# Stability of doped zirconia under extreme conditions: Toward long-term and secure storage of radioactive waste

Volodymyr Svitlyk, Stephan Weiss, Christoph Hennig

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Volodymyr Svitlyk, Stephan Weiss, Christoph Hennig. Stability of doped zirconia under extreme conditions: Toward long-term and secure storage of radioactive waste. Journal of the American Ceramic Society, 2022, 105 (12), pp.7831-7839. 10.1111/jace.18735. hal-05077635

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# Stability of doped zirconia under extreme conditions:
Toward long-term and secure storage of radioactive waste

**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, Grant/Award Number:
02NUK060

**Abstract**

Extreme temperatures and pressures were applied to systems based on stabilized zirconia, ZrO₂, doped with Ce⁴⁺ ions as surrogate for tetravalent Actinides in order to conclude on their long-term stability in deep geological underground. Both tetragonal and cubic yttrium-stabilized ZrO₂ (YSZ) exhibit excellent phase and structural stabilities up to 1150 K. In addition, incorporated guest Ce⁴⁺ did not show any increase in their mobility at elevated temperatures. Application of external pressure did not induce any structural or phase changes in cubic YSZ doped with 5 at.% Ce as well. However, a corresponding tetragonal analog with lower yttrium content exhibits a second-order phase transition toward higher cubic symmetry around 9 GPa. Remarkably, no discharge of the guest Ce⁴⁺ ions was observed throughout the transition and further upon increase in pressure. This together with *T*-dependent data indicates excellent affinity of guest Ce atoms with the host YSZ matrices. The parent YSZ phases are, therefore, promising candidates as host materials for long-term underground immobilization for radiotoxic tetravalent elements like U, Th, or Pu.

**KEYWORDS**

extreme conditions, mobility, structure, underground storage, Y-stabilized zirconia

## 1 | INTRODUCTION

Structural integrity is crucial for materials intended for secure and long-term (10⁶ years) immobilization of radiotoxic actinide elements. Zirconia, ZrO₂, is a promising prototype candidate for these applications as it is known to remain stable on geological time scale (10⁹ years).<fnref n="1" />,<fnref n="2" /> However, five different structural polymorphs of ZrO₂ can exist in a wide temperature-pressure domain.³ This requires the stabilization of a desired modification in order to prevent *in operando* structural transformations that can be accompanied by abrupt changes in volume and/or atomic rearrangements. This, in turn, could

compromise mechanical and microstructural integrity of system. Such kind of failure is not acceptable for materials hosting radioactive elements as it substantially increases a risk of discharge of incorporated species.

Doping of zirconia, ZrO₂, is widely used to stabilize a desired structural modification in diverse application fields.⁴ For instance, introduction of yttrium allows one to obtain either tetragonal or cubic form of ZrO₂, and the corresponding symmetry can be controlled by the amount of added Y.⁵ The yttrium-stabilized zirconia phases (denoted as YSZ) are used, in particular, as host matrices for radiotoxic Actinides since the YSZ family of compounds is known to incorporate substantial amount of

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2022 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society.

<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>
these species. Moreover, the structure of host YSZ matrices was found to influence maximal possible intake of tetravalent An. For instance of Th, it was shown that tetragonal-to-cubic symmetry increase in YSZ enhanced solubility by 20% with a corresponding maximal concentration of incorporated Th reaching circa 12 at.% for cubic YSZ phases.³

Solid stability of An-containing phases under elevated pressure and temperature conditions is essential to ensure secure underground storage for millions of years. Although currently most concepts for underground nuclear waste emplacements are based on a typical depth of circa 500 m, depths of up to 15 km have been considered in the past.⁶ These depths correspond to typical pressure range of circa 0.01–0.4 GPa and a temperature range of circa 300–700 K. These conditions may, however, evolve/alter with time due to possible tectonic activities or technological accidents in the underground storage, for example, fire and/or explosions.

The YSZ phases are poorly studied as a function of pressure and temperature compared to the parent ZrO₂ compound, and the reported results are sometimes contradictory. Namely, static compression experiments on the YSZ phases with Y³⁺ concentration <10 at.% suggested the existence of tetragonal-cubic (8 at.% Y and T > 1000°C⁷), irreversible monoclinic-tetragonal (3–4 at.% Y) or reversible tetragonal-disordered tetragonal (5-at.% Y)⁸ transformations at P < 5 GPa (for the sake of simplicity further in the text samples from the YSZ series will be denoted as NYSZ where N is an at.% content of Y atoms). At higher pressures, a 2YSZ biphasic sample containing both tetragonal and monoclinic phases was found to feature monoclinic-to-ortho-I transformation at T > 400°C.⁹ In addition, a possible tetragonal-ortho-I phase transformation was suggested to take place in this sample via an intermediate monoclinic structure.⁹ Formation of the monoclinic YSZ phase was also proposed during the tetragonal-ortho-II transformations observed for Y concentrations of 2 and 6 at.% at P > 16 GPa.¹⁰ For the nanocrystalline 3YSZ sample, a tetragonal phase was found to transform to monoclinic modification at P < 1 GPa and to remain stable after decompression.¹¹ Finally, no P-dependent phase transitions were reported for YSZ samples with 3- and 4-at.% Y, although presented diffraction patterns feature apparent changes in symmetry.¹² One of the factors that generates the observed inconsistencies are symmetries of the YSZ modifications used for subsequent high pressure (HP) studies. Structures of YSZ phases depend strongly on a thermal history of the corresponding high-temperature (HT) synthesis and, apparently, HT phases have not been successfully stabilized at ambient conditions for some of the reported cases.

In this work, we subjected tetragonal and cubic YSZ phases doped with tetravalent cerium ions to in situ synchrotron radiation diffraction experiments under extreme conditions of temperature and pressure. These studies allowed to track in detail phase stability and structural evolutions in Ce-YSZ and to conclude on the mobility of the incorporated guest Ce⁴⁺ ions that were used as surrogate for tetravalent An elements.

## 2 | EXPERIMENT

For this study, five CeₓY₀.₁₀Zr₀.₉₋ₓO₂₋z (x = 0–0.08, ~0.02 step) and five CeₓY₀.₁₆Zr₀.₈₄₋ₓO₂₋z (x = 0–0.09, ~0.02 step) samples have been synthesized by a coprecipitation method. These series represent YSZ phases with lower (0.10) and higher (0.16) yttrium content, respectively, in order to probe different regions of the Y-ZrO₂ phase diagram.⁵ Synthesis was performed at the laboratory of the Rossendorf Beamline (BM20) at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) and at the Institute of Resource Ecology (HZDR, Dresden, Germany). For this cerium(III) nitrate hexahydrate (Sigma-Aldrich, >99.99%) 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) using molar ratios to match targeted compositions. Precipitation of amorphous precursors was induced by a dropwise addition of ammonia solution under constant stirring until pH = 11. Obtained white slurries were sedimented by ultracentrifugation at 286 000 × g (XPN-80, Beckman Coulter). Solid residues were subsequently dried 12 h at 250°C. Final calcination was performed in air at 1673 K for 10 min using a simultaneous thermogravimetry-differential scanning calorimetry machine (STA 449 F5 Jupiter, Netzsch). This assured the oxidation of cerium ions from initial +3 to targeted +4 states. Rapid cooling with a rate of 20 K/min was further used in order to stabilize tetragonal phases with lower yttrium content and HT cubic modification.⁵ Chemical compositions of all samples were verified using inductively coupled plasma mass spectrometry (ICP-MS) technique (NexION 350x, Perkin Elmer). For this analysis, both initial solutions before precipitation and corresponding supernatants after ultracentrifugation were analyzed. From the ICP-MS measurements, a typical spread in the actual yttrium content was found to be circa 1% compared to nominal concentrations of the corresponding series.

In situ T- and P-dependent synchrotron radiation powder diffraction experiments were performed at the ROBL BM20 beamline¹³ at the ESRF. Diffraction data were collected on a multipurpose XRD-2 diffractometer equipped
<figure><img src="image_3.png" /><figcaption>FIGURE 1 (Left) Series of powder diffraction patterns of the Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> phase collected upon heating showing excellent phase stability and preservation of tetragonal symmetry; (right) unit cell volume (black datapoints) and occupancy of cerium atoms (blue datapoints) of the Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> phase as a function of temperature revealing the absence of mobility of Ce<sup>4+</sup> ions at high-temperature (HT)</figcaption></figure>

with a Pilatus3 2M photon counting detector. During the T-dependent measurements, the temperature was controlled using a Cyberstar hot gas blower furnace. Temperature on the sample position was calibrated using diffraction lines of Ag powder and all 10 samples were subsequently scanned in a T range of 300–1150 K with a typical rate of 6 K/min (Figure 1, left, data for the Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> sample are shown). Upon reaching the target, temperature samples were quickly cooled down to room temperature (RT) in order to check for possible irreversible changes. For T-dependent measurements, energy of synchrotron radiation was set to 22 keV and the beam was focused to the size of 200×200 µm². During P-dependent experiments, HP was generated using diamond anvil cells (DACs) with 600-µm diamond culets. Samples were contained in stainless steel gaskets with holes of 300 µm and a thickness of 95 µm (Figure 2, inset). One representative phase from the series with low- and high-Y content was subjected to in situ compression: Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> and Ce<sub>0.05</sub>Y<sub>0.16</sub>Zr<sub>0.79</sub>O<sub>1.92</sub>. Ne gas and silicon oil where used as pressure-transmitting media (PTM) with maximum probed pressures of 13.8 and 11.0 GPa for samples with low- and high-Y content, respectively, with a typical step of 1–1.5 GPa. Pressure inside the sample chambers was measured using a ruby fluorescence technique.<sup>14</sup> For both T- and P-dependent measurements, raw 2D diffraction data as obtained from the Pilatus3 2M detector (Figure 1, left, example of T-dependent data for Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub>) were reduced using Bubble software.<sup>15</sup> Full structural analysis, including a refinement of occupancies of Ce<sup>4+</sup> ions, was performed with FullProf package<sup>16</sup> based on least-squares minimization method (Figure 2, typical refinement is shown on the example of HP data of Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub>; diffraction data collected at ambient conditions for all the studied samples with the corresponding Rietveld refinement analysis [cell parameters,

<figure><img src="image_4.png" /><figcaption>FIGURE 2 Full structural refinement of diffraction data for the tetragonal Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> phase collected at 0.46 GPa. Experimental data are represented by red points, *I*(obs); calculated profile is shown as black continuous line, *I*(calc); continuous blue line is difference, *I*(obs) − *I*(calc); green vertical bars stand for Bragg positions. Inset: sample chamber of the diamond anvil cell (DAC) loaded with powder of the sample (center), ruby chips (on sides), and Ne gas as pressure-transmitting media (PTM) (transparent)</figcaption></figure>

R-factors] are presented in the Supporting Information section).

Mobility of incorporated guest Ce<sup>4+</sup> ions, regarded as exsolution from zirconia lattice, was tracked by monitoring effective site occupancies of these species. In order to avoid correlations during the refinement procedure, occupancies of Ce<sup>4+</sup> atoms were refined with Y/Zr stoichiometries fixed to the values as obtained from the ICP-MS analysis. For the tetragonal Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub>, the occupancy of Ce<sup>4+</sup> ions could not be refined reliably at the highest probe pressure of 14 GPa due to increased contribution to the diffraction pattern of solidified Ne gas used as PTM.
During the refinement, occupancies of oxygen atoms were set to the values that satisfy overall charge balance.

For *T*-dependent data, individual isotropic thermal displacement parameters, *B*<sub>iso</sub>, were initially refined at ambient temperature. During the following sequential refinement as a function of temperature, individual *B*<sub>iso</sub> parameters were fixed and the overall thermal displacement parameters, *B*<sub>ove</sub>, were released to account for increase in corresponding thermal vibrations of atoms upon increase in temperature. This allowed refinement of *T*-dependent changes in occupancies of guest Ce<sup>4+</sup> atoms that would be, otherwise, correlated to the corresponding individual thermal displacement parameters. For *P*-dependent data, individual thermal displacement parameters and occupancies of Ce<sup>4+</sup> ions were refined independently. For all the studied samples, background was described as linear interpolation between manually selected points between the Bragg peaks. Pseudo-Voigt profile, which is a convolution of Gaussian and Lorentzian functions, was used to describe peaks shapes for both *T*- and *P*-dependent data.

## 3 | RESULTS AND DISCUSSION

As expected, at ambient temperature and pressure conditions samples from the Ce<sub>x</sub>Y<sub>0.10</sub>Zr<sub>0.9-x</sub>O<sub>2-z</sub> and Ce<sub>x</sub>Y<sub>0.16</sub>Zr<sub>0.84-x</sub>O<sub>2-z</sub> series were found to crystallize in tetragonal and cubic symmetries, respectively. This is in agreement with the experimental Y-ZrO<sub>2</sub> phase diagram.<sup>5</sup> Stabilization of metastable tetragonal (usually denoted as *t′*) and cubic phases indicates that the cooling rate of 20 K/min is sufficient to quench the corresponding HT modifications in Ce-doped YSZ systems. This cooling rate was also reported to be sufficient for the synthesis of *t′* phases in related Th-YSZ systems.<sup>3</sup>

In situ *T*-dependent diffraction studies on tetragonal Ce<sub>x</sub>Y<sub>0.10</sub>Zr<sub>0.9-x</sub>O<sub>2-z</sub> and cubic Ce<sub>x</sub>Y<sub>0.16</sub>Zr<sub>0.84-x</sub>O<sub>2-z</sub> series in an *RT*-1150-*RT* K range revealed excellent stability for all the studied compounds. In particular, no phase separation and no structural transformations were observed in all the studied compounds despite crossing twice (on heating and on cooling) phase boundary line around 850 K, which corresponds to monoclinic-tetragonal transformation in equilibrium state of YSZ.<sup>5</sup> In addition, the occupancy of Ce<sup>4+</sup> atoms as a function of temperature does not decrease in all phases within the error range (Figure 1, right, blue datapoints, Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> phase is shown as an example) indicating that the mobility of these guest ions does not increase with temperature (no exsolution from the host zirconia matrix). Similarly, refinement of O(z) of tetragonal Ce-YSZ phases showed

<table><caption>TABLE 1 Coefficients of linear thermal expansion (×10<sup>-5</sup>) for tetragonal Ce<sub>x</sub>Y<sub>0.10</sub>Zr<sub>0.9-x</sub>O<sub>2-z</sub> (*t′*) and cubic Ce<sub>x</sub>Y<sub>0.16</sub>Zr<sub>0.84-x</sub>O<sub>2-z</sub> (*c*) Ce-yttrium-stabilized ZrO<sub>2</sub> (YSZ) phases</caption><thead><tr><th colspan="2"><b><i>t</i>'-Ce-YSZ</b></th><th colspan="2"><b><i>c</i>-Ce-YSZ</b></th></tr><tr><th><b>Ce, x</b></th><th><b>α, K<sup>-1</sup></b></th><th><b>Ce, x</b></th><th><b>α, K<sup>-1</sup></b></th></tr></thead><tbody><tr><td>0</td><td>3.66</td><td>0</td><td>3.52</td></tr><tr><td>0.02</td><td>3.62</td><td>0.02</td><td>3.52</td></tr><tr><td>0.05</td><td>3.75</td><td>0.05</td><td>3.53</td></tr><tr><td>0.07</td><td>3.72</td><td>0.07</td><td>3.53</td></tr><tr><td>0.08</td><td>3.90</td><td>0.09</td><td>3.61</td></tr></tbody></table>

that this coordinate remains constant upon heating and cooling.

Unit cell volume increases linearly for all the phases within the error range (Figure 1, right, black datapoints, Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> phase is shown as an example). Corresponding coefficients of linear thermal expansion (CLTE), defined as α = 1/V<sub>0</sub> × ((V - V<sub>0</sub>)/(T - T<sub>0</sub>)), with V<sub>0</sub>(V) and T<sub>0</sub>(T) being the initial (final) unit cell volume and the sample temperature, are listed in Table 1. Cubic Ce-YSZ samples show slightly stiffer response toward the application of high temperature than the corresponding tetragonal phases. This behavior is explained by higher yttrium content which, in turn, yields a higher bulk modulus with resulting decrease in CLTEs of YSZ.<sup>17</sup> Similar reduction in thermal expansion coefficients was observed in related Y-doped HfO<sub>2</sub> phases<sup>18</sup> upon the introduction of trivalent yttrium atoms and was shown to be related to increase in oxygen vacancies. On the contrary, doping with larger Ce<sup>4+</sup> ions induces minor but steady increase in CLTEs for both tetragonal and cubic Ce-YSZ phases (Table 1). Increase in corresponding coefficients was also reported for CeO<sub>2</sub>-ZrO<sub>2</sub> solid solution upon the introduction of cerium ions.<sup>19</sup> This behavior was explained by reduced strength in Zr-O bonding due to the substitution of larger Ce<sup>4+</sup> for smaller Zr<sup>4+</sup> in the host ZrO<sub>2</sub> lattice (ionic radii of 0.97 vs. 0.84 Å, respectively, in eight-fold coordination<sup>20</sup>). The Zr-O bonds with reduced energy are more affected by temperature increase with resulting increase in CLTEs.

Contrary to temperature, HP generated with DAC (Figure 2, inset) did induce structural transformation in the tetragonal Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> phase. Namely, tetragonal (002) and (110) doublets gradually approach upon the application of pressure and convert into one single c(200) peak around 9 GPa (Figure 3, left) indicating transition to cubic symmetry.

The P4<sub>2</sub>/nmc-Fm$\overline{3}$m is a continuous second-order transition of a displacive type, as concluded from group-subgroup analysis using the ISOTROPY Software Suite.<sup>21,22</sup> Accordingly, no discontinuity is observed in the unit cell volume at the transition point (Figure 3,
<figure><img src="image_5.png" /><figcaption>FIGURE 3 (Left) P-induced tetragonal-cubic transition in Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> as witnessed by approaching of the (0 0 2) and (1 1 0) reflections around 9 GPa yielding a single (2 0 0) cubic peak; (right) volume compression of tetragonal (black datapoints) and cubic (blue datapoints) Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> upon application of external pressure</figcaption></figure>

right). The low pressure region (P < 8 GPa) of the unit cell volume, which corresponds to the tetragonal phase (Figure 3, right, black datapoints), was fitted with the third-order Birch–Murnaghan equation of state (EOS), as shown in the following equation:

$$P(V) = \frac{3K_0}{2} \left[ \left( \frac{V_0}{V} \right)^{\frac{7}{3}} - \left( \frac{V_0}{V} \right)^{\frac{5}{3}} \right] \left[ 1 + \frac{3}{4} (K'_0 - 4) \left\{ \left( \frac{V_0}{V} \right)^{\frac{2}{3}} - 1 \right\} \right] \quad (1)$$

where $V_0$ is the zero pressure volume, $K_0$ is the bulk modulus, and $K'_0$ is its first pressure derivative. The corresponding coefficients are $V_0 = 68.757(12) \AA^3$, $K_0 = 184(4)$ GPa, and $K'_0 = 4(11)$ GPa. The EOS for the HP cubic Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> was not calculated due to a low number of experimental datapoints (Figure 3, right, blue). The bulk modulus, $K_0$, of 184 GPa for tetragonal Ce-YSZ is significantly higher than the corresponding value of 95 GPa for the pristine parent monoclinic ZrO<sub>2</sub> phase at ambient temperature.<sup>23</sup> Its hardness is, nevertheless, comparable with the one of the HP ZrO<sub>2</sub> orthorhombic-I phase with $K_0 = 220$ GPa<sup>23</sup> but significantly lower compared to ZrO<sub>2</sub> orthorhombic-II modification with $K_0$ estimated to be in the range of 280–320 GPa.<sup>24,25</sup> As expected, much harder orthorhombic-II phase is less affected by thermal expansion ($\alpha = 2.05 \times 10^{-5} \text{ K}^{-1}$)<sup>26</sup> than studied in this work tetragonal/cubic phases (Table 1). Increase in bulk moduli upon the introduction of trivalent yttrium atoms was previously observed both in experimental and theoretical studies.<sup>17,27</sup> This behavior is explained by decrease in effective Zr/Y–O distances upon the introduction of Y<sup>3+</sup> ions via a generation of oxygen vacancies, which results in the

increase of the corresponding Zr/Y–O binding energies. As a result, bulk moduli in YSZ system increase with higher Y<sub>2</sub>O<sub>3</sub> content.

Tetragonal-to-cubic transition pressure of 9 GPa found for the Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> phase is certainly influenced by the amount of the dissolved yttrium and cerium guest ions as well. Indeed, YSZ phases with various Y contents were reported to undergo P-dependent structural transformations already under pressures below 1 GPa and for some compositions even above 16 GPa (Section 1). Pristine ZrO<sub>2</sub> phase itself features monoclinic–orthorhombic-I phase transformation around 4 GPa.<sup>28,29</sup> Although the tetragonal structure of Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> is stabilized by doping, due to a relatively high content of Y<sup>3+</sup> ions, it is situated close to the stability region of the cubic YSZ phases.<sup>5</sup> In addition, doping with large Ce<sup>4+</sup> ions also favors evolution toward higher cubic symmetry.<sup>3</sup> Therefore, the application of HP on Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> pushes its tetragonal structural arrangement toward energetically closely situated and structurally similar cubic modification without a formation of orthorhombic phases observed for the pristine ZrO<sub>2</sub> phase.<sup>24,25</sup>

Unit cell of tetragonal Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> exhibits anisotropic contraction upon compression. Specifically, at the transition point, a and c parameters are reduced by 1.2% and 1.6%, respectively, compared to the values at ambient conditions (Figure 4, left). Interestingly, the application of negative chemical pressure induces opposite anisotropic response in the tetragonal YSZ phases. Here upon the introduction of large Th<sup>4+</sup> guest ions,<sup>3</sup> the volume of unit cell in the ab plane expands about twice as faster than in the c direction (0.51% vs. 0.27%, respectively).<sup>3</sup> In the latter case, strong anisotropic response helps to preserve long Zr/Y–O distances of the tetragonal YSZ system that results in a better accommodation of large tetravalent Actinide elements. The resulting flattening of the tetragonal YSZ
<figure><img src="image_6.png" /><figcaption>**FIGURE 4** (Left) Evolution of unit cell parameters of tetragonal (black and red datapoints) and cubic (blue datapoints) Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> upon compression; for plotting the values of *a* parameter (red datapoints) were multiplied by √2 to follow cubic setting; (right) *P*-dependent behavior of O(*z*) coordinate indicating onset of cubic symmetry around 9 GPa</figcaption></figure>

<table><caption>**TABLE 2** Refined structural parameters for Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> at *P* = 0.46 GPa (space group *P*4<sub>2</sub>/nmc, *a* = *b* = 3.63230(7) Å, *c* = 5.19681(19) Å, *V* = 68.565(3) Å<sup>3</sup>, *R*<sub>B</sub> = 5.11%, *R*<sub>wp</sub> = 8.99%)</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>Ce/Y/Zr</td><td>2a</td><td>3/4</td><td>1/4</td><td>1/4</td><td>0.85(2)</td></tr><tr><td>O</td><td>4d</td><td>1/4</td><td>1/4</td><td>0.021(4)</td><td>2.2(1)</td></tr></tbody></table>

lattice reduced the tetragonality of the system with a clear evolution toward higher cubic symmetry.³

In the case of application of external pressure (current study), a cubic structure is achieved by the suppression of the sole degree of structural liberty—z coordinate of oxygen atoms, O(z), provided by the 4d site of the tetragonal *P*4<sub>2</sub>/nmc structure (Table 2, refined data for tetragonal Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> at *P* = 0.46 GPa is shown as an example; a corresponding refinement is illustrated on Figure 1). Although at lower pressures below 4 GPa the O(z) coordinate is constant within the error range (Figure 4, right), it starts to gradually decrease above 4 GPa until oxygen atoms settle at the 8c position of cubic *Fm*3*m* structure around 9 GPa (Figure 4, right, blue datapoints). This continuous change in position of oxygen atoms is consistent with the second-order nature of the *P*4<sub>2</sub>/nmc-*Fm*3*m* displacive transition (see previous text).

Remarkably, for Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> within the error range, occupancies of the incorporated Ce<sup>4+</sup> ions remain constant throughout all the studied pressure range, including the transition point (Figure 5, left). In both tetragonal and cubic YSZ phases, central metal ion (M) is coordinated by eight oxygen atoms and in the corresponding 3D structures, the ZrO<sub>8</sub> polyhedra share their edges.³ However, in tetragonal YSZ, two distinct sets of M–O distances are present (Figure 5, right, blue and black solid lines inside the ZrO<sub>8</sub> polyhedra), which results in puckered

<figure><img src="image_7.png" /><figcaption>**FIGURE 5** (Left) Refined occupancy of the incorporated Ce<sup>4+</sup> ions on the M site of Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> as a function of pressure; (right) coordination of central Zr/Ce atoms in tetragonal yttrium-stabilized ZrO<sub>2</sub> (YSZ) featuring two distinct sets of M–O distances (solid blue and black lines inside the MO<sub>8</sub> polyhedra) and a resulting puckered 3D structure (*P* region below 9 GPa, left). The corresponding distances are equal in cubic YSZ (*P* region above 9 GPa, left).</figcaption></figure>

layered structures. In HP cubic modification, all eight M–O distances are equal and the corresponding ZrO<sub>8</sub> layers are flat. As the Ce<sup>4+</sup> ions are incorporated on the M site of the Ce–YSZ phase, they are also subjected to the changes in bonding and coordination found for M–O at the transition point. Nevertheless, stability in the occupancy of the guest Ce<sup>4+</sup> ions during the *P*4<sub>2</sub>/nmc-*Fm*3*m* transformation is maintained by the corresponding increase in symmetry. Indeed, cubic symmetry was found to facilitate the incorporation of large tetravalent An elements into the YSZ systems,³ and the same mechanism applies to the large Ce<sup>4+</sup> ions when introduced into the Zr<sup>4+</sup> position. In addition, the second-order nature of the tetragonal–cubic transition with the resulting continuous structural changes also contributes toward phase stability during the transformation. After compression to 14 GPa, the Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> phase brought to ambient pressure featured initial *P*4<sub>2</sub>/nmc symmetry and this behavior also corroborates with the continuous second-order nature of the transformation observed at 9 GPa.
<figure><img src="image_8.png" /><figcaption>FIGURE 6 Unit cell volume of cubic Ce<sub>0.05</sub>Y<sub>0.16</sub>Zr<sub>0.79</sub>O<sub>1.92</sub> as a function of pressure (black datapoints) fitted with the third-order Birch–Murnaghan equation of state (EOS) (red solid line).</figcaption></figure>

No phase transition was observed in cubic Ce<sub>0.05</sub>Y<sub>0.16</sub>Zr<sub>0.79</sub>O<sub>1.92</sub> up to the maximal probed pressure of 11 GPa. For this, the phase volume of the unit cell decreases gradually with pressure (Figure 6, black datapoints) and fitting of the experimental data with the third-order Birch–Murnaghan EOS (Equation 1, Figure 6, red solid line) yielded the following coefficients: V<sub>0</sub> = 138.08(16) Å<sup>3</sup>, K<sub>0</sub> = 182(34) GPa, and K'<sub>0</sub> = 39(14) GPa. Cubic YSZ phase is expected to be harder than the corresponding tetragonal Ce<sub>0.05</sub>Y<sub>0.10</sub>Zr<sub>0.85</sub>O<sub>1.95</sub> analog with lower yttrium content.<sup>17</sup> Although the values of the corresponding bulk moduli, K<sub>0</sub>, for tetragonal and cubic YSZ analogs are comparable (184(4) vs. 182(32) GPa, respectively), higher experimental error in the case of cubic phase hinders reliable comparison of these values. In the case of cubic YSZ, higher uncertainty in K<sub>0</sub> originates from a larger spread of the experimental datapoints in the V–P domain compared to the tetragonal phase (Figures 6 and 3, right, respectively). It is, in turn, explained by different hydrostaticities of the employed PTMs: Ne gas and silicon oil for tetragonal and for cubic YSZ phases, respectively. Indeed, although Ne gas results in a pressure deviation of less than 0.05 GPa within the pressure chamber for the probed pressure range, corresponding errors in pressure can be up to 10× higher for silicon oil for pressures above 4 GPa.<sup>30</sup> Advantage of silicon oil as PTM is its lower compressibility compared to condensed gasses, including Ne, which results in a larger volume of the experimental HP chamber at elevated pressures, which provides more experimental flexibilities.

Presented in this work, physical characteristics of
the tetragonal and cubic YSZ phases as obtained from
synchrotron radiation diffraction experiments, namely,
CLTE and bulk moduli, can be used as experimental
constrains for theoretical modeling for corresponding

materials under *in operando* conditions. In particular,
these data can help one to evaluate a long-term mechan-
ical stability of similar YSZ materials hosting radioactive
An elements. Possible mechanical failure, for exam-
ple, fracture of the system, would influence undergoing
physicochemical processes, for instance, leaching rate,
due to increase in the surface area. In this sense, both
tetragonal and cubic YSZ systems are rather promis-
ing as they do not exhibit sudden volume changes as
a function of temperature or pressure, even throughout
P-dependent phase transition found for the tetragonal
modification.

4 | CONCLUSIONS

Tetragonal and cubic series of YSZ phases doped with Ce⁴⁺
ions were subjected to extreme conditions of temperature
and pressure. Upon heating, both systems were found to be
stable, and the mobility of the incorporated guest cerium
ions did not increase up to T = 1150 K. Although cubic YSZ
doped with 5 at.% Ce was stable until a maximal applied
pressure of 11 GPa, its tetragonal analog transformed into
higher cubic symmetry around 9 GPa. Nevertheless, in this
phase, the occupancy of the guest Ce⁴⁺ ions remained con-
stant throughout and above the P-dependent structural
transition. No observed discharge of the incorporated Ce⁴⁺
ions in a broad T–P domain points to their high affinity
with the YSZ matrix, and these phases can be considered
possible candidates for secure immobilization of An(IV)
elements.

An-YSZ phases potentially placed in underground
depositories likely will not be exposed during the storage
period to the extreme temperature and pressure conditions
used in this study (Tmax = 1150 K and Pmax = 14 GPa).
Nevertheless, applied elevated T and P allowed one to
accelerate processes that could possibly occur in these sys-
tems deep underground over a period of millions of years.
As no discharge of the guest Ce⁴⁺ ions was observed in
this study, even during phase transition found in tetragonal
Ce-YSZ, these phases are expected to be stable during life-
time of the incorporated radiotoxic elements with respect
to harsh external temperature and pressure conditions.
Thus, we suggest in situ studies under extreme conditions
as a part of standard protocol to validate phases of inter-
est as host matrixes for radioactive elements for long-term
underground storage.

ACKNOWLEDGMENTS

This work was supported by the Bundesministerium für
Bildung und Forschung (BMBF) AcE grant (02NUK060).
We acknowledge the HZDR BM20 ROBL station at ESRF
for providing beamtime in the frame of the AcE project.
D. Naudet, J. Exner, N. Baumann, and J. Jacobs are acknowledged for computing and technical support during experiments at the 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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**How to cite this article:** Svitlyk V, Weiss S, Hennig C. Stability of doped zirconia under extreme conditions: Toward long-term and secure storage of radioactive waste. J Am Ceram Soc. 2022;105:7831-7839.

https://doi.org/10.1111/jace.18735

## SUPPORTING INFORMATION

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