# Self-healing ThSiO4-ZrSiO4 system under conditions relevant to underground nuclear waste repositories

Volodymyr Svitlyk, Stephan Weiss, Gaston Garbarino, Salim Shams Aldin Azzam, René Hübner, Andreas Worbs, Nina Huittinen, Christoph Hennig

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Volodymyr Svitlyk, Stephan Weiss, Gaston Garbarino, Salim Shams Aldin Azzam, René Hübner, et al.. Self-healing ThSiO4-ZrSiO4 system under conditions relevant to underground nuclear waste repositories. Acta Materialia, 2024, 281, pp.120357. 10.1016/j.actamat.2024.120357. hal-05118262

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# Self-healing ThSiO₄-ZrSiO₄ system under conditions relevant to underground nuclear waste repositories

Volodymyr Svitlyk<sup>a,b</sup>, Stephan Weiss<sup>a</sup>, Gaston Garbarino<sup>c</sup>, Salim Shams Aldin Azzam<sup>a</sup>, René Hübner<sup>d</sup>, Andreas Worbs<sup>d</sup>, Nina Huittinen<sup>a,e</sup>, Christoph Hennig<sup>a,b,*</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Zircon

ZrSiO₄

Thorium

Water content

Phase composition

High pressure

High temperature

## ABSTRACT

Two series of Th₁ₓZrₓSiO₄ phases were synthesized hydrothermally under weakly basic (pH = 8) and strongly acidic (pH = 1) conditions. Changes in pH were found to have a significant effect on experimental phase diagrams. Synthesis at pH = 8 favors the formation of Th-rich phases with resulting Th₁ₓZrₓSiO₄ solid solution for x = 0 – 0.5. Contrary, synthesis at pH = 1 results in the formation of pure end-members of the ThSiO₄-ZrSiO₄ pseudo-binary system separated by multiple miscibility gaps. Phases formed both under basic and acidic conditions were found to retain water, which can be discharged from the structure upon heating. A different high-pressure (HP) behaviour was found for Th-rich and Zr-rich solid solutions. While Th-rich Th₀.₉Zr₀.₁SiO₄ and Th₀.₆Zr₀.₄SiO₄ phases retain their stoichiometry and crystal structure upon compression at HP, a significant reduction of the Th occupancy related to a decrease of the Th-O distances is observed for the Th-poor Th₀.₂Zr₀.₇₄SiO₄ phase at P > 8 GPa, with the subsequent formation of a Th-rich amorphous phase. The Th diffusion between the crystalline and amorphous phases was found to be fully reversible. This unique self-healing property makes these phases promising candidates for nuclear applications under extreme pressure and temperature conditions, in particular those found in underground repositories.

## 1. Introduction

Incorporation of radiotoxic elements into host crystalline matrices for subsequent underground storage can enhance long-term safety of these nuclides and their decay products. For example, Ce doped Y-stabilized zirconia (YSZ) phases [1], analogous to Th-containing YSZ compounds [2], have been shown to retain tetravalent guest ions even under extreme temperature and pressure conditions. In addition to derivatives of zirconia, ZrO₂, other Zr-based phases may also be promising hosts for radiotoxic elements. For instance, natural zircon, ZrSiO₄, is well known to incorporate actinide elements, in particular uranium and thorium. That is why this mineral is widely used to determine the age of geological objects by analyzing the ratio of U isotopes to their stable Pb decay products (²³⁵U/²⁰⁷Pb and ²³⁸U/²⁰⁶Pb) [3–9]. Moreover, zircon is one of the most stable minerals and natural billion years-old ZrSiO₄-based rocks have been discovered [10–13]. This exceptional geological

stability calls for systematic studies on the abilities of ZrSiO₄ to incorporate specific radiotoxic elements, in particular tetravalent actinides which are omnipresent in spent nuclear fuel (SNF). Zircon is proposed to heal cracks in thermally stressed yttria partially stabilized zirconia (YPSZ) by forming a rigid crystalline zone [14]. Thorium has a stable oxidation state of +4 and is used in this study as a surrogate for other tetravalent actinides such as U(IV) and Pu(IV) to avoid problems caused by redox reactions.

Nevertheless, data available in literature are sometimes not consistent or even contradicting. One of the earliest attempts to introduce Th into ZrSiO₄ using a hydrothermal route was not successful [15]. Therefore, Th and Zr were concluded to be immiscible and the difference in the corresponding ionic radii was evoked as a reason for this behavior. The zircon mineral crystallizes in a tetragonal structure (space group I4₁/amd) and is composed of ZrO₈ and SiO₄ edge- and corner-sharing polyhedra (Fig. 1). In an 8-fold coordination the ionic radius of Zr⁴⁺ is

<footnote><sup>a</sup> Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, 01328 Dresden, Germany</footnote>

<footnote><sup>b</sup> Rossendorf Beamline (BM20), European Synchrotron Radiation Facility, 38000 Grenoble, France</footnote>

<footnote><sup>c</sup> High Pressure Beamline (ID15b), European Synchrotron Radiation Facility, 38000 Grenoble, France</footnote>

<footnote><sup>d</sup> Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Material Research, 01328 Dresden, Germany</footnote>

<footnote><sup>e</sup> Institute of Chemistry and Biochemistry, Freie Universität Berlin, 14195 Berlin, Germany</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: hennig@esrf.fr (C. Hennig).</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Crystal structure of the tetragonal Zircon/Thorite phases featuring edge- and corner-sharing Th/ZrO<sub>8</sub> (marine blue) and SiO<sub>4</sub> (green) polyhedra.</figcaption></figure>

equal to 0.84 Å and that of Th⁴⁺ is 1.05 Å [16]. Although the size mismatch is about 20 %, it is now well documented that Th can be introduced on the Zr site coordinated by eight oxygen atoms, as in the ZrO₂-ThO₂ system [17–19], for instance. The argument of the size effect is, therefore, not valid as demonstrated by other studies on the pseudo-binary zircon-thorite ((Zr,Th)SiO₄) systems. Specifically, it has been shown that up to 2 % of Th can be dissolved in zircon using the sol-gel method [20]. Up to 4 % Th in ZrSiO₄ and 6 % Zr in ThSiO₄ were reported using dry and hydrothermal synthesis procedure [21]. Natural zircon and thorite minerals were found to form stable phases with even higher solubility ranges. Here, maximal concentrations of the dissolved Th and Zr species in the host zircon and thorite phases, respectively, were found to exceed 40 % [22]. However, these natural minerals contained considerable amounts of many further elements, essentially uranium, yttrium, and rare-earth elements.

Clearly, the synthesis procedure has a significant impact on the resulting experimental ThSiO₄ – ZrSiO₄ phase diagram. In addition, chemical doping (impurities) can also influence the Th-Zr solubility limits. In this work we report on synthesis, phase and structural studies of the pseudobinary thorite-zircon system. The corresponding compounds were obtained via hydrothermal synthesis routes using high (pH = 8) and low pH (pH = 1) solutions. Selected phases were subjected to in situ structural studies with synchrotron radiation under extreme conditions of pressure (0 – 20 GPa) and temperature (300 – 1135 K). This allowed assessing the stability of the solid solutions at conditions expected in underground repositories, where radioactive materials will be stored at elevated pressures and temperatures until the radioactive material has decayed into stable products.

## 2. Experimental

### 2.1. Synthesis

Hydrothermal synthesis under different pH conditions was employed for this study in order to probe phase compositions and corresponding solubility limits in the ThSiO₄-ZrSiO₄ pseudobinary system. It is known that a change in pH during hydrothermal synthesis can influence the formation yield of phases of interest. For instance, it was reported that the ThSiO₄ phase could be obtained hydrothermally with a yield greater than 95 % for a pH ranging from 1 to 9 [23]. On the other hand, the HfSiO₄ phase, closely related to ZrSiO₄, has only been successfully synthesized at pH values of up to 1 [24]. This behavior was explained by induced hydrolysis of Hf or Th at elevated pH with a subsequent formation of HfO₂ or ThO₂ oxides with aging. In this study we performed

two series of hydrothermal synthesis at pH values of 1 and 8. This allowed to study optimal conditions for formation of ThSiO₄- and ZrSiO₄-based solid solutions and to construct corresponding phase diagrams.

For the hydrothermal synthesis at pH = 1 the following chemicals were mixed in appropriate ratios: Th(NO₄)₄·5H₂O (Merck, p.a.), Na₂SiO₃·9H₂O (J.T. Baker, p.a. grade), ZrOCl₂·8H₂O (Merck, p.a. grade). The final pH = 1 was obtained by addition of 12 M HCl acid under constant stirring and monitoring of pH. For the synthesis at pH = 8 the following chemicals were used: Na₂SiO₃·9H₂O (J.T. Baker, p.a. grade), ZrOCl₂·8H₂O (Merck, p.a. grade), Th(NO₄)₄·5H₂O (Merck, p.a. grade) and NaHCO₃ (Roth, ≥99,5 %, p.a. grade) as a buffer solution. In order to probe the corresponding phase diagrams the following compositions were targeted for the synthesis at pH 1 and 8: x = 0, 0.02, 0.04, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.96, 0.98 and 1 for the Th₁₋ₓZrₓSiO₄ series. An excess of 3 at.% of Na₂SiO₃·9H₂O was added to all the starting mixtures in order to avoid formation of thorium and zirconium oxides. Mixed solutions were placed into Teflon containers which were placed inside stainless steel autoclaves. The synthesis was performed in ovens at 220 °C during 24 h. The white precipitates were subsequently filtered, washed with distilled water and centrifuged. The washing procedure was performed twice for every sample. The resulting white compact powder was dried under air at 60 °C for 12 h. All the manipulations were performed in a controlled area following safety procedures for handling of radioactive substances.

### 2.2. Compositional analysis

Compositions of samples synthesized at pH = 1 were controlled with an inductively coupled plasma spectrometry (ICP-MS) technique (NexION 350 ×, Perkin Elmer). For this, both initial solutions before the hydrothermal precipitation and corresponding supernatants after ultracentrifugation were analyzed. The net stoichiometries of the Th₁₋ₓZrₓSiO₄ samples synthesized at pH = 1 are given according to the data obtained from the ICP-MS analysis. This analysis was not employed to control compositions of the samples obtained at pH = 8 since precipitation appeared upon mixing. To control the chemical composition of the hydrothermally precipitated samples at pH = 8, element analysis was performed using energy-dispersive X-ray spectroscopy (EDX) in a scanning electron microscope (SEM) for the Th₀.₆Zr₀.₄SiO₄ sample which is close to the equimolar part of the ThSiO₄ – ZrSiO₄ phase diagram. The SEM analysis (Fig. 1S, Supplementary Material, image of the Th₀.₆Zr₀.₄SiO₄ sample is shown) revealed a Th/Zr ratio equal to 1.4(1), which corresponds to the nominal value of 1.5 within the error range. Therefore, for the samples synthesized hydrothermally at pH = 8 the nominal compositions as prepared for the synthesis were used.

### 2.3. Synchrotron diffraction and thermogravimetry studies

High-resolution powder diffraction data of the samples synthesized at pH 1 and 8 were collected on the XRD-1 diffractometer of ROBL (European Synchrotron Radiation Facility, ESRF, Grenoble, France) [25]. This allowed to study phase compositions and to construct corresponding phase diagrams for the ThSiO₄ – ZrSiO₄ pseudo-binary system. The energy of the incoming synchrotron radiation was set to 15,957 eV, just below the Th L₃ absorption edge, to minimize the sample absorption. The beam was focused to a size of 300 × 300 µm². Samples were filled in glass capillaries of 0.3 mm diameter. The glass capillaries were placed in polyimide tubes used as confinement for the radioactive samples. The data were collected in transmission mode with an Eiger CdTe 500k detector (Dectris) and integrated using a specific Python code based on PyFAI routines adapted to treat series of images collected as a function of 2θ [26]. The powder diffraction data were analysed with FullProf suite in order to determine phase compositions of the samples and to extract structural information of interest. A typical refinement is shown in Fig. 2 for the Th₀.₆Zr₀.₄SiO₄ sample and the corresponding
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Structural refinement of diffraction data for $Th_{0.6}Zr_{0.4}SiO_4$. Experimental data are represented by red points, I(obs); the calculated profile is shown as a black continuous line, II{calc}; continuous blue line is the difference I(obs)-I{calc} and green vertical bars stand for Bragg reflex positions.</figcaption></figure>

results are summarized in Table 1.

Initially, synchrotron radiation powder diffraction data under applied external pressure were collected for selected samples on the multipurpose XRD-2 diffractometer of ROBL [25]. To generate high pressure, diamond anvil cells (DACs) with culets of 600 µm were used. Samples were contained using stainless steel gaskets within laser-drilled holes of 300 µm in diameter. The sample chambers were filled with Ne gas which was used as pressure transmitting medium (PTM). The pressure was measured using the ruby luminescence technique which relies on the calibrated P-dependent luminescence signal obtained from laser-excited ruby spheres (Al₂O₃:Cr) placed in the sample chamber [27]. The synchrotron beam was focused down to the size of 100 × 100 µm² and the energy was set to 30,491 eV. The maximum pressure that was reached was 14.4 GPa. A further P-dependent data collection was performed at the High Pressure ID15b ESRF beamline under similar experimental conditions. Namely, Ne gas was also used as a PTM and energy of the synchrotron radiation was set to 30,000 eV. The beam was focalized to the size of 10 × 10 µm² which allowed reaching higher pressures up to 20 GPa. Subsequent full structural refinement was performed with FullProf package. For the refinement initial Th/Zr ratios were fixed to the values obtained from the ICP-MS analysis (pH = 1 series) or nominal compositions (pH = 8 series). The occupancies of Th atoms were subsequently freely refined in order to follow P-dependent evolution of Th concentration. Structural refinement of high pressure (HP) data is shown in Supplementary Section on the example of the $Th_{0.6}Zr_{0.4}SiO_4$ sample collected at 4.1 GPa (Fig. 2S).

T-dependent powder diffraction data for the ZrSiO₄ sample were collected on the XRD-2 diffractometer. For the measurements, ZrSiO₄ powder was loaded in a quartz capillary of 300 µm diameter. The temperature was controlled with a Cyberstar hot gas blower furnace and scanned in a range of 300 – 1135 K with a rate of 4 K/min on heating and 8.5 K/min on cooling. The temperature on the sample position was calibrated using the diffraction lines of silver powder.

Thermogravimetry and differential scanning calorimetry (DSC) data

<caption>**Table 1**</caption>

Structure refinement and crystal data of $Th_{0.6}Zr_{0.4}SiO_4$ at ambient conditions (Fig. 2). Space group I4₁/amd, $a = b = 7.0389(3)$ Å, $c = 6.2214(3)$ Å, V = 308.25 (3) Å³, $R_B = 0.3\%$, $R_f = 0.2\%$.

<table><thead><tr><th>Atom</th><th>Site</th><th>x/a</th><th>y/b</th><th>z/c</th><th>B, Å<sup>2</sup></th></tr></thead><tbody><tr><td>Th<sub>0.6</sub>Zr<sub>0.4</sub></td><td>4a</td><td>0</td><td>¾</td><td>1/8</td><td>1.50(2)</td></tr><tr><td>Si</td><td>4b</td><td>0</td><td>¼</td><td>3/8</td><td>3.9(1)</td></tr><tr><td>O</td><td>16h</td><td>0</td><td>0.0639(4)</td><td>0.1961(3)</td><td>0.72(7)</td></tr></tbody></table>

were collected on the STA 449 F5 Jupiter, Netzsch machine in a temperature range of 300 – 1673 K. These measurements allowed to reveal the amount of water incorporated into the hydrothermally synthesized samples and to track the corresponding water discharge upon heating. This information complemented the T-dependent diffraction studies on the ZrSiO₄ sample.

### 3. Results and discussion

Phase analysis based on X-ray powder diffraction data from the $Th_{1-x}Zr_xSiO_4$ series of samples hydrothermally synthesized at pH 1 and 8 were used to construct corresponding pseudo-binary phase diagrams. It was found that synthesis performed under weakly basic conditions (pH = 8) resulted in a successful formation of pure thorite phase, ThSiO₄, and thorite-based $Th_{1-x}Zr_xSiO_4$ solid solution (denoted as TH, Fig. 3, top). The solubility limit of Zr in the thorite phase situates at the equimolar part of the $ThSiO_4-ZrSiO_4$ phase diagram and corresponds to the $Th_{0.5}Zr_{0.5}SiO_4$ composition. In the ThSiO₄ - $Th_{0.5}Zr_{0.5}SiO_4$ region the unit cell volume of the $Th_{1-x}Zr_xSiO_4$ phases decreases linearly upon introduction of the smaller $Zr^{4+}$ ions, as expected (Fig 3S, Supplementary Material). Upon further increase of the Zr content amorphous $ThO_2-ZrO_2$ oxides are formed (denoted as TZO, Fig. 3, top). Formation of pure zircon phase, ZrSiO₄, was not observed during synthesis at pH = 8.

Compared to synthesis at pH = 8, hydrothermal synthesis under strongly acidic conditions at pH = 1 resulted in a more complex phase diagram. Firstly, while the pure ThSiO₄ phase was also formed (Fig 3, bottom), >10% of Zr atoms could be introduced onto the Th site of the thorite phase. Therefore, the $Th_{0.9}Zr_{0.1}SiO_4$ composition corresponds to the Zr-rich limit of the thorite-based solid solution at pH = 1. Upon further introduction of Zr, a mixture of amorphous $ThO_2-ZrO_2$ oxides, thorite- and zircon-based solid solutions (denoted as ZR, Fig. 3, bottom) was formed. The amorphous $ThO_2-ZrO_2$ oxides can be recognized by several very broad humps with maxima at scattering angles of the crystalline phases. The complementary SiO₂ is most likely also amorphous, as no diffraction peaks of SiO₂ can be recognized. However, this assumption cannot be verified as the scattering signal of amorphous SiO₂ overlaps with background scattering of the glass capillary. At the equimolar section of the $ThSiO_4-ZrSiO_4$ phase diagram a mixture of thorite- and zircon-based solid solutions precipitates, but no amorphous mixture of oxides can be observed. While the thorite and zircon phases are isostructural and crystallize in the tetragonal I4₁/amd structural arrangement (Fig. 1), they can be distinguished due to a difference in unit cell volumes and resulting separation of diffraction peaks. Upon further increase in Zr concentration a single-phase $Th_{1-x}Zr_xSiO_4$ solid solution rich in Zr (ZR) persists up to the $Th_{0.26}Zr_{0.74}SiO_4$ composition. Further increase in Zr results in formation of an amorphous mixture of $ThO_2-ZrO_2$ (TZO) oxides. Finally, the zircon phase precipitates at the Zr-rich side of the phase diagram.

It is evident that differences in the synthesis process significantly

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Experimental phase diagrams of the $ThSiO_4-ZrSiO_4$ system synthesized at pH = 8 (top) and pH = 1 (bottom). TH – solid solution based on the thorite phase; ZR – solid solution based on the zircon phase; TZO – mixture of amorphous $ThO_2-ZrO_2$ oxides.</figcaption></figure>
influence the observed solubility limits for the ThSiO₄-ZrSiO₄ system. (Fig. 3). Indeed, independent studies of this system have resulted in yet different results to those reported here, even claiming complete immiscibility between the thorite and zircon phases (see the introduction section). Formation and mixing enthalpies were shown to determine phase stabilities and miscibility properties of thorite-based and related systems, and of the corresponding oxides [28–32]. From the current study it can be concluded that a change in the pH has a significant influence on the corresponding formation and mixing enthalpies of Th₁-xZrₓSiO₄. The crystallite size of Th₁-xZrₓSiO₄ was extracted for all crystalline phases synthesized at pH = 1 and pH = 8 from the experimental XRD data (Tab. S1). The crystallite size of the samples prepared at pH 1 ranges from 17 to 83 nm and those obtained at pH = 8 from 7 to 14 nm. The samples prepared at pH= 1 show slightly larger crystallites. Within the series there is no clear dependence of the crystallite size on the Th/Zr ratio. Nevertheless, it is possible that the experimental phase diagrams have been influenced by the crystallite size, which can affect the surface energy of the precipitates. Such a correlation is documented for other oxide systems [33,34]. Studies of the corresponding thermodynamic properties are, however, beyond the scope of this paper and remain to be measured and modelled.

Four Th₁-xZrₓSiO₄ samples were subjected to in situ synchrotron radiation diffraction studies under HP: Th₀.₉Zr₀.₁SiO₄ (pH = 8), Th₀.₆Zr₀.₄SiO₄ (pH = 8), Th₀.₂₆Zr₀.₇₄SiO₄ (pH = 1) and ZrSiO₄ (pH = 1). The Th-rich Th₀.₉Zr₀.₁SiO₄ sample was compressed up to a pressure of 14.4 GPa. As expected, Bragg reflections shift to higher 2θ angles upon compression due to a reduction in unit cell volume (Fig. 4, left, blue and red lines). However, diffraction peaks exhibit significant broadening with increasing pressure. For instance, the full width at half maximum (FWHM) of the (200) peak almost doubles (+91 %) at the maximum reached pressure of 14.4 GPa (Fig. 4, right, red curve). This may indicate the presence of induced strain and/or a reduction in size of the crystalline domains. Surprisingly, on decompression, the sample fully recovers and the diffraction profile becomes identical to the initial one before compression (Fig. 4, left, green line; right, red and black curves for the FWHM of (200) and (101) peaks, respectively). This complete reversibility excludes a reduction in the crystallinity during compression and indicates the presence of induced internal strain which disappears when the pressure is released. At HP, peak broadening due to increased internal strain can be caused by non-hydrostatic compression conditions. Here this factor can be excluded as the Ne gas employed in this study as a PTM preserves excellent hydrostaticity at these pressures. Specifically, while the first signs of non-hydrostaticity of Ne gas appear at 15 GPa [35] it is still highly hydrostatic even up to 50 GPa where the standard deviation in pressure is <1 %. Therefore, the observed significant peak broadening for the Th₀.₉Zr₀.₁SiO₄ sample is due to internal

P-induced strain.

Due to the high P-induced strain and a resulting peak broadening, full structural analysis of HP data of Th₀.₉Zr₀.₁SiO₄ was limited to a maximal pressure of 10 GPa. For the other studied Th₁-xZrₓSiO₄ phases certain datapoints corresponding to the highest probed pressures were omitted during analysis for the same reason. Compressibilities of this and other samples were evaluated using a 3rd order Birch-Murnaghan equation of state (EOS) (Eq. (1)), where V₀ is a zero-pressure volume, K₀ is a bulk modulus and K'₀ is its first pressure derivative.

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

The Th₀.₉Zr₀.₁SiO₄ phase was found to be rather soft (Fig. 5, left) with a corresponding bulk modulus equal to 120(6) GPa. Beside the unit cell parameters, there are only two free structural parameters in the tetragonal thorite/zircon-based structures – y and z coordinates of oxygen atoms. Upon application of external pressure, the value of the y(O) coordinate steadily increases (Fig. 5, right, black curve). Contrary, the z (O) coordinate remains constant within the error range up to the highest probed pressure of 10 GPa (Fig. 5, right, red curve). Corresponding bonding evolutions will be discussed in detail further in the text.

Finally, the site occupancy of Th atoms as a function of external pressure was monitored in the Th₁-xZrₓSiO₄ series. For Th₀.₉Zr₀.₁SiO₄ the occupancy of Th remains constant within the error range upon increase in pressure (Fig. 6). This indicates an excellent phase stability of Th₀.₉Zr₀.₁SiO₄ when subjected to extreme pressure conditions. This is an important property for a material that may be a candidate for long-term underground storage of nuclear material.

The P-dependent behavior of Th₀.₆Zr₀.₄SiO₄ is similar to Th₀.₉Zr₀.₁SiO₄ and, therefore, all the corresponding illustrations are presented in the Supplementary Material. The Th₀.₆Zr₀.₄SiO₄ phase is rather soft with a bulk modulus equal to 101(2) GPa (Fig. 4S, left). The y (O) of Th₀.₆Zr₀.₄SiO₄ steadily increases (Fig. 4S, right, black curve) whereas the z(O) coordinate (Fig. 4S, right, red curve) does not feature any systematic change. The Th content shows up to the applied pressure of 13 GPa a slight decreasing tendency (Fig. 5S). Nevertheless, while the experimental diffraction data are affected by P-induced peak broadening, the magnitude of the corresponding effect is in Th₀.₆Zr₀.₄SiO₄ smaller than in Th₀.₉Zr₀.₁SiO₄ (Fig. 4, right, blue and orange curves for the FWHM of (200) and (101) peaks, respectively). For instance, the (200) peak shows an increase in FWHM by only 22 % for Th₀.₆Zr₀.₄SiO₄ compared to 91 % for Th₀.₉Zr₀.₁SiO₄.

In contrast to Th₀.₉Zr₀.₁SiO₄ and Th₀.₆Zr₀.₄SiO₄, experimental diffraction data of the Zr-rich Th₀.₂₆Zr₀.₇₄SiO₄ sample contained an additional broad peak around 2θ = 8° (Fig. 7, left, marked with a red

<figure><img src="image_7.png" /><figcaption>Fig. 4. Left: Diffraction profile of Th₀.₉Zr₀.₁SiO₄ upon compression and decompression (blue line – starting pressure; green line – after decompression); right: P-dependent evolution of FWHM of (101) and (200) peaks of the Th₁-xZrₓSiO₄ series of samples.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5. Left: evolution of the unitcell volume, *V*, of Th<sub>0.9</sub>Zr<sub>0.1</sub>SiO<sub>4</sub> upon compression (black squares) and a corresponding fit with the EOS (red solid line); right: *P*-dependent evolution of *y* (black curve) and *z* (red curve) coordinates of oxygen atoms of Th<sub>0.9</sub>Zr<sub>0.1</sub>SiO<sub>4</sub>.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. *P*-dependent refined occupancy of Th atoms of the Th<sub>0.9</sub>Zr<sub>0.1</sub>SiO<sub>4</sub> phase.</figcaption></figure>

arrow). Remarkably, the intensity of this amorphous signal increased with the application of HP (Fig. 7, left, red curve) and then returned close to its original intensity upon pressure release (Fig. 7, green line).

The Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> was also found to be harder than its Th-rich analogues and features a bulk modulus of 161(4) GPa (Fig. 7, right).

While the behavior of the *z*(O) coordinate is virtually independent of the pressure (Fig. 8, left, red curve), the *y*(O) coordinate remains constant until 8 GPa and then features a rapid decrease (Fig. 8, left, black curve). Thus, application of HP triggers in part opposite structural response of the Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> phase compared to its Th-rich Th<sub>0.9</sub>Zr<sub>0.1</sub>SiO<sub>4</sub> and Th<sub>0.6</sub>Zr<sub>0.4</sub>SiO<sub>4</sub> analogues (Fig. 5, right, Fig. 4S, right). The occupancy of Th atoms also exhibits an anomaly around 8 GPa. While the refined Th concentration at lower pressures stays very close to its initial value of 0.26 (Fig. 8, right, black curve), it starts to decrease above 8 GPa and reaches a value of 0.12 around 16 GPa. Thus, about half of the Th atoms initially contained in the Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> phase were found to be discharged from the host crystalline matrix at HP, as revealed from the structural refinement. Remarkably, the behavior of the amorphous peak of Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> (Fig. 7, left, marked with a red arrow) also exhibits an anomaly at 8 GPa. Specifically, its intensity starts to increase above 8 GPa (Fig. 8, right, blue curve) and this behavior is correlated with a decrease in the Th concentration. We would like to emphasize that the *P*-dependent behavior of this broad peak (Fig. 8, right, blue curve) was obtained via individual intensity integration for every pressure point and this procedure was performed completely independently from the full profile structural refinement. The presence of this broad peak in addition to well-defined Bragg

<figure><img src="image_10.png" /><figcaption>Fig. 7. Left: Diffraction profile of Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> upon compression and decompression (blue line – starting pressure; green line – after decompression); right: evolution of the unitcell volume, *V*, of Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> upon compression (black squares) and a corresponding fit with the EOS (red solid line).</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Left: *P*-dependent evolution of *y* (black curve) and *z* (red curve) coordinates of oxygen atoms of Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub>; right: *P*-dependent refined occupancy of Th atoms of the Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> phase (black curve, red datapoint corresponds to the decompressed state, *P* = 0 GPa) and intensity of the amorphous peak observed on the experimental diffraction data around 8° 2θ (blue curve).</figcaption></figure>

reflections indicate that the main crystalline phase, Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub>, coexists with a minor amorphous phase with a mass fraction of a few percent. As a secondary minor phase was not clearly visible on experimental SEM images of Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> (Fig. 6S), one can speculate that this phase can be precipitated between grain boundaries of the major phase. The trend shown in Fig. 8 (right) may indicate that upon application of HP the Th atoms diffuse away from the Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> lattice and precipitate in an amorphous form around the parent crystalline domains. However, this statement is difficult to prove experimentally since upon decompression the Th occupancy returns to its original values (Fig. 8, right, red dot). This observed diffusion process is therefore reversible.

As it was mentioned above, the principal difference in *P*-dependent structural response of the Zr-rich Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> phase compared to the Th-rich Th<sub>0.9</sub>Zr<sub>0.1</sub>SiO<sub>4</sub> and Th<sub>0.6</sub>Zr<sub>0.4</sub>SiO<sub>4</sub> analogues lies in the behavior of the oxygen *y* coordinates, *y*(*O*). Whereas in the Th<sub>0.9</sub>Zr<sub>0.1</sub>SiO<sub>4</sub> (Fig. 5, right) and Th<sub>0.6</sub>Zr<sub>0.4</sub>SiO<sub>4</sub> phases (Fig. 4S, right) *y*(*O*) steadily increases with pressure, this coordinate decreases for Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> at HP (Fig. 8, left). While application of HP induces an overall decrease of unit cell volume, the increase in *y*(*O*) coordinate provides locally more space for central atoms of the Th/ZrO<sub>8</sub> polyhedra. There are two sets of symmetry-independent Th/Zr-O distances in the Th/ZrO<sub>8</sub> polyhedra of tetragonal Th<sub>1-x</sub>Zr<sub>x</sub>SiO<sub>4</sub> (Fig. 9, marked with black solid and dashed

lines). While the Th/Zr-O bonds of the first set decrease upon application of external pressure (Fig. 9, left, black solid lines), the other four distances increase (Fig. 9, left, black dashed lines). For the instance of the Th<sub>0.6</sub>Zr<sub>0.4</sub>SiO<sub>4</sub> phase, the corresponding relative changes in bond lengths amount to -5% and +10%, respectively, for a pressure change of 0.4-12.7 GPa. Contrary, the decrease in the *y*(*O*) coordinate of Th-poor Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> leads to a stabilization of the Th/Zr-O distances. Here the first set of the Th/Zr-O bonds decreases by 9% for a pressure change of 0.4-15.5 GPa (Fig. 9, middle, black solid lines). The other Th/Zr-O distances remain unchanged (Fig. 9, middle, black dashed lines). As a result, the large Th atoms in Th-poor Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> have a smaller volume available at HP compared to the Th-rich phases, and therefore discharge from these sites.

This difference in the observed structural response to HP and the associated phase properties within the Th<sub>1-x</sub>Zr<sub>x</sub>SiO<sub>4</sub> series as a function of Th content (Fig. 9) is quite unexpected. To confirm these findings, we performed independent in situ diffraction studies under HP on the Th<sub>0.26</sub>Zr<sub>0.74</sub>SiO<sub>4</sub> sample at the ID15B High Pressure Beamline, ESRF. For this experiment, we followed experimental procedures that closely resembled those from the study conducted at ROBL. Similar to data collected at ROBL (Fig. 7, left), a broad peak around 2θ = 8° was observed at the HP data obtained at ID15B (Fig. 7S, left) and its intensity also increases upon pressure increase. The compressibility of

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Evolution of the Th/Zr-O bonding in Th<sub>1-x</sub>Zr<sub>x</sub>SiO<sub>4</sub> upon application of HP. Four Th/Zr-O bonds of the Th-rich phase decrease, the other four distances increase under pressure, whereas the Zr-rich phase shows exclusively a compression which becomes more pronounced with increasing pressure. Data from ROBL: left and middle, from ID15B: right side.</figcaption></figure>
Th₀.₂₆Zr₀.₇₄SiO₄ was also comparable to the one previously observed. Specifically, the calculated bulk modulus, K₀, of 152.7(2) GPa (Fig. 7S, right) is equivalent within the experimental uncertainty to the value of 162(4) GPa initially obtained at ROBL (Fig. 7, right). Within the same pressure range also the structural parameters exhibit consistent behavior. Namely, the y(O) coordinate is constant at lower pressures and starts to decrease at *P* > 8 GPa (Fig. 10, left, black curve). However, the *z* (O) coordinate started to steadily decrease above 13 GPa (Fig. 10, left, red curve). No firm conclusions for this *HP* range can be drawn from data collected at ROBL since only two experimental datapoint are available. Nevertheless, a possible tendency towards a decrease in *z*(O) can be observed as well on the original data at *P* > 12 GPa (Fig. 8, left, red curve). This structural behavior at higher pressures (Fig. 10, Fig. 7S, right) results in a complete compression of the Zr/ThO₈ polyhedra (Fig. 9, right). Finally, the occupancy of Th atoms exhibits a similar decrease upon increase in pressure (Fig. 10, right). Moreover, this behavior is reversible and the decompressed Th₀.₂₆Zr₀.₇₄SiO₄ sample features Th content equivalent to its initial state. Full reversibility was also observed for data collected at ROBL (Fig. 8, right). Thus, the two independent diffraction experiments confirm the existence of reversible *P*-induced diffusion of Th atoms within the Th₀.₂₆Zr₀.₇₄SiO₄ phase.

This self-healing behavior of the Th₀.₂₆Zr₀.₇₄SiO₄ phase is of great potential value in the context of long-term immobilization of nuclear waste. When placed in underground repositories, phases containing radioactive isotopes are subjected to conditions of elevated pressure and temperature. For example, pressures and temperatures in a storage facility located below 500 m of bedrock would reach ca. 100 bar (0.01 GPa) and 35 °C, respectively. Deeper installations would yield even more extreme conditions. Nuclear materials sealed in a 15 km borehole [36] would be exposed to pressures of ca. 4000 bars (0.4 GPa) and temperatures on the order of 400 °C. Furthermore, potential accidents in a nuclear waste repository such as fires or explosions could further exacerbate these conditions. Finally, formation of He gas bubbles as a result of α decay in the solid waste form [37] can result in local pressure surges reaching 10 GPa [38]. Therefore, robust and accident-tolerant materials would improve long-term safety of nuclear facilities, for instance underground waste repositories.

The thorium-free zircon phase, ZrSiO₄, was found to be the hardest among the pressurized samples. Its bulk modulus, B₀, was found in this study to be equal to 176(4) GPa (Fig. 8S, left) for the 0–15 GPa pressure range. However, previous independent *HP* studies on the zircon phase revealed even higher values of bulk modulus close to 200 [39,40] or even 225 GPa [41,42]. Origin of this effective softness of the here studied ZrSiO₄ will be discussed below. No zircon-reidite phase

transition was observed up to 15 GPa in this study. Oxygen *y* and *z* coordinates of ZrSiO₄ were found to exhibit similar *P*-dependent behavior found in thorium-rich Th₀.₉Zr₀.₁SiO₄ and Th₀.₆Zr₀.₄SiO₄ phases. Namely, the y(O) coordinate increase and *z*(O) coordinate remains unchanged upon increase in pressure (Fig. 8S, right).

It is well documented that hydrothermally synthesized zircon and silicate phases with a zircon-type structure can retain residual water and/or hydroxyl groups (OH⁻). These species can be adsorbed on the surface, confined internally or even incorporated as structural units (occupying specific crystallographic sites/voids) [21,43–45]. It has been hypothesized that water stabilizes these otherwise metastable phases by lowering their formation enthalpies [31,46]. Presence and amount of residual water in this structure family can be investigated with *T*-dependent diffraction and DSC. The heating originates water discharge from the structure and affects the thermal expansion properties of the host materials. Anomalies in the unit cell parameters (XRD data) and the mass loss (DSC data) can be used to quantify different stages of water release [29,31,47]. Powder diffraction data of ZrSiO₄ collected upon heating and cooling exhibits indeed multiple anomalies indicative of water discharge (Fig. 11, left and Fig. S9, left). At low-temperature in the range between 300 and 500 K the *a* unit cell parameter of zircon initially slightly decreases and then remains unchanged (Fig. 11, left marked with *). This behavior can be associated with the evaporation of weakly bound surface water. The second region with *T* ranging from ca. 500 to 950 K (Fig. 11, left, marked with **) initially features a slight linear increase in the *a* unit cell parameter. This is a typical thermal expansion behavior of material subjected to heating. However, at the *T* of ca. 600 K the structure starts to shrink along the *a* direction, thus indicating an important release of water stored along the [001] channels of ZrSiO₄. Similar strong anomalies in the *a* unit cell parameter were observed for isostructural zircon-type phases upon heating [29,31] and were interpreted to be related to confined molecular water along the [001] channels. Therefore, the activation energy for the release of water trapped along *a* direction was reached at 600 K. Finally, above 950 K (Fig. 11, left, marked with **) the *a* parameter increases linearly which may indicate a region where already a stable and water-free ZrSiO₄ phase exists. However, upon cooling from the maximum reached temperature of 1135 K the *a* unitcell parameter does not follow this original path. This implies that upon heating above 950 K yet another phase change occurred in ZrSiO₄. Since the corresponding structural expansion is linear and requires high activation energy (high temperature), the underlying process is likely related to a gradual discharge of residual structural water/hydroxyl groups incorporated into the zircon phase. Models proposed for the location of hydroxyl

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Left: *P*-dependent pressurisation and depressurisation cycles of *y* (black curve) and *z* (red curve) coordinates of oxygen atoms of Th₀.₂₆Zr₀.₇₄SiO₄; right: *P*-dependent refined occupancy of Th atoms of the Th₀.₂₆Zr₀.₇₄SiO₄ phase. The curves with higher number of data points correspond to pressure increase. Decompression was performed with fewer data points. Data from ID15B.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Left: *T*-dependent evolution of the *a* unitcell parameter of ZrSiO₄. Right: experimental thermogravimetry (black line) and DSC (blue line) signals of ZrSiO₄ upon heating.</figcaption></figure>

groups in a crystalline zircon phase suggest their association with the SiO₄-vacant sites [48,49]. However, these sub-structures do not produce pronounced long-range order correlations and are difficult to observe directly using diffraction techniques. Indeed, the experimental data throughout the studied temperature range (RT – 1135 K) corresponds well to the zircon-type structural arrangement. Anomalies are also present along the *c* direction, although less pronounced (Fig. 9S, left). As a result, *T*-dependent behavior of the unitcell volume is affected predominantly by the *a* unitcell parameter (Fig. 9S, right). In summary, the presence and location of residual water in ZrSiO₄ could be revealed by analyzing the behavior of the unit cell parameters.

Water release upon heating can be also tracked and even quantified with DSC scans combined with thermogravimetry data. Specifically, in the RT – 500 K range the ZrSiO₄ phase rapidly lost almost 2 % of its initial mass (Fig. 11, right, black curve, marked with *) as water adsorbed at the surface. After a reduction in the rate of water release at approximately 500 K, ZrSiO₄ once again actively discharges water from 600 K to 950 K, albeit at a slower rate than at lower temperatures. The corresponding mass loss of ca. 2.5 % can be attributed to water discharged from the [001] channels. Finally, above 950 K the water content decreases gradually and at the maximum measured temperature of 1673 K the total mass loss for the ZrSiO₄ sample is equal to 5.1 %. The corresponding calculated stoichiometry of the sample is equal to ZrSiO₄·0.55H₂O.

For the other compressed Th₁₋ₓZrₓSiO₄ samples in this study, the corresponding stoichiometries revealed from the experimental thermogravimetric data are the following: Th₀.₉Zr₀.₁SiO₄·1.02H₂O, Th₀.₆Zr₀.₄SiO₄·1.75H₂O and Th₀.₂₆Zr₀.₇₄SiO₄·0.79H₂O. It can be seen that the Th₀.₆Zr₀.₄SiO₄ sample contains the largest amount of water and also exhibits the lowest bulk modulus, B₀, equal to 101(2) GPa (Fig. 4S). This property may be related to the incorporated residual water that results in higher compressibility of the corresponding host phases. Similarly, the lower bulk modulus found for the Th-free ZrSiO₄ zircon phase compared to the previously reported values (see above) can be also linked to the presence of interstitial water in the host crystalline matrix. On the other side, no correlation between the water content and the observed *P*-dependent peak broadening in the Th₁₋ₓZrₓSiO₄ series (Fig. 4, right) was observed. This indicates that incorporated water has no significant influence on the *P*-induced strain in Th₁₋ₓZrₓSiO₄.

## 4. Conclusions

The phase diagrams of hydrothermally synthesized Th₁₋ₓZrₓSiO₄ series were found to be highly influenced by the employed experimental synthetic conditions. Synthesis under weakly basic conditions (pH = 8)

resulted in a formation of pure thorite phase, ThSiO₄, and a thorite-based solid solution extending up to the Th₀.₅Zr₀.₅SiO₄ stoichiometry. Contrary, both pure thorite and zircon phases were successfully obtained via synthesis under strongly acidic conditions (pH = 1). Nevertheless, the resulting experimental ThSiO₄-ZrSiO₄ phase diagram features multiple miscibility gaps. The Th₁₋ₓZrₓSiO₄ phases hydrothermally synthesized both at pH = 1 and pH = 8 were found to retain water which can be released upon heating during a multi-step process.

Pressurized Th-rich Th₀.₉Zr₀.₁SiO₄ and Th₀.₆Zr₀.₄SiO₄ phases were found to be structurally stable and no discharge of Th atoms at *HP* was observed. Accommodation of large Th⁴⁺ ions was found to be facilitated by a partial increase in bond distances to the coordinating oxygen atoms upon pressure increase. Contrary, the Th-poor Th₀.₂₆Zr₀.₇₄SiO₄ phase releases incorporated Th atoms upon compression and this discharge is triggered by the reduction in the available volume due to a decrease in the Th/Zr-O distances. Nevertheless, this diffusion process is fully reversible, as concluded from independent in situ *HP* diffraction experiments. This self-healing behavior is particularly valuable in the context of application of these systems as hosts for radiotoxic elements intended to be stored underground under elevated pressures and temperatures for extended periods of time. Further studies on the tolerance of the Th₁₋ₓZrₓSiO₄ phases against internal irradiation and resistance against leaching are needed to validate them for nuclear applications.

## CRediT authorship contribution statement

**Volodymyr Svitlyk:** Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft. **Stephan Weiss:** Data curation, Formal analysis, Investigation, Visualization, Writing – original draft. **Gaston Garbarino:** Data curation, Formal analysis, Writing – original draft. **Salim Shams Aldin Azzam:** Data curation, Formal analysis. **René Hübner:** Data curation, Formal analysis, Writing – original draft. **Andreas Worbs:** Data curation, Formal analysis. **Nina Huittinen:** Funding acquisition, Investigation, Writing – review & editing. **Christoph Hennig:** Conceptualization, Funding acquisition, Project administration, Writing – review & editing.

## Declaration of competing interest

The authors declare no competing interests for this manuscript.

## Acknowledgements

This work was supported by the Bundesministerium für Bildung und Forschung (BMBF) AcE grant (02NUK060). We acknowledge the HZDR
BM20 ROBL beamline at ESRF for providing beamtime in the frame of the AcE project. D. Naudet, J. Exner, N. Baumann, J. Jacobs and P. Colompe are acknowledged for computing and technical support during experiments at the BM20 beamline. Support by the Structural Characterization Facilities at the HZDR Ion Beam Center (IBC) is gratefully acknowledged.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.actamat.2024.120357.

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