to be more limited when zirconia is not stabilized with yttrium atoms. Specifically, solubilities less than 5 at.% Th were reported in ThO₂-ZrO₂ systems.²⁶⁻²⁸ Contrary, the addition of small amount of yttrium was found to increase the corresponding solubility limit of Th up to 7 at.% in YSZ doped with 8 at.% Y²⁹ (8YSZ, further in the text corresponding systems will be denoted as NYSZ, where N = at.% Y). Remarkably, undoped tetragonal zirconia phases were reported to be stabilized at ambient conditions by low concentrations of Th atoms,³⁰ even down to 1 at.%³¹

Th incorporation was found to influence certain microstructural effects of ZrO₂ as well. For instance, doping with Th improved thermal stability and increased the surface area of mesoporous ZrO₂.³⁰ Finally, the introduction of Th induced systematic structural changes in tetragonal 8YSZ phases. Specifically, the tetragonality of ZrO₂ was reported to decrease with the increase of Th content.²⁹ Certainly, this structural effect is a convolution of the local coordination environment of ZrO₈ polyhedra in tetragonal ZrO₂ with the introduction of guest Th atoms that are larger than the volume provided by crystallographic sites of smaller Zr atoms.

In order to obtain more details on the influence of Th incorporation on the structure of the tetragonal YSZ series, samples with finer doping steps have been studied. They correspond to series II Zr₀.89-y Y₀.11 ThyO₂-z with y = 0.008, 0.014, 0.021, 0.027, 0.037, 0.044, 0.051, 0.064, and 0.069. Typical structural refinement for samples from this series is shown in Figure 3 in the example of phase with Th concentration on the metal Zr/Y site of 0.8 at.%. Here the value of Th content, which corresponds to a fraction y = 0.008, was rounded to 0.01 in the corresponding formula for representation purpose. Details of structural refinement and structural parameters for the Zr₀.88 Y₀.11 Th₀.01 O₁.₉₅ phase (Figure 3) are listed in Table 1. All other refined X-ray diffraction patterns for the series II and all other phases studied in this work are provided in the Supplementary Material section.

As expected, the unitcell volume for the Zr₀.89-y Y₀.11 ThyO₂-z series increases nearly linearly upon introduction of larger Th⁴⁺ ions (Figure 4, left;

<figure><img src="image_5.png" /><figcaption>FIGURE 3 Structural refinement of diffraction data for the Zr₀.88 Y₀.11 Th₀.01 O₁.₉₅ phase (y = 0.008 for the series II). Experimental data are represented by red points, I(obs); a calculated profile is shown as a black continuous line, I; continuous blue line is the difference, I(obs)-I; green vertical bars stand for Bragg positions</figcaption></figure>

<table><caption>TABLE 1 Refined structural parameters for
Zr₀.₈₈ Y₀.₁₁ Th₀.₀₁ O₁.₉₅ (space group P4₂/nmc, a = b = 3.61622(1) Å, c = 5.16644(2) Å, V = 67.5618(5) Å³, Rₐ = 2.20%, Rₑ = 3.31%)</caption><thead><tr><th>Atom</th><th>Site</th><th>x/a</th><th>y/b</th><th>z/c</th><th>B<sub>iso</sub> (Å<sup>3</sup>)</th></tr></thead><tbody><tr><td>Zr/Y/Th<sup>a</sup></td><td>2a</td><td>3/4</td><td>1/4</td><td>1/4</td><td>0.628 (3)</td></tr><tr><td>O<sup>a</sup></td><td>4d</td><td>1/4</td><td>1/4</td><td>0.0356 (2)</td><td>1.69 (2)</td></tr></tbody></table>

corresponding refined values are listed in Table S2). Interestingly, this increase is accompanied by a systematic decrease in the z coordinate of the oxygen atoms (Figure 4, right; corresponding refined values are listed in Table S2). In tetragonal ZrO₂ structures, oxygen atoms occupy 4d sites (Table 1) with a resulting eightfold coordination of Zr atoms (Figure 5). In the corresponding ZrO₈ polyhedra, two distinct sets of Zr-O bonds are present: Zr-O' and Zr-O'' (Figure 5, equal to 2.1204(5) and 2.3338(7) Å, respectively, for tetragonal Zr₀.₈₈ Y₀.₁₁ Th₀.₀₁ O₁.₉₅). The observed decrease in the oxygen z coordinate (Figure 4, right) results in symmetry-induced antiparallel movements of the O' and O'' atoms in planes orthogonal to the ab plane of tetragonal YSZ. As a result, the corresponding Zr-O' and Zr-O'' distances are expected to increase and decrease, respectively. Although the Zr-O' distances indeed feature an increase upon introduction of Th (Figure 6, left, blue line), the Zr-O'' bonds remain unchanged within the experimental error range (Figure 6, left, black line).

<footnote>ªDuring refinement effective occupancies on the metal sites were set to the values obtained from the ICP-MS analysis; oxygen occupancies were set to the values which satisfy overall charge balance.</footnote>
<figure><img src="image_6.png" /><figcaption>FIGURE 4 Increase in the unitcell volume of tetragonal Zr<sub>0.89-y</sub>Y<sub>0.11</sub>Th<sub>y</sub>O<sub>2-z</sub> series upon introduction of Th (left) and associated decrease of the oxygen z coordinate (right)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>FIGURE 5 Structural evolution of tetragonal ZrO<sub>2</sub> upon introduction of Th atoms. Left: Zr–O' distances (solid blue lines inside the ZrO<sub>8</sub> polyhedra) increase, whereas the Zr–O'' distances (solid black lines inside the ZrO<sub>8</sub> polyhedra) are “stabilized” by faster lattice expansion in the ab plane. Right: flattened Zr/ThO<sub>8</sub> polyhedra. The relative scale of the presented flattening is exaggerated for illustration purposes</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>FIGURE 6 Evolution of bonding in the ZrO<sub>8</sub> polyhedra (left) and decrease in tetragonality (right) in the Zr<sub>0.89-y</sub>Y<sub>0.11</sub>Th<sub>y</sub>O<sub>2-z</sub> series upon introduction of Th. In the latter figure, the *a* lattice parameter was multiplied by √2 to follow the cubic setting of the unitcell</figcaption></figure>
The observed behavior in the Zr–O’ and Zr–O” bondings as a function of Th intake results from a highly anisotropic volume expansion in the tetragonal Zr<sub>0.89</sub>–<sub>y</sub>Y<sub>0.11</sub>Th<sub>y</sub>O<sub>2</sub>–<sub>z</sub> series. Specifically, in the *ab* plane, the unitcell expands about twice faster than that in the orthogonal *c* direction (0.51% vs. 0.27%, respectively). This anisotropy is a result of structural adaptation following the introduction of bigger Th atoms onto the 2*a* site of smaller Zr atoms of the parent ZrO<sub>2</sub> phase. This overcomes the symmetry-imposed shortening of the Zr–O” distances following the decrease in the *z*(O) coordinate, and these bonds are effectively stabilized. In addition, this bonding behavior allows one to preserve the average valence of 3.9 on the metal site upon introduction of Th, as concluded from bond valence sums (BVS) calculations performed for this data series. Bonding behavior was also found to be linked to effective BVS for central metal site upon introduction of other tetravalent ions into tetragonal zirconia.<sup>32</sup> The resulting Zr/ThO<sub>8</sub> polyhedra become flatter (Figure 5, right) compared to those in the parent pristine ZrO<sub>2</sub> phase.

It is clear that the introduction of Th induces structural evolution toward a higher (cubic) symmetry via flattening of the ZrO<sub>8</sub> polyhedra. In the origin of this, structural response lies a larger ionic radius of Th<sup>4+</sup> and this, in turn, allows to accommodate eight coordinating oxygen atoms with higher symmetry (change from 2*mm*. site symmetry to –43*m* site symmetry). This mechanism based on size effect is analogous to the one in the origin of the temperature-dependent increase in symmetry for pristine zirconia. Here, thermal expansion of the lattice initially allows a change from sevenfold coordination in monoclinic P2<sub>1</sub>/<i>c</i> zirconia to an eightfold coordination in tetragonal modification at *T* = 1170°C. Further increase in temperature provides additional volume around the central Zr atom, and all eight oxygen atoms are able to position in a more symmetrical arrangement with all Zr–O distances equal with an onset of cubic *Fm*–3*m* symmetry at *T* > 2370°C. The observed tendency toward increase in symmetry (Figure 5) is also reflected in the behavior of the unitcell parameters. Indeed, the introduction of Th induces decrease in tetragonality of ZrO<sub>2</sub> (Figure 6, right) with a clear tendency toward cubic structural arrangement, and this behavior is directly related to the decrease in the *z* coordinate of the oxygen atoms (Figure 4, right).<sup>33</sup> Consequently, one can assume that ZrO<sub>2</sub>-based phases with cubic symmetry may accommodate a larger amount of Th atoms than their tetragonal analogs.

Indeed, phases of a cubic symmetry that corresponds to elevated Y content<sup>19</sup> feature a higher intake of Th atoms. Specifically, samples from series III Zr<sub>0.79–<sub>y</sub></sub>Y<sub>0.21</sub>Th<sub>y</sub>O<sub>2–<sub>z</sub></sub> (<i>y</i> = 0–0.16, ~0.03 steps) exhibit linear increase in unitcell volume up to Th concentration on the metal site of

<figure><img src="image_9.png" /><figcaption>**FIGURE 7** Evolution of unitcell volume in cubic Zr<sub>0.79–<sub>y</sub></sub>Y<sub>0.21</sub>Th<sub>y</sub>O<sub>2–<sub>z</sub></sub> phases (series III, *y* < 0.2) and cubic phases of (Zr<sub>0.57</sub>Y<sub>0.17</sub>)<sub>–<sub>y</sub></sub>Th<sub>y</sub>O<sub>2–<sub>z</sub></sub> series IV (for *y* > 0.2) upon Th intake indicating a maximum solubility limit of ca. 12.3 at.% Th</figcaption></figure>

11 at.% (Figure 7; corresponding refined values are listed in Table S3). The following datapoint with *y* = 0.16 does not feature increase in volume within the error range and, in addition, the precipitation of ThO<sub>2</sub> was observed in this sample. Accordingly, samples from the series IV (Zr<sub>0.57</sub>Y<sub>0.17</sub>)<sub>–<sub>y</sub></sub>Th<sub>y</sub>O<sub>2–<sub>z</sub></sub> with higher Th content (*y* = 0.25–0.90, ~0.10 steps) were also phase separated and were composed of mixtures of cubic Th–YSZ and ThO<sub>2</sub> phases throughout all the compositional range. Nevertheless, certain samples from this series do feature compositions with even higher amount of incorporated Th. Specifically, the highest volume in series IV is observed for the sample with *y* = 0.34, and through linear extrapolation the amount of the dissolved Th in the corresponding cubic phase was estimated to be equal to ca. 12.3 at.% (Figure 7). In this sample, a weight fraction of the corresponding cubic Th–YSZ phase is equal to 70(1) mass% and ThO<sub>2</sub> phase accounts for the remaining 30(1) mass%, as concluded from quantitative phase analysis of the corresponding diffraction data (Figure S21).

Our study shows that the evolution in symmetry and the observed solubility limits in Th–YSZ agree qualitatively with the corresponding properties found in benchmark Y<sub>2</sub>O<sub>3</sub>–CeO<sub>2</sub>–ZrO<sub>2</sub> systems. Indeed, the *c/a* ratio was also reported to decrease in CeO<sub>2</sub>–ZrO<sub>2</sub> upon the introduction of larger cerium ions (*r*(Ce<sup>4+</sup>) = 1.11 Å vs. *r*(Zr<sup>4+</sup>) = 0.98 Å in eightfold coordination<sup>24</sup>), and cubic symmetry was obtained for high concentrations of Ce<sup>4+</sup> ions.<sup>34,35</sup> In addition, the introduction of oversized trivalent yttrium ions into a ZrO<sub>2</sub> matrix (*r*(Y<sup>3+</sup>) = 1.16 Å in eight-fold coordination<sup>24</sup>) is known to generate oxygen vacancies around Zr atoms that stabilize structural
arrangements of higher symmetry, namely, tetragonal and cubic.36 Extended but limited (20–30 at.%) solid solutions were reported in Y₂O₃–CeO₂–ZrO₂ system, and the solubility limits were also found to be higher for phases based on cubic ZrO₂ than on its tetragonal analog.37 As expected, in a pseudo-binary CeO₂–ZrO₂ system, almost complete miscibility is achieved due to a smaller size of Ce⁴⁺ ions compared to Y³⁺ as the constituents CeO₂ and ZrO₂ oxides feature analogs fluorite-type structural arrangements.35,38,39 On the contrary, the studied here ThO₂-YSZ solid solutions are less stable than the solid solutions based on CeO₂ due to the larger ionic radius of Th⁴⁺ (*r* = 1.19 Å) that fits less good into the crystal lattice of ZrO₂.

The maximum Th intake in cubic YSZ phases of ca. 12.3 at.% was found in a phase-separated sample with *y* = 0.34. This indicates that the observed Th incorporation does not correspond to equilibrium phase conditions. Remarkably, Th intake of up to ca. 15.8% was reported for the cubic 25YSZ sample doped with 40 at.% of Th obtained by the calcination of a mixture of the corresponding oxides.40 This sample was also 25YSZ-ThO₂ phase separated. Clearly, a higher solubility limit of Th in cubic 25YSZ could originate from higher Y content present in this phase, and this behavior was indeed observed in this study and was previously reported in the literature (previously mentioned). However, as in the current study and in the case of the Th-25YSZ system,40 both samples featuring maximum Th intake are phase separated, and synthesis conditions may be in the origin of the underlying nonequilibrium conditions and the resulting phase compositions. Specifically, annealing temperature and time, cooling rate, and even synthetic procedure in general may be adjusted to synthesize monophasic samples with the observed maximum Th concentrations.

4 | CONCLUSIONS

Thorium was successfully introduced into tetragonal and cubic yttria-stabilized ZrO₂ phases. For the tetragonal 14YSZ series, the corresponding maximum solubility limit was found to be ca. 10.3 at.% Th on the Zr/Y metal site. Cubic YSZ phases, which were stabilized by higher Y content, featured higher Th solubility limits. Specifically, a monophasic 21YSZ sample with 11 at.% Th has been synthesized. However, nonequilibrium phase with higher Th intake of up to ca. 12.3 at.% was observed in the cubic ThO₂-YSZ pseudo-binary system in a phase-separated sample with Th content of 34 at.%. Corresponding monophasic samples with higher Th concentrations are expected to be obtained by adjusting experimental synthetic parameters.

Increased Th solubility range in cubic phases with higher Y content was found to be favored by a corresponding increase in symmetry. This behavior was clearly observed in the tetragonal 11YSZ series in which the introduction of Th induced flattening of the ZrO<sub>8</sub> polyhedra, as concluded from synchrotron radiation powder diffraction data. This structural evolution was also accompanied by decrease in tetragonality of the corresponding unitcells of 11YSZ. Accordingly, tetragonal-cubic symmetry change upon doping with Y facilitates the incorporation of larger Th atoms via a more symmetrical arrangement of the coordinating O atoms, in analogy to T-dependent behavior in pristine ZrO<sub>2</sub>. Cubic zirconia, therefore, may be more favorable toward the incorporation of tetravalent actinide ions.

The applied experimental procedure for the incorporation of tetravalent An into the YSZ phases is technically rather straightforward for potential large-scale applications. Initially, the radioactive material of interest in a solid form should be transformed into a salt solution with a pH < 7, for example, dissolved in an acidic media. The following incorporation procedures via coprecipitation technique do not require sophisticated and expansive equipment, and a full incorporation cycle can be completed in a few days (the complex differential scanning calorimetry system used in this work can be replaced with ovens adapted for rapid cooling of samples). This makes these phases potentially attractive for the large-scale industrial transformation of radioactive waste into corresponding zirconia-based solid solutions. Nevertheless, further studies on structural and phase stabilities of these materials or corresponding analogs with surrogate atoms are needed to evaluate them as potential candidates for long-term and secure storage of radioactive waste. Specifically, their irradiation stability and behavior under extreme conditions of temperature and pressure are required to validate studied YSZ phases as efficient host matrixes for tetravalent radiotoxic elements.

ACKNOWLEDGMENTS

This project was supported by the Bundesminis-
terium für Bildung und Forschung (BMBF) AcE grant
(No. 02NUK060). We also acknowledge the HZDR BM20
ROBL station at ESRF for providing beamtime in the
frame of the AcE project. D. Naudet, J. Exner, and N.
Baumann are acknowledged for computing and technical
support during experiments at BM20 beamline.

Open Access funding enabled and organized by Projekt
DEAL.

ORCID

Volodymyr Svitlyk https://orcid.org/0000-0001-5449-8009
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## SUPPORTING INFORMATION

Additional supporting information can be found online in the Supporting Information section at the end of this article.

**How to cite this article:** Svitlyk V, Weiss S, Hennig C. Immobilization of radiotoxic elements with Y-stabilized zirconia: The thorium case. J Am Ceram Soc. 2022;105:5975-5983. https://doi.org/10.1111/jace.18543