Investigation of Radiation Damage in the
Monazite-Type Solid Solution La1-xCexPO4

Theresa Lender, Gabriel Murphy, Elena Bazarkina, Andrey Bukaemskiy, Sara
Gilson, Maximilian Henkes, Christoph Hennig, Alexander Kaspor, Julien
Marquardt, Jonas Nießen, et al.

► To cite this version:

Theresa Lender, Gabriel Murphy, Elena Bazarkina, Andrey Bukaemskiy, Sara Gilson, et al. Investigation of Radiation Damage in the Monazite-Type Solid Solution La1-xCexPO4. Inorganic Chemistry, 2024, 63 (38), pp.17525-17535. 10.1021/acs.inorgchem.4c02041. hal-05098766

HAL Id: hal-05098766

https://hal.science/hal-05098766v1

Submitted on 5 Jun 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Investigation of Radiation Damage in the Monazite-Type Solid Solution La₁⁻ₓCeₓPO₄

Theresa Lender, Gabriel Murphy,* Elena Bazarkina, Andrey Bukaemskiy, Sara Gilson, Maximilian Henkes, Christoph Hennig, Alexander Kaspor, Julien Marquardt, Jonas Nießen, Lars Peters, Jenna Ponoosamy, André Rossberg, Volodymyr Svitlyk, Kristina O. Kvashnina, and Nina Huittinen

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

Cite This: *Inorg. Chem.* 2024, 63, 17525–17535

<figure><img src="image_4.png" /></figure>

Read Online

## ACCESS

<figure><img src="image_5.png" /></figure>

Metrics & More

<figure><img src="image_6.png" /></figure>

Article Recommendations

<figure><img src="image_7.png" /></figure>

Supporting Information

**ABSTRACT:** Crystalline materials such as monazite have been considered for the storage of radionuclides due to their favorable radiation stability. Understanding their structural chemical response to radiation damage as solid solutions is a key component of determining their suitability for radionuclide immobilization. Herein, high-resolution structural studies were performed on ceramics of the monazite solid solution La₁⁻ₓCeₓPO₄ (x = 0.25, 0.5, 0.75, 1) in order to understand the role of structural chemistry on irradiation stability. Ceramic samples were irradiated with 14 MeV Au ions with 10¹⁴ ions/cm² and 10¹⁵ ions/cm² to simulate the recoil of daughter nuclei from the alpha decay of actinide radionuclides. The extent of radiation damage was analyzed in detail using scanning electron microscopy (SEM), Raman spectroscopy, grazing incidence X-ray diffraction (GI-XRD), and high-energy-resolution fluorescence detection extended X-ray absorption fine structure (HERFD-EXAFS) spectroscopy. SEM and Raman spectroscopy revealed extensive structural damage as well as the importance of grain boundary regions, which appear to impede the propagation of defects. Both radiation-induced amorphization and recrystallization were studied by GI-XRD, highlighting the ability of monazite to remain crystalline at high fluences throughout the solid solution. Both, diffraction and HERFD-EXAFS experiments show that while atomic disorder is increased in irradiated samples compared to pristine ceramics, the short-range order was found to be largely preserved, facilitating recrystallization. However, the extent of recrystallization was found to be dependent on the solid solution composition. Particularly, the samples with uneven ratios of solute cations, La₀.75Ce₀.25PO₄ and La₀.25Ce₀.75PO₄ were observed to exhibit the least apparent radiation damage resistance. The findings of this work are discussed in the context of the monazite solid solution chemistry and their appropriateness for radionuclide immobilization.

<figure><img src="image_8.png" /></figure>

## 1. INTRODUCTION

Monazite is an important natural source of rare earth elements¹,² and a major source of thorium.³ Monazite-type phosphates have a wide range of technical applications including catalysis, proton conduction, fluorescent labeling in cell biology, or as thermal barrier coatings.⁴⁻⁷ Due to its high chemical durability and the frequent occurrence of uranium in the monazite structure, it is commonly used for geo-chronological studies.⁸,⁹ These properties also substantiate the ongoing interest in monazite as a potential ceramic host for high-level nuclear waste.¹⁰⁻¹² The particularly high chemical flexibility of monazite has been underlined by numerous studies that have shown perfect miscibility between the lanthanides La–Gd¹³⁻¹⁵ as well as successful incorporation of the actinides Pu, Am, Cm, Bk, Cf, and Es.¹⁶⁻¹⁸ Substitution of further elements like S, As, F and V has been reported among others by Aldred et al.¹⁹ and Ondreijka et al.²⁰ The exceptional durability of monazite is evidenced by the presence of monazite in beach sands and placer deposits that have been produced by weathering of granitic host rocks.²¹

The general formula for monazite-type compounds is LnPO₄ (Ln = La–Gd). It crystallizes in the monoclinic space group P2₁/n (Z = 4). The structure is based on distorted LnO₉ polyhedra which are connected to PO₄ tetrahedra in the [001] direction, the structure is generally described as a phosphate framework (see Figure 1).²² Every LnO₉ polyhedron shares one oxygen atom with five surrounding chains in the ⃗a–⃗b–plane, respectively. This has been called the locking effect.²³

Resistance against radiation damage is one of the most fundamental requirements of host phases for the long-term storage of radioactive waste. Most of this damage is introduced by alpha decay of radionuclides generating alpha particles (He²⁺), recoil of the formed daughter nuclei, and, to some

Received: May 17, 2024

Revised: August 9, 2024

Accepted: August 13, 2024

Published: September 4, 2024

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 1.** Monazite unit cell showing the framework consisting of LnO₉ polyhedra and PO₄ tetrahedra.</figcaption></figure>

extent, by gamma emission.²⁴ Alpha particles transfer 99.8% of their energy via ionization and electronic excitation processes and only introduce a few hundred atomic displacements.²⁵ In contrast, alpha-recoil cascades contain 1000–2000 atomic displacements.²⁴ This damage is known to introduce structural changes in crystalline materials such as swelling,²⁶,²⁷ decreased hardness,²⁵ decreased thermal conductivity,²⁸ and increased diffusion coefficients, both due to percolating clusters forming within the material and due to amorphization.²⁹⁻³¹ It is therefore essential for possible waste matrices to show high resistance toward radiation damage as well as good recrystallization properties.

While it is possible to amorphize monazite under ion irradiation³²,³³ or by incorporation of potent alpha-emitters like²⁴¹Am and ²³⁸Pu,¹⁷,³⁴,³⁵ there appears to be no report about natural metamict samples regardless of high levels of radionuclides commonly encountered in these samples. In literature this fact has been attributed to the exceptional damage recovery based on a combination of low critical temperatures (as low as 60 °C in synthetic LaPO₄³⁶) and the capacity for radiation-induced recovery.³⁷⁻³⁹ This remarkable stability is one of the main reasons why monazite is being considered as one of the most promising candidates as a high-level waste storage matrix. So far, studies have focused on the various monazite end members,¹¹,⁴⁰⁻⁴² although nuclear waste forms are likely to be far more complex in terms of elemental composition. Accordingly, understanding the structural–chemical response of monazite solid solutions is crucial to assessing their applicability in waste form application and actinide immobilization.

Irradiation with heavy ions makes it possible to attain considerable damage levels within short time frames with a high degree of control over the experimental conditions. Gold ions have been used before to simulate recoil atoms from alpha-decay⁴³ even though the energy fraction transferred via inelastic processes is higher compared to recoil nuclei.²⁵ The avoidance of radioactive species facilitates sample handling and subsequent analytical investigation of radiation damage. It must be considered though, that the stability of monazite against ion implantation has been shown to differ from the behavior observed in case of self-irradiation.⁴⁴ The high dose

rates applied in heavy ion irradiation generally overwhelm the thermal recovery taking place on longer time scales.²⁴ On the other hand, irradiation has been shown to facilitate recombination of point defects in damaged areas.³⁹ An important factor to consider is the material type for irradiation. In the context of monazite materials, ceramics are the likely option due to their ease of processing and generation, particularly compared to single crystals. Although single-phase ceramic materials should be homogeneous in terms of chemical composition, the inherent occurrence of variably sized grains and grain boundaries enhances the material heterogeneity. This can be further amplified by irradiation damage effects. This has been shown previously in a variety of compounds.⁴⁵⁻⁴⁷ Particularly, it has been shown that grain boundaries can act as sinks toward the accumulation of radiation defects in the case of zirconia materials.⁴⁸ For monazite materials, similar observations have been made in CePO₄.⁴⁰ In such studies, radiation damage also appears to be linked to grain boundaries. Less clear is how radiation damage affects the material as a solid solution, i.e., a matrix closer to a real waste form with dissolved radioactive constituents within the structure, which contribute to solid solution mixing effects as seen for instance in pyrochlores among others. Indeed there is a relative paucity of information regarding the structural response of monazite solid solutions to radiation damage relevant to radionuclide disposal.

In an attempt to address this dearth of information and shed further light on the irradiation response of monazite solid solutions, ceramic samples with various compositions encompassing the solid solution La₁⁻ₓCeₓPO₄ (x = 0.25, 0.5, 0.75, and 1) were investigated with regard to their susceptibility to radiation damage. The samples were irradiated with 14 MeV Au ions at two different fluences: 10¹⁴ ions/cm² (only CePO₄ and La₀.₅Ce₀.₅PO₄) and 10¹⁵ ions/cm². While the former is slightly below the critical amorphization dose of CePO₄ and should therefore not lead to full amorphization, the latter exceeds it and should result in the complete destruction of long-range order if no recrystallization occurs. Scanning electron microscopy (SEM) was used to assess topological effects of the irradiation. Raman measurements were performed to probe changes to local bonding environments and chemistry. Finally, the long- and short-range order of the irradiated compounds were probed by grazing-incidence X-ray diffraction (GI-XRD) and high energy resolution fluorescence detection extended X-ray fine structure (HERFD-EXAFS), respectively.

## 2. METHODS

**2.1. Samples.** Precursor rhabdophane powders (LnPO₄ × 0.67 H₂O) for both end members and solid solution compounds were prepared according to methods described in the literature.⁴⁹ Targeted stoichiometric additions of cerium nitrate (99.99%, Sigma-Aldrich) and lanthanum nitrate (99.98%, Alfa Aesar) salts were dissolved in deionized water. Precipitation of rhabdophane was induced by the addition of phosphoric acid (85%, Sigma-Aldrich). The slurry solution was heated at 90 °C for approximately 12 h to allow full conversion to rhabdophane. The solid was separated and washed several times using deionized water with subsequent separation via centrifugation. For the final step, the solid was separated and suspended in 0.1 M HNO₃ and left to sit overnight according to the procedure of Babelot et al.⁵⁰ The acid was then decanted off and the solid calcined for 3 h at 120 °C in air, prior to further calcination at 500 °C in air for 2 h to form monazite. The calcined products were ground to a fine powder by using an agate mortar and pestle. Thereafter, the calcined products
<figure><img src="image_12.png" /><figcaption>**Figure 2.** SEM images of the boundary between the irradiated (top) and pristine pellet (bottom). CePO₄ and La₀.₅Ce₀.₅PO₄ irradiated with 10¹⁴ ions/cm² (a, b) and 10¹⁵ ions/cm², respectively (c, d) as well as La₀.₂₅Ce₀.₇₅PO₄ and La₀.₇₅Ce₀.₂₅PO₄ irradiated with 10¹⁵ ions/cm² (e, f). Radiation damage is clearly visible in all cases due to swelling of the crystallites, highlighting the grain boundaries. The strongest effect is visible in CePO₄. Further images are shown in the SI (S4–S9).</figcaption></figure>

were pressed into green pellets using an Oehlglass, Hahn and Kolb MP12 uniaxial cold press by applying a force of 38 kN (p = 450 MPa) to approximately 500 mg of material as reported previously.⁵⁰ The obtained pellets were weighed geometrically to calculate the green densities. Pellets were then sintered in a tube furnace in air at 1400 °C for 5 h. Densities of the crystalline targets were calculated through geometric and hydrostatic weighing as dictated by Archimedean and modified Archimedean methods⁵⁰ (for more information see Supporting Information, Density Measurements). As a final step, targets were polished using abrasive silicon carbide paper and 1 µm diamond paste on an automatic polishing table for approximately 15 min.

**2.2. Irradiation.** The ceramic pellets were mounted on a Si wafer where one half was covered with Al foil to protect it from irradiation (compare with Figure S1). The implantation chamber was evacuated to approximately 3 × 10⁻⁷ mbar and cooled to 77 K with liquid nitrogen to reduce the thermal load at the sample surface during irradiation. The samples were irradiated with 14 MeV Au ions with fluences of 10¹⁴ (CePO₄ and La₀.₅Ce₀.₅PO₄) and 10¹⁵ ions/cm² (all samples). The irradiations were performed at the Ion Beam Center at the Helmholtz-Zentrum Dresden-Rossendorf, Germany (HZDR) using the 3 MV Tandetron Ion Implanter. The penetration depth of the Au ions into the monazite ceramics was calculated to be approximately 2 µm for all samples using the Stopping Range of Ions in Matter (SRIM) Monte Carlo simulation code⁵¹ (for more information see Supporting Information, Penetration Depth).

**2.3. SEM and EDX.** Microstructural and chemical characterization was carried out by electron microscopy (SEM) and energy dispersive spectroscopy (EDX) (FESEM Gemini 500, Zeiss, Oberkochen, Germany; EDX detector X-Max80, Oxford Instruments, Abingdon, Oxfordshire, UK). Due to subsequent surface-sensitive measurements, samples were not coated for the analyses. Operating at 1 kV acceleration yielded high-quality secondary electron images. However, employing the variable pressure mode to capture backscattered images with an acceleration voltage of 15 kV was essential. The increased chamber pressure allowed for charge compensation by gas molecules. Large-field EDX scans were used to determine the precise composition of the pellets.

**2.4. Raman Spectroscopy.** Raman spectroscopic mapping was performed using an automated inverted microscope, specifically a Witec alpha300 Ri Inverted Confocal Raman Microscope. The microscope is composed of an inverted base microscope, the Nikon Ti-2 U, equipped with a 70 mW Nd:YAG laser (λ = 532 nm), a thermoelectrically cooled charge-coupled device (CCD) and a 50x

objective. For the measurements, the laser power was set to 30 mW and a grating of 600 grooves/mm was chosen.

Raman maps of 100 µm × 100 µm size were recorded with a setp size of 1 µm in the x-y direction and with an integration time of 0.5 s. The maps were analyzed using the Raman Tool Set software from Candeloro et al.⁵² with regard to the intensity and fwhm of the ν1 PO₄ stretching band.

**2.5. Grazing Incidence X-ray Diffraction.** Diffraction experiments in grazing incidence (GI) mode were performed on the BM20 Rossendorf Beamline (ROBL, ESRF, Grenoble, France). For this, a special GI module developed at the Rossendorf beamline was used, which allows alignment of the sample surface with respect to the incoming beam. Details on the GI module and the corresponding alignment procedure can be found in Svitlyk et al.⁵³ Data collection was performed at an incidence angle α = 1° chosen to achieve the desired X-ray penetration depth. The latter depends both on the chemical composition of the studied material and on the energy of the incident synchrotron radiation. For the monazite samples studied here, penetration depths as a function of α was calculated using the GIXA⁵⁴,⁵⁵ package which is based on X-ray scattering tables published in Henke et al.⁵⁶ For the used energy of 12 keV (λ = 1.0332 Å), the corresponding X-ray penetration depth is equal to 0.4 µm (see Figure S3). This ensured that only irradiated material was probed. During data collection, samples were oscillated by 20° in order to improve diffraction averaging. Full 360° rotation was not possible since the studied pellets were half-irradiated and full rotation would result in the simultaneous measurement of pristine and irradiated parts.

The GI module was mounted on to the rotation axis of the heavy-duty XRD2 multipurpose diffractometer of ROBL.⁵⁷ Calibration of the GI diffraction module with respect to the incoming beam and detector position was performed with a standard NIST 660c LaB₆ powder, corresponding experimental procedure is described in Svitlyk et al.⁵³ The size of the incoming synchrotron beam was set to 0.3 mm (horizontal) and 0.03 mm (vertical), and 2D data were recorded on a Pilatus 2 M detector. Data were subsequently reduced to 1D powder patterns with the Dioptas software⁵⁸ and Fourier transformed using PDFgetX3.⁵⁹ The 2θ range of all measurements was 1°–42.5° (λ = 1.0332 Å).

**2.6. HERFD-XANES and HERFD-EXAFS.** Ce L₃-edge HERFD-XANES and HERFD-EXAFS measurements of the pristine and irradiated CePO₄ and La₀.₅Ce₀.₅PO₄ pellets in grazing incidence were performed at the ROBL beamline (BM20) of the ESRF in Grenoble, France.⁵⁷ The storage ring was operated at 6 GeV with a ≈ 200 mA current in 7/8 + 1 filling mode. The incident photon energies were selected using a liquid nitrogen-cooled double Si(111) crystal
<figure><img src="image_13.png" /><figcaption>**Figure 3.** 2-dimensional mappings of the intensity of the $v_1$ PO$_4$ stretching mode measured by confocal Raman spectroscopy, shown for CePO$_4$ and La$_{0.5}$Ce$_{0.5}$PO$_4$ irradiated with $10^{15}$ ions/cm$^2$. Lower intensities in the irradiated areas indicate the distortion of PO$_4$ tetrahedra due to radiation damage. Intensity differences in irradiated samples appear to be linked to the individual grains. Mappings of all compositions can be found in the SI (Figure S10).</figcaption></figure>

monochromator, and higher harmonics were suppressed by two Si mirrors operating in total reflection mode. The vertically focused beam size was $50 \times 2000 \ \mu\text{m}^2$. The incident energy was calibrated by using a standard crystalline powder of CeO$_2$ pressed into a pellet; the Ce L$_3$ edge energy position was set to 5723.2 eV as the maximum of the first derivative of the main-edge HERFD-XANES spectrum. Both the pristine (masked side) and the irradiated parts of the pellet were measured in fluorescence mode with a 9° incidence angle for HERFD-EXAFS measurements and 45° for HERFD-XANES. A Johann-type X-ray emission spectrometer in a vertical Rowland geometry available at BM20$^{60}$ was equipped with five spherically bent crystal Ge(331) analyzers with a 1 m bending radius, and a silicon drift X-ray detector (©Ketek). For HERFD measurements at the Ce L$_3$ edge (5723 eV), the spectrometer was aligned at the maximum of the Ce Lα1 emission line (4839.2 eV) using the (331) reflection and the 80.8° Bragg angle. Six to eight spectra were collected for each sample and then averaged. Data reduction and shell fits were conducted with EXAFSPAK.$^{61}$ CePO$_4$ and CeO$_2$ were used as the Ce(III) and Ce(IV) standards for HERFD-XANES interpretation, respectively. In order to calculate the electron wave vector (k), the Ce L$_3$-edge ionization potential was arbitrarily defined as $E_0$ = 5723 eV. For shell fitting theoretical scattering phases and amplitudes were calculated using the ab initio code FEFF8.20$^{62}$ based on a La-substituted CePO$_4$ monazite structure. During fitting, scaling factor S0$_2$ was fixed to 0.9.

An impediment for the analysis of the mixed composition is the small energy difference between the La L$_2$ (5891 eV) and Ce L$_3$-edges (5723 eV), reducing the interpretable EXAFS region to approximately $k$ = 6 Å$^{-1}$. In the case of the short k-range spectra the fitted EXAFS parameter like radial distances (R), coordination numbers (CN) and Debye-Waller factors (DW) correlate strongly with the shift in energy threshold ($\Delta E_0$) which is unknown a-priory. Thus, in order to avoid unreliable structural parameter, shell fits were performed on the

CePO$_4$ end member first in the complete EXAFS range (up to 10 Å$^{-1}$) to determine the shift in energy threshold $\Delta E_0$, which was then fixed for all fits using a common, shorter k-range of 2–6 Å$^{-1}$.

Based on the crystal structure of monazite, only the Ln-O shell and a Ln-P shell with 4 P atoms at an average distance of 3.6 Å were considered in the shell fit. The coordination number of these shells had to be fixed during fitting, as it is strongly correlated to the Debye-Waller factors.

### 3. RESULTS

#### 3.1. SEM and EDX.
SEM images were recorded at the interface between the irradiated and masked areas of the pellets to visually assess irradiation-induced changes in the sample topography. As shown in Figure 2, the interface between the pristine and irradiated part is clearly visible in all samples, indicated by swelling of the damaged region. This effect is more pronounced in samples irradiated with the higher fluence as well as strongly accentuated and rounded grain boundaries that might result from local melting or from swelling. The strongest topographical effect is observed in CePO$_4$. In this case, individual grains seem to have swollen to the point of bursting. This indicates that the structural response of the end member toward irradiation is more severe than that of the solid solution members. As SRIM calculations do not show significant differences in structural damage introduced into the samples (see Supporting Information for detailed information), it may be concluded that the observed variations between the samples are based on various degrees of recrystallization. It is apparent that the grain boundaries play an important role in defect migration, as swelling is most
pronounced at the centers of crystallites. EDS measurements confirmed the expected compositions (for more information, see Supporting Information).

**3.2. Raman Spectroscopy.** Raman maps were generated in both the pristine and irradiated areas of each pellet and analyzed to identify spectral differences that point toward radiation damage. Ruschel et al.⁶³ recommended the fwhm of the $v_1$ PO₄ stretching mode as the most suitable spectral parameter for the estimation of radiation damage in monazite since disorder commonly results in band-broadening and Raman stretching modes are particularly sensitive to disorder of neighboring atoms. However, no significant differences could be discerned with respect to this feature between the pristine and irradiated areas. By way of contrast, the intensity of the $v_1$ mode is significantly lower in damaged areas compared to undamaged material as shown in Figure 3. These results are particularly notable, since Nasdala et al.⁶⁴ argued that confocal Raman spectrometers do not have a sufficient depth resolution in order to probe only the damaged surface area. They concluded that thorough analysis is possible only using thin lamellae. Since the peak height is dependent on the number of PO₄ tetrahedra, conclusions can still be drawn about the degree of radiation damage using confocal Raman spectroscopy based on the relative change of intensity of the $v_1$ stretching mode without the necessity of preparing FIB lamellae.

The lack of band broadening can be traced back to the high degree of damage introduced by the high fluences used in this study. Nasdala et al.⁶⁴ showed that at a fluence of 5.1 * 10¹⁴ ions/cm² no significant Raman signal was obtained from the monazite lamella. Thus, at least in the case of the samples irradiated with 10¹⁵ ions/cm² the irradiated layer is damaged to such an extent that it does not contribute to the Raman spectrum.

Upon closer inspection of the Raman maps, it becomes apparent that the range of intensities observed in the irradiated sections is larger than in the pristine areas, indicating regions with different levels of damage and that significant differences occur between individual grains. This suggests that grain boundaries act as barriers to the propagation of lattice defects. The ability of grain boundaries to act as defect sinks has been established before⁶⁵ and becomes apparent again in these measurements.

Comparisons between different samples to assess the severity of the irradiation damage are not possible as the peak intensities depend on various factors, including crystallite orientation. Therefore, the absolute intensities between samples vary dramatically, irrespective of the degree of structural damage.

**3.3. Grazing Incidence X-ray Diffraction.** Grazing incidence X-ray diffraction experiments were carried out to determine the level to which long-range order and crystallinity were preserved or regained in the damaged layer. The low incidence angles allow probing depths in the micrometer range, eliminating the influence of the underlying, undamaged bulk material. Detector images of a pristine CePO₄ pellet as well as pellets irradiated with both fluences measured with an incidence angle $\alpha$ of 1° are shown in Figure 4. All of the pristine samples were fully crystalline. Due to the large crystallite sizes in the ceramics, their diffraction images exhibit single crystal diffraction spots in addition to Debye–Scherrer rings. The CePO₄ pellet irradiated with 10¹⁴ ions/cm² is almost completely amorphized with three broad amorphous

<figure><img src="image_14.png" /><figcaption>**Figure 4.** Diffraction images of a CePO₄ pellet, left: pristine, middle: irradiated with 10¹⁴ ions/cm², right: irradiated with 10¹⁵ ions/cm². At 10¹⁴ ions/cm² the sample is almost completely amorphous. Single crystal reflections are observed at 10¹⁵ ions/cm² indicating a high degree of recrystallization.</figcaption></figure>

rings being visible and very few crystalline reflections, which results in a very low crystallinity index of 12% (see Table 1).

<table><caption>**Table 1.** Crystallinity Index of Four Different Compositions Irradiated with Two Fluences</caption><thead><tr><th rowspan="2">composition</th><th colspan="2">fluence</th></tr><tr><th>10<sup>14</sup> ions/cm<sup>2</sup></th><th>10<sup>15</sup> ions/cm<sup>2</sup></th></tr></thead><tbody><tr><td>CePO<sub>4</sub></td><td>12%</td><td>39%</td></tr><tr><td>La<sub>0.25</sub>Ce<sub>0.75</sub>PO<sub>4</sub></td><td></td><td>21%</td></tr><tr><td>La<sub>0.5</sub>Ce<sub>0.5</sub>PO<sub>4</sub></td><td>6%</td><td>30%</td></tr><tr><td>La<sub>0.75</sub>Ce<sub>0.25</sub>PO<sub>4</sub></td><td></td><td>22%</td></tr></tbody></table>

Debye–Scherrer rings at the same distances are also visible in the pellet irradiated with 10¹⁵ ions/cm², but the intensity is significantly lower, and broadened single crystal reflections can be observed on top of the amorphous rings. These show strong variations in intensity indicating a broader distribution of crystallite sizes. The crystallinity index was determined to be 21%, considerably exceeding the crystallinity of the lower fluence sample.

Similarly, La₀.₅Ce₀.₅PO₄ exhibits a higher degree of damage at the intermediate fluence than at the highest fluence with crystallinity indices of 6 and 30%, respectively. These results underline the extent to which self-annealing of the monazite structure is possible given a sufficiently high energy input. Short local temperature spikes have been reported in ion-irradiated samples that enable annealing of structural defects.⁶⁶ These, of course, depend on the fluence. With a fluence of 10¹⁴ ions/cm² no evidence of recrystallization of the introduced damage is observable. In contrast, with 10¹⁵ ions/cm² enough energy is introduced for damaged areas to recrystallize, resulting in the growth of large crystallites as evidenced by the bright spots in the diffraction images shown in Figure 5. In literature, dose rate effects have already been described with respect to damage generation in monazite.¹¹,⁶⁷ Based on our observations, we propose the same for self-annealing.

Notably, the two compositions La₀.₂₅Ce₀.₇₅PO₄ and La₀.₇₅Ce₀.₂₅PO₄ are more severely damaged with 10¹⁵ ions/cm² than CePO₄ and La₀.₅Ce₀.₅PO₄ with crystallinity indices of 21% and 22%, respectively. One might expect CePO₄ to recrystallize more readily considering the higher degree of order inherent to the end member structure. However, this does not explain the differing behavior between the La₀.₅Ce₀.₅PO₄ on the one side and La₀.₂₅Ce₀.₇₅PO₄ and La₀.₇₅Ce₀.₂₅PO₄ on the other hand. This might be indicative of a possible asymmetry in the mixing enthalpy of the solid solution, as observed in La₁-xNdₓPO₄⁶⁸ and La₁-xPrₓPO₄⁶⁹ as well as pyrochlores,⁷⁰ possibly resulting in a lower radiation
<figure><img src="image_15.png" /><figcaption>**Figure 5.** Diffraction images of CePO₄ and La₀.₅Ce₀.₅PO₄ (left top and bottom) irradiated with 10¹⁴ ions/cm² and CePO₄, La₀.₂₅Ce₀.₇₅PO₄, La₀.₅Ce₀.₅PO₄, and La₀.₇₅Ce₀.₂₅PO₄ (middle top and bottom and right top and bottom) irradiated with 10¹⁵ ions/cm², respectively. The damage is most severe in both samples irradiated at 1 × 10¹⁴ ions/cm². The highest degree of crystallinity is observed in CePO₄ at 10¹⁵ ions/cm².</figcaption></figure>

resistance of these compositions. It cannot be excluded, though, that the differences arise from the fact that the different samples were obtained from independent synthesis runs. This is supported both by our SRIM calculations and by the study by from Picot et al.²⁵ who did not see any significant differences between the susceptibility of LaPO₄ and La₀.₇₃Ce₀.₂₇PO₄ toward radiation damage. However, Burakov et al.³⁵ describe clear differences between (La,Pu)PO₄ and PuPO₄ with respect to their resistance to self-irradiation. Indeed, a more pronounced effect might be observable in solid solutions containing cations with stronger variations of the ionic radius than La (1.216 Å) and Ce (1.196 Å), e.g., La₁⁻ₓGdₓPO₄.

After carefully masking the Bragg reflections, the amorphous rings were integrated and smoothed to allow for a more detailed analysis of the amorphized material. All spectra show three broad signals at 1.5 Å⁻¹, 2 Å⁻¹, and 3 Å⁻¹, as shown in Figure 6 on the left. Additionally, small features can be observed in CePO₄ and La₀.₅Ce₀.₅PO₄ irradiated at both fluences. These indicate that a higher degree of short-range order is retained than that in the other two solid solution samples.

Information about the atomic distances can be gained from the Fourier transforms of these spectra. Due to the low Q-range inherent to GI-XRD data, which is measured at low energies to reduce the penetration depth, no full analysis of the pair distribution function (PDF) was possible. Nevertheless, the same three dominant distances were identified in all irradiated samples as shown in Figure 6 on the right. These distances match closely some atomic distances found in the monazite structure: the average Ce–O distance in the CeO₉ polyhedron (2.5 Å), the average of the Ce–Ce distance between individual LnO₉–PO₄ chains (4.2 Å), and the Ce–Ce distance corresponding to the unit cell parameter in the ⃗a -direction (6.7 Å), respectively (compare Table 2). This shows

<table><caption>**Table 2.** Atomic distances Extracted from Amorphised Material Compared to those in CePO₄ as Published by Bevara et al⁷¹</caption><thead><tr><th>atomic distance</th><th>irradiated</th><th>pristine</th></tr></thead><tbody><tr><td>Ce–O</td><td>2.45 Å</td><td>2.446 Å - 2.767 Å</td></tr><tr><td>Ce–Ce along [1 3/8 1]</td><td>4.21 Å</td><td>4.2874(5) Å</td></tr><tr><td>Ce–Ce along [1 0 0]</td><td>6.69 Å</td><td>6.7657(7) Å</td></tr></tbody></table>

that although the long-range order is lost due to the irradiation damage, the short-range order is largely retained. Especially, the preservation of LnO₉ polyhedra can be observed.

<figure><img src="image_16.png" /><figcaption>**Figure 6.** Left: Scattering of the amorphous fraction of irradiated pellets occurred in the Q-range up to 4 Å⁻¹. Right: The dominant interatomic distances in the amorphous fraction correspond to the indicated atomic distances in the monazite structure on the right.</figcaption></figure>
Additionally, a high degree of stability along the $\bar{a}$-direction can be inferred.

The single crystal reflections visible in Figure 5 for CePO₄ irradiated with 10¹⁵ ions/cm² were integrated and compared to monazite diffraction patterns. The peak positions were found to coincide, verifying that the recrystallized phase is indeed monazite.

### 3.4. HERFD-XANES and HERFD-EXAFS.
HERFD-XANES measurements on the Ce L₃-edge were carried out to investigate a possible change in oxidation state of cerium due to the irradiation.⁷²,⁷³ No such change was observed and cerium was trivalent in all samples.⁷⁴ A broadening of the white line can be attributed to the partial amorphization of the sample. The spectra are shown in Figure S12.

HERFD-EXAFS measurements were performed to study changes in the short-range order arising from irradiation. The compositions CePO₄ and La₀.₅Ce₀.₅PO₄ were analyzed both in the pristine state and after irradiation with 10¹⁴ ions/cm². To increase comparability, both compositions were then analyzed in the short-range up to 6 Å⁻¹ available for the mixed sample. The fits are shown in Figure 7, and the fit parameters are summarized in Table 3. Details regarding the fitting strategy and the fixed and fitted parameters are given in the Methods section.

<figure><img src="image_17.png" /><figcaption>**Figure 7.** Shell fits (red) of the HERFD-EXAFS measurements (black) of pristine and irradiated CePO₄ and La₀.₅Ce₀.₅PO₄.</figcaption></figure>

Strong peak broadening in the irradiated samples becomes immediately apparent, as indicated by the increased Debye-Waller factors. This indicates a higher degree of disorder in the local environment due to irradiation-induced defects. Additionally, the average Ln-O distances decrease during irradiation, which corroborates the results obtained from the PDF analysis, Table 2. The Ln-P distances increase when the shorter k-range is considered. For the full range, however, the distance appears to be very similar, especially when considering the rather large error associated with this Ln-P distance.

<table><caption>**Table 3.** Shell Fit EXAFS Structural Parameters for CePO₄ and La₀.₅Ce₀.₅PO₄ᵃ</caption><thead><tr><th>path</th><th>CN</th><th>R [Å]</th><th>DW [Å<sup>2</sup>]</th><th>ΔE<sub>0</sub> [eV]</th></tr></thead><tbody><tr><td colspan="5">CePO<sub>4</sub> pristine, full range</td></tr><tr><td>O</td><td>9f</td><td>2.472(9)</td><td>0.026(1)</td><td>6.9(3)</td></tr><tr><td>P</td><td>4f</td><td>3.76(3)</td><td>0.023(5)</td><td>6.9/</td></tr><tr><td>Ce1</td><td>3f</td><td>4.023(7)</td><td>0.0016(9)</td><td>6.9/</td></tr><tr><td>Ce2</td><td>3f</td><td>4.216(8)</td><td>0.0016/</td><td>6.9/</td></tr><tr><td colspan="5">CePO<sub>4</sub> irradiated, full range</td></tr><tr><td>O</td><td>9f</td><td>2.43(1)</td><td>0.049(3)</td><td>6.9f</td></tr><tr><td>P</td><td>4f</td><td>3.70(3)</td><td>0.029(6)</td><td>6.9/</td></tr><tr><td>Ce1</td><td>3f</td><td>4.01(2)</td><td>0.014(2)</td><td>6.9/</td></tr><tr><td>Ce2</td><td>3f</td><td>4.56(2)</td><td>0.014/</td><td>6.9/</td></tr><tr><td colspan="5">CePO<sub>4</sub> pristine, short range</td></tr><tr><td>O</td><td>9f</td><td>2.464(5)</td><td>0.027(1)</td><td>6.9f</td></tr><tr><td>P</td><td>4f</td><td>3.68(7)</td><td>0.06(2)</td><td>6.9/</td></tr><tr><td colspan="5">CePO<sub>4</sub> irradiated, short range</td></tr><tr><td>O</td><td>9f</td><td>2.426(9)</td><td>0.051(3)</td><td>6.9f</td></tr><tr><td>P</td><td>4f</td><td>3.72(9)</td><td>0.07(3)</td><td>6.9/</td></tr><tr><td colspan="5">La₀.₅Ce₀.₅PO₄ pristine</td></tr><tr><td>O</td><td>9f</td><td>2.484(7)</td><td>0.029(2)</td><td>6.9f</td></tr><tr><td>P</td><td>4f</td><td>3.53(2)</td><td>0.014(3)</td><td>6.9/</td></tr><tr><td colspan="5">La₀.₅Ce₀.₅PO₄ irradiated</td></tr><tr><td>O</td><td>9f</td><td>2.454(7)</td><td>0.038(2)</td><td>6.9f</td></tr><tr><td>P</td><td>4f</td><td>3.80(7)</td><td>0.06(2)</td><td>6.9/</td></tr></tbody></table>

ᵃCN = coordination number, R = radial distance, DW = Debye-Waller factor, ΔE₀ = shift in energy threshold. Fixed parameters are marked with f, and linked parameters with /.
Standard deviations as estimated by EXAFSPAK are given in parentheses.

Thereby, definite conclusions about this second scattering shell cannot be drawn from the fitted data.

A possible explanation for the decreasing Ln-O bond distance can be found in the SRIM calculations. The vast majority of displacements introduced by ion bombardment are oxygen vacancies, while phosphorus atoms are significantly less likely to be displaced. Thus, the latter bond distances are less affected by the irradiation, or they even become slightly longer, as expected in a distorted structure, while the former may be separated into a minority of displaced atoms with very long Ln-O distances and a majority of oxygen atoms experiencing a trend toward 8-fold coordination. A very asymmetric Ln-O bond length distribution in the irradiated materials would also explain the FT's of the CePO₄ sample (Figure 7, right), where the first scattering shell shows an apparent splitting (full range fit) or a clear asymmetry (short-range fit) following irradiation. A radiation-induced reduction of oxygen coordination has been observed before in perovskites⁷⁵ and zirconolites.⁷⁶ The Ce peak at R+Δr = 3.5 Å, which is clearly visible in pristine CePO₄, has almost disappeared in the irradiated CePO₄ sample. The absence of the Ce peak indicates larger differences among the Ce-Ce distances, which result in destructive interferences of the corresponding backscattering contributions. This effect is well-known for highly disordered structures.⁷⁷

A further pertinent observation from the present investigation is the reduced radiation resistance of the solid solution members La₀.₂₅Ce₀.₇₅PO₄ and La₀.₇₅Ce₀.₂₅PO₄ compared to the end and middle member under the fluence 10¹⁵ ions/cm² (Figure 5). Simultaneously, these two compositions under the same fluence appear to exhibit a relatively minimized swelling (Figure 2). This variability and nonlinearity of structural
behavior in the solid solutions from irradiation damage has been previously noted in titanate perovskite solid solutions. Smith et al.⁷⁸ pointed to the significance of defect-assisted recovery and enhanced amorphization from ion-irradiation depending on composition and position of the solid solution.⁷⁸ The salience of defect processes in radiation damaged solid solutions has also been highlighted in pyrochlore oxides, where Lumpkin et al.⁷⁹ highlighted the significant role of cation/anion energetics in inducing disorder. With the present data and lack of simulation methods applied, it is difficult to draw definitive conclusions in the present case for the apparent reduced radiation resistance and minimized swelling of La<sub>0.25</sub>Ce<sub>0.75</sub>PO<sub>4</sub> and La<sub>0.75</sub>Ce<sub>0.25</sub>PO<sub>4</sub> compared with other studied members. However, it is postulated that the uneven configuration of La and Ce cations in these samples (compared to LaPO<sub>4</sub> and La<sub>0.5</sub>Ce<sub>0.5</sub>PO<sub>4</sub>) and particularly the susceptibilities to displacement of different individual cations from ion irradiation are linked.⁸⁰ Grain boundaries have been previously described as sinks toward the migration and accumulation of radiation defects and potential recrystallization.⁴⁸ Any heterogeneity of this process, particularly variability in the accumulation of unlike cations (La and Ce) due to their different susceptibilities to displacement from ion irradiation, can potentially impact recrystallization and in turn may reduce swelling. This description lacks a theoretical basis to properly describe it. Nevertheless, it is apparent from this study that “uneven” solid solutions in which the solution cations are not present in equal quantities appear to reduce the radiation tolerance of monazite materials. This accordingly may impact the design and application of such or similar materials for actinide immobilization and waste form purposes.

## 4. CONCLUSION

Ceramic pellets of different compositions of the La<sub>1-x</sub>Ce<sub>x</sub>PO<sub>4</sub> solid solution (x = 0.25, 0.5, 0.75, and 1) were irradiated with 14 MeV Au ions using 10<sup>14</sup> ions/cm<sup>2</sup> and 10<sup>15</sup> ions/cm<sup>2</sup> to simulate the effect of alpha-recoil from radionuclides in order to examine the changes to structure and disorder within variable solid solutions. It was found that macroscopic changes like swelling and loss of structural integrity generally increase in severity with increasing fluence. However, the damage appeared strongest in the solid solution members La<sub>0.25</sub>Ce<sub>0.75</sub>PO<sub>4</sub> and La<sub>0.75</sub>Ce<sub>0.25</sub>PO<sub>4</sub> in which it is argued that the uneven composition of the solid solution solutes, La and Ce, and their variability to displacement from ion irradiation<sup>80</sup> contribute strongly to their heightened radiation intolerance. Raman mappings showed significant reductions to intensities of the ν<sub>1</sub> symmetric PO<sub>4</sub> stretching vibration on all examined compositions due to ion irradiation. However, both grazing incidence X-ray diffraction and HERFD-EXAFS showed that the short-range ordering was preserved within the compounds even after almost full amorphization, facilitating recrystallization. As demonstrated by GI-XRD, crystallinity could be partially restored under irradiation with 10<sup>15</sup> ions/cm<sup>2</sup> in all compositions. Recrystallization was found to be most effective in the end member CePO<sub>4</sub>, and less so in solid solution compounds and accordingly has implications for the immobilization of actinides within monazite. Overall, the study demonstrates the importance of structural chemistry of solid solution materials, the role of long- and short-range order in addition to the presence of grain boundaries in determining the bulk response and stability of a material to radiation damage.

## ■ ASSOCIATED CONTENT

### Data Availability Statement

The SEM images (Figure 1), Raman spectra (Figure 2), GI-XRD measurements (Figure 4) and HERFD-EXAFS data (Figure 6) that support the findings of this study have been deposited in the data repository Zenodo under the link 10.5281/zenodo.11127328.

### ■ Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c02041.

Additional experimental details, calculations, and methods, including density data, photographs of the experimental setup, further SEM images and Raman and HERFD-XANES spectra are provided in the Supporting Information file (PDF)

## ■ AUTHOR INFORMATION

### Corresponding Author

Gabriel Murphy - Institute of Fusion Energy and Nuclear Waste Management (IFN-2), Forschungszentrum Jülich GmbH, Jülich 52428, Germany; orcid.org/0000-0003-3239-9725; Email: g.murphy@fz-juelich.de

### Authors

Theresa Lender - Institute of Crystallography, RWTH Aachen University, Aachen 52066, Germany

Elena Bazarkina - Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01328, Germany; The Rossendorf Beamline at ESRF, The European Synchrotron, Grenoble 38043, France

Andrey Bukaemskiy - Institute of Fusion Energy and Nuclear Waste Management (IFN-2), Forschungszentrum Jülich GmbH, Jülich 52428, Germany

Sara Gilson - Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01328, Germany

Maximilian Henkes - Institute of Fusion Energy and Nuclear Waste Management (IFN-2), Forschungszentrum Jülich GmbH, Jülich 52428, Germany

Christoph Hennig - Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01328, Germany; The Rossendorf Beamline at ESRF, The European Synchrotron, Grenoble 38043, France; orcid.org/0000-0001-6393-2778

Alexander Kaspor - Institute of Fusion Energy and Nuclear Waste Management (IFN-2), Forschungszentrum Jülich GmbH, Jülich 52428, Germany

Julien Marquardt - Institut für Geowissenschaften, Goethe-Universität Frankfurt, Frankfurt am Main 60438, Germany

Jonas Nießen - Institute of Mineral Engineering, RWTH Aachen University, Aachen 52074, Germany

Lars Peters - Institute of Crystallography, RWTH Aachen University, Aachen 52066, Germany; orcid.org/0000-0002-4677-850X

Jenna Poonoosamy - Institute of Fusion Energy and Nuclear Waste Management (IFN-2), Forschungszentrum Jülich GmbH, Jülich 52428, Germany; orcid.org/0000-0003-0090-1522

André Rossberg - Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01328, Germany; The Rossendorf Beamline at ESRF, The European Synchrotron, Grenoble 38043, France
**Volodymyr Svilytky** — *Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01328, Germany; The Rossendorf Beamline at ESRF, The European Synchrotron, Grenoble 38043, France; orcid.org/0000-0001-5449-8009*

**Kristina O. Kvashnina** — *Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01328, Germany; The Rossendorf Beamline at ESRF, The European Synchrotron, Grenoble 38043, France; orcid.org/0000-0003-4447-4542*

**Nina Huittinen** — *Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01328, Germany; Institute of Chemistry and Biochemistry, Freie Universität Berlin, Berlin 14195, Germany; orcid.org/0000-0002-9930-2329*

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.4c02041

## Author Contributions

T.L. analyzed and interpreted SEM, GI-XRD and Raman data, performed SRIM calculations and prepared the manuscript. G.M. took part in conceptualization, synthesis method development, review and editing of the manuscript. E.B. and K.O.K. performed HERFD-XANES and HERFD-EXAFS measurements. A.B. contributed to the development of the synthesis method and sample preparation. S.G. took part in conceptualization, sample preparation, review and editing. M.H. performed sample preparation. C.H. and V.S. took part in GI-XRD measurements. A.K. and J.P. took part in Raman measurements. J.M. performed Raman and GI-XRD measurements. J.N. provided SEM measurements and took part in GI-XRD measurements. L.P. was responsible for supervision, project administration, review and editing. A.R. performed the analysis of HERFD-EXAFS data. N.H. was responsible for project administration, analysis of HERFD-EXAFS data, review and editing. All authors read and approved the final manuscript.

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

The idea for this study originates from FZJ-IFN2. We gratefully acknowledge the BMBF for funding through grant number 02NUK060. The BMBF played no role in study design, data collection, analysis and interpretation of data, or the writing of this manuscript. We also thank the Ion Beam Center, HZDR for irradiations. GI-XRD and XAS measurements were performed at the ROBL beamline at ESRF under the proposal numbers A20-1-839 and A20-1-843, respectively. The authors thank the FAME beamline at ESRF for providing the Ge(331) crystals for this study.

## REFERENCES

(1) Pillai, P. Naturally occurring radioactive material (norm) in the extraction and processing of rare earths. *Proceedings Of The International Symposium On NORM*; Citeseer, 2007, 197221.

(2) Clavier, N.; Mesbah, A.; Szenknect, S.; Dacheux, N. Monazite, rhabdophane, xenotime and churchite: Vibrational spectroscopy of gadolinium phosphate polymorphs. *Spectrochim. Acta, Part A* **2018**, *205*, 85.

(3) Mesbah, A.; Clavier, N.; Elkaim, E.; Gausse, C.; Kacem, I. B.; Szenknect, S.; Dacheux, N. Monoclinic form of the rhabdophane compounds: REEPO₄·0.667H₂O. *Cryst. Growth Des.* **2014**, *14*, 5090.

(4) Takita, Y.; Sano, K.-I.; Muraya, T.; Nishiguchi, H.; Kawata, N.; Ito, M.; Akbay, T.; Ishihara, T. Oxidative dehydrogenation of isobutane to iso-buteneII. Rare earth phosphate catalysts. *Appl. Catal., A* **1998**, *170*, 23.

(5) Norby, T.; Christiansen, N. Proton conduction in Ca-andSr-substitutedLaPO₄. *Solid State Ionics* **1995**, *77*, 240.

(6) Patra, C. R.; Bhattacharya, R.; Patra, S.; Basu, S.; Mukherjee, P.; Mukhopadhyay, D. Lanthanide phosphate nanorods as inorganic fluorescent labels in cell biology research. *Clinical Chem.* **2007**, *53*, 2029.

(7) Sudre, O. H.; Marshall, D. B.; Morgan, P. E. Monazite-based thermal barrier coatings US 6,863,999 B1, 2005.

(8) Harrison, T. M.; Catlos, E. J.; Montel, J.-M. U-Th-Pb dating of phosphate minerals. *Rev. Mineral. Geochem.* **2002**, *48*, 524.

(9) Williams, M. L.; Jercinovic, M. J.; Hetherington, C. J. Microprobe monazite geochronology: Understanding geologic processes by integrating composition and chronology. *Annu. Rev. Earth Planet. Sci.* **2007**, *35*, 137.

(10) Dacheux, N.; Clavier, N.; Podor, R. Monazite as a promising long-term radioactive waste matrix: Benefits of high-structural flexibility and chemical durability. *Am. Mineral* **2013**, *98*, 833.

(11) Leys, J. M.; Ji, Y.; Klinkenberg, M.; Kowalski, P. M.; Schlenz, H.; Neumeier, S.; Bosbach, D.; Deissmann, G. Monazite-type SmPO₄ as potential nuclear waste form: Insights into radiation effects from ion-beam irradiation and atomistic simulations. *Materials* **2022**, *15*, 3434.

(12) Qin, D.; Shelyug, A.; Szenknect, S.; Mesbah, A.; Clavier, N.; Dacheux, N.; Navrotsky, A. Chemical and environmental stability of monazite-cheralite solid solutions Ln₁₋₂Ca₅Th₅PO₄ (Ln = Pr,Nd; x = 0-0.15): A thermodynamic study. *Appl. Geochem.* **2023**, *148*, 105504.

(13) Terra, O.; Clavier, N.; Dacheux, N.; Podor, R. Preparation and characterization of lanthanum-gadolinium monazites as ceramics for radioactive waste storage. *New J. Chem.* **2003**, *27*, 957.

(14) Thust, A.; Hirsch, A.; Haussühl, E.; Schrodt, N.; Loison, L.; Schott, P.; Peters, L.; Roth, G.; Winkler, B. Physical properties and microstructures of La $\$_{1}\{-x}\$ $\$_{x}$\$ 1 - x$ Pr $\$_{1}\{-x}\$\$ x$ PO $\$_{4}\$ 4 monazite-ceramics. *Phys. Chem. Miner.* **2017**, *45*, 323.

(15) Schlenz, H.; Dellen, J.; Kegler, P.; Gatzen, C.; Schreinemachers, C.; Shelyug, A.; Klinkenberg, M.; Navrotsky, A.; Bosbach, D. Structural and thermodynamic mixing properties of La1-xNdPO4 monazite-type solid solutions. *J. Solid State Chem.* **2019**, *270*, 470.

(16) Keller, C.; Walter, K. Darstellung, Gitterkonstanten und chemische Eigenschaften einiger ternärer Oxide des Plutoniums, *Ameri- ciums und Curiums vom Typ MeIIIXVO₄*. *J. Inorg. Nucl. Chem.* **1965**, *27*, 1253.

(17) Bregiroux, D.; Belin, R. C.; Valenza, P. J.; Audubert, F.; Bernache-Assollant, D. Plutonium and americium monazite materials: Solid state synthesis and X-ray diffraction study. *J. Nucl. Mater.* **2007**, *366*, 52.

(18) Hobart, D. E.; Begun, G. M.; Haire, R. G.; Hellwege, H. E. Raman spectra of the transplutonium orthophosphates and trimetaphosphates. *J. Raman Spectrosc.* **1983**, *14*, 59.

(19) Aldred, A. T. Crystal chemistry of ABO₄ compounds In *Geochemical Behavior of Disposed Radioactive Waste*; ACS publication, 1983.

(20) Ondrejka, M.; Uher, P.; Pršek, J.; Odzín, D. Arsenian monazite-(Ce) and xenotime-(Y), REE arsenates and carbonates from the Tisovec-Rejkovo rhyolite, Western Carpathians, Slovakia: Composition and substitutions in the (REE,Y)XO₄ system (X = P, As, Si, Nb, S. Lithos *2007*, *95*, 116).

(21) Boatner, L. A. Synthesis, structure, and properties of monazite, pretulite, and xenotime. *Rev. Mineral. Geochem.* **2002**, *48*, 87.

(22) Clavier, N.; Podor, R.; Dacheux, N. Crystal chemistry of the monazite structure. *J. Eur. Ceram. Soc.* **2011**, *31*, 941.

(23) Mullica, D.; Milligan, W.; Grossie, D. A.; Beall, G.; Boatner, L. Ninefold coordination LaPO4: Pentagonal interpenetrating tetrahedral polyhedron. *Inorg. Chim. Acta* **1984**, *95*, 231.

(24) Weber, W. J.; Ewing, R. C.; Catlow, C.; De La Rubia, T. D.; Hobbs, L. W.; Kinoshita, C.; Motta, A.; Nastasi, M.; Salje, E.; Vance,
E.; et al. Radiation effects in crystalline ceramics for the immobilization of high-level nuclear waste and plutonium. *J. Mater. Res.* **1998**, *13*, 1434.

(25) Picot, V.; Deschansels, X.; Peuget, S.; Glorieux, B.; Seydoux-Guillaume, A.-M.; Wirth, R. Ion beam radiation effects in monazite. *J. Nucl. Mater.* **2008**, *381*, 290.

(26) Hobbs, L. W.; Clinard, F. W.; Zinkle, S. J.; Ewing, R. C. Radiation effects in ceramics. *J. Nucl. Mater.* **1994**, *216*, 291.

(27) Deschansels, X.; Seydoux-Guillaume, A.; Magnin, V.; Mesbah, A.; Tribet, M.; Moloney, M.; Serruys, Y.; Peuget, S. Swelling induced by alpha decay in monazite and zirconolite ceramics: A xrd and tem comparative study. *J. Nucl. Mater.* **2014**, *448*, 184.

(28) Schmidt, H. E.; Richter, J.; Matzke, H.; Van Geel, J. The effect of self-irradiation on the thermal conductivity of plutonium and americium oxides. *Thermal Conductivity* **1993**, *22*, 920.

(29) Matzke, H. Radiation enhanced diffusion in UO₂ and (U, Pu)O₂. *Radiation Effects* **1983**, *75*, 317.

(30) Geisler, T.; Trachenko, K.; Ríos, S.; Dove, M. T.; Salje, E. K. Impact of self-irradiation damage on the aqueous durability of zircon (ZrSiO₄): Implications for its suitability as a nuclear waste form. *J. Phys.: Condens. Matter* **2003**, *15*, L597.

(31) Trachenko, K. Understanding resistance to amorphization by radiation damage. *J. Phys.: Condens. Matter* **2004**, *16*, R1491.

(32) Karioris, F. G.; Gowda, K. A.; Cartz, L. Heavy ion bombardment of monoclinic ThSiO₄, ThO₂ and monazite. *Radiat. Eff. Defects Solids* **1981**, *58*, 1.

(33) Lenz, C.; Thorogood, G.; Aughterson, R.; Ionescu, M.; Gregg, D. J.; Davis, J.; Lumpkin, G. R. The quantification of radiation damage in orthophosphates using confocal µ-luminescence spectroscopy of Nd3+. *Front. Chem.* **2019**, *7*, 13.

(34) Popa, K.; Vigier, J.-F.; Martel, L.; Manara, D.; Colle, J.-Y.; Blanco, O. D.; Wiss, T.; Freis, D.; Konings, R. J. Synthesis, characterization, and stability of americium phosphate, AmPO₄. *Inorg. Chem.* **2020**, *59*, 6595.

(35) Burakov, B. E.; Yagovkina, M. A.; Zamoryanskaya, M. V.; Garbuzov, V. M.; Zirlin, V. A.; Kitsay, A. A. Self-irradiation of ceramics and single crystals doped with Pu-238: Summary of 5 years of research of the VG Khlopin Radium Institute. *MRS Online Proceedings Library (OPL)* **2008**, *1107*, 381.

(36) Meldrum, A.; Boatner, L. A.; Ewing, R. C. Displacive radiation effects in the monazite- and zircon-structure orthophosphates. *Phys. Rev. B* **1997**, *56*, 13805.

(37) Meldrum, A.; Wang, L. M.; Ewing, R. C. Ion beam induced amorphization of monazite. *Nuclear Instruments And Methods In Physics Research Section B: beam Interactions With Materials And Atoms*; Elsevier, 1996, *116*, 220. Radiation Effects in Insulators.

(38) Seydoux-Guillaume, A.-M.; Deschansels, X.; Baumier, C.; Neumeier, S.; Weber, W. J.; Peuget, S. Why natural monazite never becomes amorphous: Experimental evidence for alpha self-healing. *Am. Mineral.* **2018**, *103*, 824.

(39) Nasdala, L.; Akhmadaliev, S.; Burakov, B. E.; Chanmuang, C. N.; Skoda, R. The absence of metamictisation in natural monazite. *Sci. Rep.* **2020**, *10*, 14676.

(40) Lu, F.; Shen, Y.; Sun, X.; Dong, Z.; Ewing, R. C.; Lian, J. Size dependence of radiation-induced amorphization and recrystallization of synthetic nanostructured cepo4 monazite. *Acta Mater.* **2013**, *61*, 2984.

(41) Grechanovsky, A.; Eremin, N.; Urusov, V. Radiation resistance of LaPO₄ (monazite structure) and YbPO₄ (zircon structure) from data of computer simulation. *Phys. Solid State* **2013**, *55*, 1929.

(42) Overstreet, C.; Cooper, J.; O'Quinn, E.; Cureton, W.; Palomares, R.; Leys, J.; Deissmann, G.; Neumeier, S.; Chen, C.-H.; Lang, M. Structural stability of REE-PO₄ (REE= Sm, Tb) under swift heavy ion irradiation. *Nucl. Instrum. Methods Phys. Res., Sect. B* **2022**, *S27*, 34.

(43) Rafiuddin, M. R.; Seydoux-Guillaume, A.-M.; Deschansels, X.; Mesbah, A.; Baumier, C.; Szenknect, S.; Dacheux, N. An in-situ electron microscopy study of dual ion-beam irradiated xenotime-type ErPO₄. *J. Nucl. Mater.* **2020**, *S39*, 152265.

(44) Bakker, K.; Hein, H.; Konings, R.; van der Laan, R.; Matzke, H.; van Vlaanderen, P. Thermophysical property measurements and ion-implantation studies on CePO₄. *J. Nucl. Mater.* **1998**, *252*, 228.

(45) Edmondson, P.; Zhang, Y.; Moll, S.; Namavar, F.; Weber, W. J. Irradiation effects on microstructure change in nanocrystalline ceria-phase, lattice stress, grain size and boundaries. *Acta Mater.* **2012**, *60*, 5408.

(46) Kurpaska, L.; Jagielski, J. Mechanical properties of irradiated gd2Zr2O7 pyrochlores as studied by nanoindentation technique-effect of grains and grain boundaries. *Nucl. Instrum. Methods Phys. Res., Sect. B* **2016**, *379*, 107.

(47) Barr, C. M.; El-Atwani, O.; Kaoumi, D.; Hattar, K. Interplay between grain boundaries and radiation damage. *JOM* **2019**, *71*, 1233.

(48) Dey, S.; Mardinly, J.; Wang, Y.; Valdez, J. A.; Holesinger, T. G.; Uberuaga, B. P.; Ditto, J. J.; Drazin, J. W.; Castro, R. H. Irradiation-induced grain growth and defect evolution in nanocrystalline zirconia with doped grain boundaries. *Phys. Chem. Chem. Phys.* **2016**, *18*, 16921.

(49) Huittinen, N.; Arinicheva, Y.; Schmidt, M.; Neumeier, S.; Stumpf, T. UsingEu3+ as an atomic probe to investigate the local environment in LaPO₄–GdPO₄ monazite end-members. *J. Colloid Interface Sci.* **2016**, *483*, 139.

(50) Babelot, C.; Bukaemskiy, A.; Neumeier, S.; Modolo, G.; Bosbach, D. Crystallization processes, compressibility, sinterability and mechanical properties of La-monazite-type ceramics. *J. Eur. Ceram. Soc.* **2017**, *37*, 1681.

(51) Ziegler, J. F.; Ziegler, M.; Biersack, J. SRIM – The stopping and range of ions in matter (2010). *Nucl. Instrum. Methods Phys. Res., Sect. B* **2010**, *268*, 1818. 19th International Conference on Ion Beam Analysis.

(52) Candeloro, P.; Grande, E.; Raimondo, R.; Di Mascolo, D.; Gentile, F.; Coluccio, M. L.; Perozziello, G.; Malara, N.; Francardi, M.; Di Fabrizio, E. Raman database of amino acids solutions: A critical study of extended multiplicative signal correction. *Analyst* **2013**, *138*, 7331.

(53) Svitlyk, V.; dos Santos, L. B. F.; Niessen, J.; Gilson, S.; Marquardt, J.; Findeisen, S.; Richter, S.; Akhmadaliev, S.; Huittinen, N.; Hennig, C. Grazing-incidence synchrotron radiation diffraction studies on irradiated Ce-doped and pristine Y-stabilized ZrO₂ at the rossendorf beamline. *J. Synchrotron Radiat.* **2024**, *31*, 355–362.

(54) Van Den Hoogenhof, W.; De Boer, D. Glancing-incidence X-ray analysis. *Spectrochim. Acta, Part B* **1993**, *48*, 277.

(55) Ingerle, D.; Pepponi, G.; Meirer, F.; Wobrauschek, P.; Streli, C. JGIXA – A software package for the calculation and fitting of grazing incidence X-ray fluorescence and X-ray reflectivity data for the characterization of nanometer-layers and ultra-shallow-implants. *Spectrochim. Acta, Part B* **2016**, *118*, 20.

(56) Henke, B. L.; Gullikson, E. M.; Davis, J. C. X-ray interactions: Photoabsorption, scattering, transmission, and reflection at E = 50–30,000 eV, Z = 1–92. *At. Data Nucl. Data Tables* **1993**, *54*, 181.

(57) Scheinost, A. C.; Claussner, J.; Exner, J.; Feig, M.; Findeisen, S.; Hennig, C.; Kvashnina, K. O.; Naudet, D.; Prieur, D.; Rossberg, A.; et al. ROBL-II at ESRF: A synchrotron toolbox for actinide research. *J. Synchrotron Radiat.* **2021**, *28*, 333.

(58) Prescher, C.; Prakapenka, V. B. DIOPTAS: A program for reduction of two-dimensional X-ray diffraction data and data exploration. *High Pressure Res.* **2015**, *35*, 223.

(59) Juhás, P.; Davis, T.; Farrow, C. L.; Billinge, S. J. PDFgetX3: A rapid and highly automatable program for processing powder diffraction data into total scattering pair distribution functions. *J. Appl. Crystallogr.* **2013**, *46*, 560.

(60) Kvashnina, K. O.; Scheinost, A. C. A johann-type x-ray emission spectrometer at the rossendorf beamline. *J. Synchrotron Radiat.* **2016**, *23*, 836.

(61) George, G. N.; Pickering, I. J. Exafspak: A suite of computer programs for analysis of x-ray absorption spectra, 1995.

(62) Ankudinov, A. L.; Ravel, B.; Rehr, J.; Conradson, S. Real-space multiple-scattering calculation and interpretation of x-ray-absorption near-edge structure. *Phys. Rev. B* **1998**, *58*, 7565.
(63) Ruschel, K.; Nasdala, L.; Kronz, A.; Hanchar, J. M.; Toebbens, D. M.; Škoda, R.; Finger, F.; Möller, A. ARaman spectroscopic study on the structural disorder of monazite—(Ce. Mineral. Petrol. 2012, 105, 41.

(64) Nasdala, L.; Grötzschel, R.; Probst, S.; Bleisteiner, B. Irradiation damage in monazite-(Ce): An example to establish the limits of Raman confocality and depth resolution. Can. Min. 2010, 48, 351.

(65) Was, G. S. Radiation-enhanced diffusion and defect reaction rate theory. In Fundamentals of radiation materials science: Metals and alloys; Springer, 2007.

(66) Miotello, A.; Kelly, R. Revisiting the thermal-spike concept in ion-surface interactions. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 122, 458.

(67) Burakov, B. E.; Yagovkina, M. A.; Garbuzov, V. M.; Kitsay, A. A.; Zirlin, V. A. Self-irradiation of monazite ceramics: Contrasting behavior of PuPO₄ and (La, Pu) PO₄ doped with pu-238. MRS Online Proc. Libr. 2004, 824, 179–184.

(68) Schlenz, H.; Dellen, J.; Kegler, P.; Gatzen, C.; Schreinemachers, C.; Shelyug, A.; Klinkenberg, M.; Navrotsky, A.; Bosbach, D. Structural and thermodynamic mixing properties of La₁ₓNdₓPO₄ monazite-type solid solutions. J. Solid State Chem. 2019, 270, 470–478.

(69) Hirsch, A.; Kegler, P.; Alencar, I.; Ruiz-Fuertes, J.; Shelyug, A.; Peters, L.; Schreinemachers, C.; Neumann, A.; Neumeier, S.; Liermann, H.-P.; et al. Structural, vibrational, and thermochemical properties of the monazite-type solid solution La₁₋ₓPrₓPO₄. J. Solid State Chem. 2017, 245, 82.

(70) Pilania, G.; Puchala, B.; Uberuaga, B. P. Distortion-stabilized ordered structures in A₂BB'O₇ mixed pyrochlores. Npj Comput. Mater. 2019, 5, 7.

(71) Bevara, S.; Mishra, K. K.; Patwe, S. J.; Ravindran, T.; Gupta, M. K.; Mittal, R.; Krishna, P. S. R.; Sinha, A. K.; Achary, S. N.; Tyagi, A. K. Phase transformation, vibrational and electronic properties of K₂Ce(PO₄)₂: A combined experimental and theoretical study. Inorg. Chem. 2017, 56, 3335.

(72) Kvashnina, K. O.; Butorin, S. M.; Glatzel, P. Direct study of the f-electron configuration in lanthanide systems. J. Anal. At. Spectrom. 2011, 26, 1265.

(73) Zasimov, P.; Amidani, L.; Retegan, M.; Walter, O.; Caciuffo, R.; Kvashnina, K. O. HERFD-XANES and RIXS study on the electronic structure of trivalent lanthanides across a series of isostructural compounds. Inorg. Chem. 2022, 61, 1817.

(74) Kvashnina, K. O. Electronic structure interpretation: How much do we understand the Ce L₃ XANES? Chem.– Euro. J. 2024, 30, No. e202400755.

(75) Townsend, L. T.; Corkhill, C. L.; Hewitt, D. R.; Gandy, A. S.; Hyatt, N. C.; Stennett, M. C. Analysis of the structure of heavy ion irradiated lafeo3 using grazing angle x-ray absorption spectroscopy. Inorg. Chem. 2024, 63, 8531–8536.

(76) Reid, D.; Stennett, M.; Ravel, B.; Woicik, J.; Peng, N.; Maddrell, E.; Hyatt, N. The structure of ion beam amorphised zirconolite studied by grazing angle x-ray absorption spectroscopy. Nucl. Instrum. Methods Phys. Res., Sect. B 2010, 268, 1847.

(77) Hennig, C.; Weiss, S.; Banerjee, D.; Brendler, E.; Honkimäki, V.; Cuello, G.; Ikeda-Ohno, A.; Scheinost, A. C.; Zänker, H. Solid-state properties and colloidal stability of thorium(IV)–silica nano-particles. Geochim. Cosmochim. Acta 2013, 103, 197.

(78) Smith, K. L.; Lumpkin, G. R.; Blackford, M. G.; Colella, M.; Zaluzec, N. J. In situ radiation damage studies of LaₓSr₁₋₃x/₂TiO₃ perovskites. J. Appl. Phys. 2008, 103, 083531.

(79) Lumpkin, G. R.; Smith, K. L.; Blackford, M. G.; Whittle, K. R.; Harvey, E. J.; Redfern, S. A.; Zaluzec, N. J. Ion irradiation of ternary pyrochlore oxides. Chem. Mater. 2009, 21, 2746.

(80) Ji, Y.; Kowalski, P. M.; Neumeier, S.; Deissmann, G.; Kulriya, P. K.; Gale, J. D. Atomistic modeling and experimental studies of radiation damage in monazite-type LaPO₄ ceramics. Nucl. Instrum. Methods Phys. Res., Sect. B 2017, 393, 54.