# Micrometric drilling of (meta-)studtite square platelets formed by pseudomorphic conversion of UO₂ under high-frequency ultrasound

Julien Margate,¹ Matthieu Virot,¹* Thomas Dumas,² Christophe Jégou,³ Tony Chave¹, Manon Cot-Auriol,¹ Ange Alves,¹ Sergey I. Nikitenko¹

[1] ICSM, Univ Montpellier, CEA, CNRS, ENSCM, Marcoule, France.

[2] CEA, DES, ISEC, DMRC, Univ Montpellier, Marcoule, France.

[3] CEA, DES, ISEC, DPME, Univ Montpellier, Marcoule, France.

* matthieu.virot@cea.fr

## Abstract

Pseudomorphic transformations are related to chemical conversions of materials while conserving their shape and structural features. Structuring ceramic shapes this way can be used to tailor the physico-chemical properties of materials that can benefit particular applications. In the context of spent nuclear fuel storage interacting with radiolysis products, the sonochemical behavior of powdered UO₂ was investigated in dilute aqueous solutions saturated with Ar/(20%)O₂ (20 °C). Optimized parameter settings enabled the complete conversion of UO₂ micrometric platelets into uranyl peroxide precipitates, referred to as (meta-)studtite [((UO₂(O₂)(H₂O)₂)·xH₂O) with x = 2 or 4. While the most acidic conditions yielded elongated crystal shapes in agreement with a dissolution/reprecipitation mechanism, softer conditions allowed the pseudomorphic transformation of the platelet shape oxide suggesting a complex formation mechanism. For specific conditions, this unprecedented morphology was accompanied with the formation of a hole in the platelet center. Investigations revealed that the formation of the drilled polymorphs is related to a perfect blend of H⁺, in-situ generation of H₂O₂ and high-frequency ultrasound, and is most probably related to the sono-capillary effect. These insights pave the way for new sonochemical approaches dedicated to the preparation of material polymorphs tailoring specific structural properties.

## Environmental implication

Radioactive uranium oxide (UO₂) plays a central role for current and future nuclear energetics. A comprehensive knowledge about its behaviour in the presence of hydrogen peroxide is of paramount interest in order to understand its fate in storage conditions and the highly questioned environmental migration schemes. Assessing the precipitation processes for uranium-based peroxides in relation to the morphology of the precipitates and the physicochemical formation conditions remains an important issue. The physical and chemical effects driven by the acoustic cavitation phenomenon offer a promising alternative for nuclear chemistry studies including the in-situ generation of H₂O₂ as an alternative to radiolysis.
# 1. Introduction

Understanding the mechanisms of uranium peroxides formation is of prime importance as these compounds are involved both in the front-end and back-end of the nuclear fuel cycle. Assessing the peroxide precipitation processes in relation to the morphology of the precipitates and the conditions responsible for their formation remains an important issue. In many established scenarios (failure of irradiated fuel rods in storage pool, deep geological disposal, nuclear accident, etc.), [1–3] uranium peroxide formation may particularly occur as a result of the corrosion and alteration of spent uranium dioxide nuclear fuels made up of about 95 wt% of $UO_2$ and 5 wt% of fission products and transuranic elements. [4] The formation of these secondary phases is highly dependent upon the surrounding conditions and the nature of the involved reactive species. The formation of uranium oxides or hydrated uranyl oxides arising from $UO_2$ oxidative behavior has been extensively studied [5,6] and is discussed in a review by Shoesmith. [7] Due to the radioactive materials contained in the storage matrix, several chemically reactive species can be formed as a result of water radiolysis: $e^{-}_{aq}$, $H^{+}$, $H_2$ and $HO^{-}$, $HO^{\bullet}$ and $H_2O_2$. The latter has been demonstrated to be one of the most important generated reactants, even in anoxic conditions. [1,8,9] In disposal conditions, the generation of oxidants may contribute in the oxidation of U(IV) to U(VI) and slow corrosion of the $UO_2$ matrix. [4,7,10]

In natural conditions, studtite [$(UO_2(O_2)(H_2O)_2) \cdot 2H_2O$] and metastudtite [$(UO_2(O_2)(H_2O)_2)$, are the only peroxide minerals that have been reported. The structure of studtite has been resolved by Burns et al. and the one of meta-studtite was proposed by Weck et al. using density functional theory. [11,12] Studtite, is an uranyl peroxide that can lose water molecules in air-conditions to form metastudtite, it contains $U(VI)O_2^{2+}$ cations bonded with 4 oxygen atoms from peroxide groups and 2 from water molecules that polymerize into chains sharing peroxide groups. Both phases can be obtained depending on the experimental conditions. Some authors reported the formation of studtite [13,14] while others obtained metastudtite. [4] Additionally, a change in the uranyl covalence during dehydration has been reported using spectroscopic and theoretical methods. [15] Nevertheless, some structural variations can be easily identified according to XRD and Raman spectroscopy in order to distinguish both phases. [16]

Since the early 90s', many studies have focused on the oxidative dissolution of $UO_2$ by $H_2O_2$ under different conditions (e.g. presence of $CO_3^{2-}$, $Ca^{2+}$, $SiO_4^{2-}$) and the slow formation of hyperstoichiometric uranium oxides ($UO_{2+x}$) on the surface of the starting oxide was particularly reported. [5,7,14,17–19] Studies focusing the evolution of the concentration of uranium produced from $UO_2$ dissolution with $H_2O_2$ in aqueous solution evidenced the formation of secondary phases attributed to studtite. [13,20,21] Several mechanisms have been proposed to understand the impact of aqueous radiolytic oxidants on the dissolution of $UO_2$. $H_2O_2$ can be catalytically decomposed on oxide surfaces into $O_2$ and $H_2O$ in aqueous solution with the formation of hydroxyl radicals bound to the surface. (Eq. 1-3) [22,23] or it can play a major role in the oxidative dissolution of $UO_2$-based material leading to the matrix dissolution described by (Eq. 4-7). [18,19,24,25]
E. Ekeroth et al. have described the oxidation step as the limiting step in the conversion of $UO_2$ to uranyl peroxide.[24] In a system containing $UO_2$ and $H_2O_2$, Pablo et al. demonstrated that the consumption of $H_2O_2$ is faster than the dissolution of $UO_2$.[13] The authors concluded that this observation results from the formation of oxidized products of uranium that remains on the solid surface. Using atomic force microscopy, they also characterized the formation of uranyl peroxide structure found on $UO_2$ surface. In the literature, some detailed mechanisms about surface oxidation have also been proposed in aqueous solution in order to explain the formation of peroxo-species. [18,26,27]

$$H_2O_2 \rightarrow 2 HO^\bullet \qquad \text{Eq. 1}$$

$$HO^\bullet + H_2O_2 \rightarrow HO_2^\bullet + H_2O \qquad \text{Eq. 2}$$

$$HO_2^\bullet + HO_2^\bullet \rightarrow H_2O_2 + O_2 \qquad \text{Eq. 3}$$

$$\frac{1}{2} H_2O_2 + UO_{2(s)} \rightarrow UO_{2(s)} -- HO^\bullet \qquad \text{Eq. 4}$$

$$H_2O_2 + UO_{2(s)} -- HO^\bullet \rightarrow HO_2^\bullet + UO_2 + H_2O \qquad \text{Eq. 5}$$

$$UO_{2(s)} -- HO^\bullet \rightarrow HO^- + UO_2^+(s) \qquad \text{Eq. 6}$$

$$UO_2^+(s) + UO_2^+(s) \rightarrow UO_2^{2+}(aq) + UO_{2(s)} \qquad \text{Eq. 7}$$

Sonochemistry, which deals with the effect of ultrasonic waves on chemical reactions, has proven to be an innovative approach for the study of homogeneous and heterogeneous solid/liquid systems that considered actinides.[28–30] It also remains an interesting and an original way to simulate the products of water radiolysis under alpha and beta gamma irradiation. The observed effects are attributed to the acoustic cavitation phenomenon, which gathers the successive nucleation, growth and rapid implosion of gas and vapour-filled microbubbles. At the final stage of collapse, plasma-chemical conditions have been demonstrated to be at the origin of the formation of excited species and radicals. Hence, $H_2O_2$ is known to accumulate in aqueous solutions (and $H_2$ in the gas phase) submitted to ultrasound as a result of water molecule splitting (Eq. 8-9).[31–33]

$$H_2O \rightarrow H^\bullet + H^\bullet \qquad \text{Eq. 8}$$

$$HO^\bullet + HO^\bullet \rightarrow H_2O_2 \qquad \text{Eq. 9}$$

$$O_2 + H^\bullet \rightarrow HO_2^\bullet \qquad \text{Eq. 10}$$

$$HO_2^\bullet + HO_2^\bullet \rightarrow H_2O_2 + O_2 \qquad \text{Eq. 11}$$

Under oxygenated argon atmosphere (e.g. Ar/(20%)$O_2$) the quantity of $H_2O_2$ produced increases dramatically due to the scavenging of $H^\bullet$ radicals by the dioxygen molecules (Eq. 10). This reaction prevents the recombination of the $H^\bullet$ and $HO^\bullet$ radicals formed by the reaction Eq. 8 and enable the production of hydroperoxyl radicals, regenerating $O_2$ and accumulating $H_2O_2$ (Eq. 11). At high ultrasonic frequency (100 kHz-1 MHz), $H_2O_2$ formation rate can be dramatically enhanced compared to 20 kHz ultrasound due to the dissociation of $O_2$ molecules inside the acoustic bubbles.[34] In addition to the above reported chemical effects, the implosion of the cavitation bubbles generated by low-frequency ultrasound (20-40 kHz) causes the propagation of spherical shock waves that can significantly affect solid materials. When imploding at the vicinity of an extended solid surface, a micro-jet of liquid
directed towards the surface with a high velocity may form (ca. 150 m.s⁻¹ at 20 kHz). [35] Several effects have been reported in consequence including surface cleaning, depassivation, particle dispersion and fragmentation, etc. [29,36,37] The combined physical and chemical effects driven by the acoustic cavitation phenomenon are therefore promising for nuclear chemistry studies. Sonochemistry has been already applied for the dissolution or activation of refractory actinide materials including for instance UC, UO₃, UO₂, ThO₂ or PuO₂.[38,39] Particularly, 20 kHz sonication of ThO₂ in dilute sulphuric medium saturated with Ar/(20%)O₂ recently led to its significant dissolution and partial conversion into a Th(IV) peroxide compound which structure was later resolved.[40,41]

In the present work, the sonochemical behaviour of UO₂ and its conversion into uranyl peroxide was investigated in pure water and dilute aqueous solutions of H₂SO₄. The pH remains a key parameter involved in the speciation of uranium and the precipitation of peroxides. Continuous monitoring of the solution allowed to follow the dissolution of uranium dioxide and the accumulation/consumption of hydrogen peroxide. Further analyses of the solid residues led to new questions about the behaviour of UO₂ in aqueous solution with respect to the sonochemical conditions that significantly affected the morphology of the as-formed uranyl peroxides. In particular, optimized conditions enabled the polymorphic transformation of uranyl peroxides showing an unprecedented morphology which formation mechanism was deciphered.
## 2. Experimentals

Solutions were prepared with Milli-Q water having a resistivity higher than 18.2 MΩ.cm at 25°C. The various reagents used in this work were all of analytical grade and purchased from Sigma Aldrich. Uranium metal were supplied by CETAMA. The experiments were performed under an Ar/(20%)O₂ gas mixture provided by Air Liquide.

### 2.1. UO₂ synthesis

UO₂ was synthesized using the oxalic route. Briefly, uranium metal was dissolved in 6 M HCl and stabilized at the tetravalent state U(IV) in 1 M HCl solution before being added dropwise to a solution of oxalic acid under strong agitation. After 1 h, the obtained precipitate was centrifuged and washed three times with pure water before being dried overnight under Ar flow. This oxalate precursor was then fired at 485 °C (4 h) or 800 °C (2 h) in a reducing atmosphere Ar/(4%)H₂ with a 1 h step at 100 °C.[42] The obtained oxide powders were characterized by Powder X-ray Diffraction (PXRD) and Scanning Electron Microscopy (SEM) which evidenced the preparation of UO₂ exhibiting a typical square plate morphology (Fig. S1 and S2, ESI).[42]

### 2.2. Sonochemical experiments

Low frequency (20 kHz) ultrasonic treatment was performed in a 50 mL thermostated batch reactor equipped with a cooling system and a main central opening. The latter allows maintaining the titanium-based sonotrode (tip of 1 cm²) with a PTFE seal. The power supply was provided by a 750 W generator (Sonics & materials Vibracell VCX 750) used at a 30% amplitude involving an acoustic power of 0.36 W·mL⁻¹. For 346 and 1046 kHz, ultrasonic treatments were performed in a cylindrical reactor of 250 mL mounted on top of 25 cm² high-frequency transducers (L3 communication ELAC Nautik) connected to a 125 W generator (LVG 60 RF-generator). A mechanical stirring of 300 rpm was applied during high frequency sonolyses to obtain a homogeneous system. For 100 and 550 kHz frequency ultrasound, the cylindrical reactor was mounted on top of stainless-steel transducers purchased from SinapTec (France). Transducers were connected to a generator (SinapTec Lab500) having a maximum electrical power of 120 W for 100 kHz and 100 W for 550 kHz. Both high and low frequency reactors were equipped with side inlets for the temperature control, the circulation of a bubbling gas and sample collections. The temperature inside both reactors was maintained at 20 °C during ultrasonic treatment using a cryostat (Lauda Eco RE 1050) and a thermocouple immersed in the solution.

For all of the sonication experiments, the power (W) delivered to the solution was estimated by the calorimetric method.[43] The calculated acoustic power (W·mL⁻¹) was then determined through normalization by taking into account the sonicated volume of the solution. Typical experiments involved the sonication of 200 mg of UO₂ powder dispersed in 50 mL (20 kHz) or 250 mL (100 kHz – 1 MHz) of pure water or dilute sulfuric acid (100 kHz – 1 MHz). The pH of the solutions before sonication was pH= 5.5 for pure water, pH= 2.2 for 0.5 M H₂SO₄ and pH= 3.2 for 0.05 M H₂SO₄. Control experiments were carried out under similar conditions without ultrasound. The potential presence of carbonates in the sonicated dispersions was rejected before and after sonication using a Shimadzu COT-meter which revealed a concentration below 3 ppm. During ultrasound exposure, the evolution of the reaction media was characterized after filtration on 0.2 µm PTFE filters by ICP/AES and UV-
Visible absorption spectroscopy. Solution aliquots were also centrifuged to characterize the solid phase evolution (after several washing with pure water) with SEM, AFM and PXRD.

## 2.3. Analyses and characterizations

### UV-visible absorption spectroscopy

During the experiments, solution aliquots (1 mL) were regularly sampled from the reactor, filtered with 0.2 µm PTFE filters and diluted with a Ti(IV) solution for analysis with UV-Vis absorption spectroscopy to follow H₂O₂ formation rates (Thermo Evolution 220, Thermo scientific). The addition of the Ti(IV) solution allows the formation of a Ti(IV)-H₂O₂ complex absorbing at 410 nm[44] which allowed to deduce the concentration of H₂O₂ in solution using the Beer-Lambert law. Absorption spectra were measured in a 1 cm quartz cell from 300 to 750 nm. The Ti(IV) solution was prepared by dissolving 0.322 g of TiOSO₄ (2.10⁻² M) in 0.5 M H₂SO₄ under stirring and gentle heating. A calibration curve was previously prepared with a standard H₂O₂ solution to determine the molar extinction coefficient of the complex (ε= 721.4 L·cm⁻¹·mol⁻¹ at λmax = 410 nm). Statistical error is estimated to 10%.

### Inductively coupled plasma-optical emission spectroscopy (ICP-OES)

The concentration of selected elements in solution over time was determined using an ICAP 7000 series model (Thermo Scientific). Samples were prepared by diluting solution aliquots in a 0.3 M HNO₃ solution. The concentrations of the target elements were obtained against an external calibration curve prepared with standards (SCP Science) between 0.5 and 15 mg·L⁻¹. Results obtained at λ = 385.958 nm for uranium were considered. The UO₂ dissolution rate was estimated with statistical error lower than 10%.

### Powder X-ray diffraction (PXRD)

PXRD diagrams were acquired using a Bruker D8 Advance X-Ray diffractometer equipped with a linear Lynx-eye detector (Cu Kα₁,₂ radiation, λ = 1.51184 Å). The data were collected at room temperature with a 2θ value ranging between 5° and 120°, a step of 0.019° and a counting time of 1.78 s per step. The samples were deposited on a low background support with a drop of absolute ethanol and confined in a dome to avoid the potential contamination of the surroundings. The diffraction patterns were identified with the International Centre for Diffraction Data database (ICDD) using DIFFRAC.EVA software (Bruker, Germany). GSAS-II software was used to discriminate the proportion of U₃O₈, UO₂ and peroxides phases in the pattern.

### Scanning electron microscopy (SEM)

SEM was used to study the morphology evolution of the solids during ultrasonic treatment using a FEI Quanta 200 ESEM FEG electron microscope or with a FEI Helios 600 Nanolab dual beam. The samples were deposited onto carbon tapes and analysed without any specific preparation. The 3D reconstruction images were made with Alicaña MeX V5.1 software using tilted images acquired by SEM as described previously.[45]

### Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM)

An ultrafine lamellae of our samples was obtained with a focused ion beam (FIB), FEI Helios 600 Nanolab (CP2M, Marseille, France). After SiO and Pt deposit layers, thin micrometer sections were lifted in situ and transferred onto a half-copper grid using an Omniprobe 200
micromanipulator up to electron transparency (final thickness of about 100 nm). This cutting
process took place at a reduced acceleration voltage of 5 kV to reduce beam damage.

**Transmission electron microscopy (TEM)**

FIB lamella was analyzed by transmission electron microscopy TEM on FEI Tecnai G2 with a LaB₆ source operating at 200 kV equipped with a CCD TVIPS 1k*1k camera.

**Atomic Force Microscopy (AFM)**

UO₂ sample morphologies were studied by AFM using a MULTIMODE 8 AFM device supplied
with Nanoscope 5 software from Bruker (Germany). Typically, droplets of solid residues
dispersed in absolute ethanol were deposited onto Cu disk. The images were acquired in
tapping mode (2 khz) using SNL tips (k= 0.35 N/m)

**CHESS (Chemical Equilibrium of Species and Surfaces) code**

CHESS code developed by the Ecole des Mines de Paris was used to calculate chemical
speciation in aqueous solution. Computation was performed at thermodynamic equilibrium
using CHESS v3.0 and the EQ3/6 database.[46]
# 3. Results

## 3.1. Sonochemical conversion of UO₂ into uranyl peroxides

**Fig. 1.a** displays the dissolution kinetics of uranium in pure water and sulfuric media observed during the 346 kHz sonication of UO₂ fired at 800°C. **Fig. 1.b** illustrates the corresponding sonochemical kinetics related to the accumulation of H₂O₂ in solution. The accumulation rates for uranium was found higher in acidic media than in pure water with similar sonochemical conditions. The highest dissolution rate was observed in concentrated 5 mM H₂SO₄ medium (Fig 1.a). Some authors reported an increase of UO₂ dissolution in aqueous solution when decreasing the pH in agreement with the reaction Eq. 12.[21,47] For a H₂O₂ concentration of ca. 10⁻³ M, Clarens et al. stated that the dissolution rate does not depend on the pH in the acidic range and attributed this difference to the precipitation of an uranium solid phase (uranyl peroxide, Eq. 13).[48] Such a behavior could explain the shape of the accumulation curve observed for uranium in the 5 mM H₂SO₄ aqueous solution.

$$H_2O_2 + UO_2 + 2 H^+ \rightarrow 2 H_2O + UO_2^{2+} \qquad \text{Eq. 12}$$

$$UO_2^{2+} + H_2O_2 + xH_2O \rightarrow UO_2(O_2) \cdot xH_2O(s) + 2 H^+ \qquad \text{Eq. 13}$$

Whatever the medium, sonochemical accumulation rates of H₂O₂ observed in the presence of UO₂ powder were found lower than the reference experiment carried out without solid (Fig. 1.b). Table 1 compares H₂O₂ formation yields for the various systems, which agree with these observations and also suggest a higher consumption of H₂O₂ in acidic conditions. In agreement with the literature, the differences observed in these conditions can originate three processes: oxidation of U(IV) into U(VI), precipitation of secondary phases, catalytic decomposition of H₂O₂ at UO₂ surface.[7] Generally, the pH observed for our experiments carried out in acidic conditions was stable (pH = 2.2 for 5 mM H₂SO₄ and pH = 3.2 for 0.5 mM H₂SO₄), whereas it significantly decreased during sonolysis in pure water (ranging from 5.8 to 3.7). Such a pH decrease can be attributed to the possible surface hydrolysis of UO₂ that may be promoted under ultrasound irradiation.[49,50] In the global conversion process (Eq. 12-13), protons play the role of a catalyst. Note that for similar other conditions carried out in the absence of ultrasound but with addition of H₂O₂, the pH of the solution only decreased from 5.8 to 5.4 during 6 h experiments evidencing the dramatic effect of the acoustic cavitation and in-situ generation of hydrogen peroxide. Furthermore, the colour of the sonicated suspension became lighter during the experiment indicating the significant modification of the solid phase (Fig. S3).
<table><caption>Table 1: H₂O₂ formation yields (G (H₂O₂) (nanomol/J) observed during the sonolysis of H₂O and H₂SO₄ under Ar/(20%)O₂ atmosphere at 20 °C as a function of the applied acoustic frequency.</caption><thead><tr><th rowspan="2">f (kHz)</th><th colspan="2">H₂O</th><th colspan="2">0.5 mM H₂SO₄</th></tr><tr><th>Pure</th><th>With UO₂ (200mg)</th><th>Pure</th><th>With UO₂ (200mg)</th></tr></thead><tbody><tr><td>20</td><td>0.1 ± 0.05</td><td>-</td><td>0.2 ± 0.1</td><td>-</td></tr><tr><td>100</td><td>1.3 ± 0.1</td><td>-</td><td>-</td><td>-</td></tr><tr><td>346</td><td>2.6 ± 0.3</td><td>1.6 ± 0.3</td><td>2.9 ± 0.3</td><td>1.9 ± 0.3</td></tr><tr><td>1047</td><td>1.9 ± 0.2</td><td>1.2 ± 0.1</td><td>2.2 ± 0.2</td><td>0.6 ± 0.1</td></tr></tbody></table>

Formation yields were calculated using the following formulae $\frac{W_o (H_2O_2)}{P_{ac} \times 60}$ where P<sub>ac</sub> is the acoustic pressure.

<figure><img src="image_1.png" /><figcaption>Figure 1: (a.) Concentration of uranium accumulated in solution as a function of time during the sonication of 200 mg UO₂ (fired at 800°C, 3 mM) in H₂O, 5 mM H₂SO₄ and 0.5 mM H₂SO₄ (346 kHz, 0.2 W.mL⁻¹, 20°C, Ar/(20%)O₂). (b.) Evolution of H₂O₂ concentration in solution for similar conditions.</figcaption></figure>

PXRD analyses of the solid residues recovered after sonication are illustrated in Fig. 2.a and confirmed the *in-situ* formation of meta-studtite [(UO₂)O₂(H₂O)₂] in agreement with the literature (ICSD 01-081-9033).[11] Data processing allowed us to estimate the percentage of conversion of UO₂ into uranyl peroxide which varied as a function of the aqueous medium and treatment duration. In the experimental domain studied (6 h sonication), a complete conversion was observed for the most acidic conditions (pH = 2.2) which was explained by a better dissolution of UO₂ (Fig. 1.a) followed by the precipitation of accumulated uranyl ions into (meta-)studtite[27]. In these conditions, the behaviour of UO₂ agrees with the literature and the reactions Eq. 12 and Eq. 13.[51] For softer acidic conditions, the conversion of UO₂ was not complete after 6 h sonication and reached ca. 88 and 77 wt.% of meta-studtite for 0.5 mM H₂SO₄ and H₂O, respectively. The absence of significant accumulation of uranium in solution for these conditions (Fig. 1.a,b) combined to the significant conversion of UO₂ to uranyl peroxide suggest a different behaviour directly related to the studtite solubility, the pH and H₂O₂ concentration. This behaviour is similar to that observed during the alteration of irradiated fuels under gamma irradiation.[52] An acidic pH combined with the formation of hydrogen peroxide by water radiolysis leads to an increase in the solubility of uranium
according to the solubility product $K_{sp}$ of Eq. 10 ($K_{sp} = [UO_2^{2+}][H_2O_2]/[H^+]^2$). The precipitation of uranium peroxide then occurs for high uranium concentrations and for significant oversaturations in solution. The concentrations in solution pass through a maximum above the solubility limit of uranyl peroxide and then decrease rapidly when the rate of condensation and growth of the nuclei increase. Conversely at a higher pH (5.8) the solubility of uranium is much lower, and the precipitation of uranyl peroxide occurs at low concentrations.

<figure><img src="image_2.png" /><figcaption>Figure 2: XRD diffractograms acquired on the solid phases after sonication of UO₂ (fired at 800°C, 3 mM) during 6 h at (a.) 346 kHz (0.2 W.mL⁻¹, 20°C, Ar/(20%)O₂) and (b.) 1046 kHz (0.16 W.mL⁻¹, 20°C, Ar/(20%)O₂).</figcaption></figure>

Table 2 gathers thermodynamic data calculated for 346 kHz experiments. In pure water, the uranyl concentration in solution was found to be low ($10^{-6}$ M) whatever the sonication duration. The associated strongly positive value of the studtite saturation index ($SI>2$) from the beginning of the experiment agree with a quick and local re-precipitation of studtite due to the poor solubility of UO₂ and studtite in these conditions. The very low concentration of calculated uranium aqueous species ($< 10^{-6}$ M) is in agreement with the absence of uranium accumulating in solution for our experimental conditions. On the other hand, for 5 mM H₂SO₄ (pH = 2.2), the uranium rises to $10^{-4}$ M. In comparison to the experiment carried out in pure water, the saturation index value is lower (around $SI= 1$) and particularly low during the first 40 min of sonication). Under these conditions, uranium solubility is therefore higher ($>10^{-4}$ M) as observed in the experiment. Nevertheless, this rise in concentration for aqueous uranium from low to higher acidic conditions may also be explained by the presence of small colloids of hydrated uranium including peroxides that would increase the apparent solubility of aqueous uranium. [53] Fig. 3 gathers solubility diagrams in the three studied media using 346 kHz experiment data with the same H₂O₂ concentration ($2 \ 10^{-3}$ M). Pure water conditions favor studtite formation whereas sulfuric conditions describe a system at the solubility limit with the presence of UO₂SO₄(aq) in solution (Fig. 3.a vs. Fig 3.c). Finally, Fig. 3.b depicts an intermediate system between the two previous ones.
<table><caption>Table 2: Concentration of uranium species released in solution as a function of H₂O₂ concentrations determined in solution during 346 kHz sonolysis in H₂O and H₂SO₄ under Ar/(20%)O₂ atmosphere at 20°C. Thermodynamic calculations for uranium were performed with CHESS, at the thermodynamic equilibrium and saturation indexes (SI) for Studtite (with Kₛ = 1.34 x 10⁻³ at 25°C)[54]</caption><thead><tr><th>Time (min)</th><th>[U<sub>tot</sub>]<sup>exp</sup> (M)</th><th>[H<sub>2</sub>O<sub>2</sub>]<sup>exp</sup> (M)</th><th>pH</th><th>[UO<sub>2</sub>]<sup>2+</sup> (M)</th><th>UO<sub>2</sub>(SO<sub>4</sub>)<sup>2-</sup> (M)</th><th>UO<sub>2</sub>SO<sub>4(aq)</sub> (M)</th><th>SI (Studtite) Log Q/K<sub>s</sub></th></tr></thead><tbody><tr><td colspan="8" style="text-align: left; font-weight: bold;">H<sub>2</sub>O</td></tr><tr><td>0</td><td>2 x 10<sup>-5</sup></td><td>0</td><td>5.8</td><td>1.68 x 10<sup>-7</sup></td><td>-</td><td>-</td><td>none*</td></tr><tr><td>300</td><td>2 x 10<sup>-6</sup></td><td>9 x 10<sup>-3</sup></td><td>3.7</td><td>5.97 x 10<sup>-9</sup></td><td>-</td><td>-</td><td>2.51</td></tr><tr><td colspan="8" style="text-align: left; font-weight: bold;">5 10<sup>-4</sup> M H<sub>2</sub>SO<sub>4</sub></td></tr><tr><td>0</td><td>2 x 10<sup>-5</sup></td><td>0</td><td>3.2</td><td>1.41 x 10<sup>-5</sup></td><td>2.24 x 10<sup>-8</sup></td><td>5.74 x 10<sup>-6</sup></td><td>none</td></tr><tr><td>20</td><td>10<sup>-4</sup></td><td>4 x 10<sup>-4</sup></td><td>3.2</td><td>1.99 x 10<sup>-6</sup></td><td>3.23 x 10<sup>-9</sup></td><td>8.21 x 10<sup>-7</sup></td><td>1.67</td></tr></tbody></table>
<table><tbody><tr><td>40</td><td>5 x 10<sup>-5</sup></td><td>9 x 10<sup>-4</sup></td><td>3.2</td><td>7.08 x 10<sup>-7</sup></td><td>1.15 x 10<sup>-9</sup></td><td>2.92 x 10<sup>-7</sup></td><td>1.72</td></tr><tr><td>170</td><td>8 x 10<sup>-6</sup></td><td>2 x 10<sup>-3</sup></td><td>3.2</td><td>3.02 x 10<sup>-7</sup></td><td>4.93 x 10<sup>-10</sup></td><td>1.25 x 10<sup>-7</sup></td><td>1.27</td></tr><tr><td>270</td><td>5 x 10<sup>-6</sup></td><td>6 x 10<sup>-3</sup></td><td>3.1</td><td>1.60 x 10<sup>-7</sup></td><td>2.52 x 10<sup>-10</sup></td><td>6.47 x 10<sup>-8</sup></td><td>1.34</td></tr><tr><td colspan="9">5 10<sup>-3</sup> M H<sub>2</sub>SO<sub>4</sub></td></tr><tr><td>0</td><td>2 x 10<sup>-5</sup></td><td>0</td><td>2.2</td><td>7.13 x 10<sup>-6</sup></td><td>3.61 x 10<sup>-7</sup></td><td>1.24 x 10<sup>-5</sup></td><td>none</td></tr><tr><td>40</td><td>9 x 10<sup>-4</sup></td><td>6 x 10<sup>-4</sup></td><td>2.2</td><td>2.37 x 10<sup>-4</sup></td><td>1.02 x 10<sup>-5</sup></td><td>3.84 x 10<sup>-4</sup></td><td>0.41</td></tr><tr><td>80</td><td>2 x 10<sup>-3</sup></td><td>1 x 10<sup>-3</sup></td><td>2.2</td><td>4.66 x 10<sup>-4</sup></td><td>1.71 x 10<sup>-5</sup></td><td>6.9 x 10<sup>-4</sup></td><td>1.02</td></tr><tr><td>120</td><td>1 x 10<sup>-3</sup></td><td>2 x 10<sup>-3</sup></td><td>2.2</td><td>6.69 x 10<sup>-5</sup></td><td>3.24 x 10<sup>-6</sup></td><td>1.11 x 10<sup>-4</sup></td><td>0.99</td></tr><tr><td>240</td><td>7 x 10<sup>-4</sup></td><td>4 x 10<sup>-3</sup></td><td>2.2</td><td>2.35 x 10<sup>-5</sup></td><td>1.17 x 10<sup>-6</sup></td><td>4.10 x 10<sup>-5</sup></td><td>1.12</td></tr><tr><td>300</td><td>4 x 10<sup>-4</sup></td><td>6 x 10<sup>-3</sup></td><td>2.2</td><td>1.40 x 10<sup>-5</sup></td><td>7.09 x 10<sup>-7</sup></td><td>2.45 x 10<sup>-5</sup></td><td>1.04</td></tr></tbody></table>

* Presence of schoepite
<figure><img src="image_3.png" /><figcaption>Figure 3: Solubility diagrams calculated for UO₂ as a function of pH at 25°C using CHESS software and data measured during 346 kHz sonolysis of UO₂ (0.2 W.mL⁻¹, 20°C, Ar/(20%)O₂) with H₂O₂ values: 2.10⁻³ M in (a.) H₂O, (b.) 0.5 mM H₂SO₄ and (c.) 5 mM H₂SO₄</figcaption></figure>

$$UO_2^{2+} + 2 SO_4^{2-} \rightarrow UO_2(SO_4)_2^{2-} \quad \text{Eq. 11}$$

To evaluate the influence of the ultrasonic frequency, the sonication of UO₂ powder was performed under similar conditions at 1047 kHz (20 °C, 6 h). Fig. 4.a displays the related accumulation kinetics for uranium in H₂O and 0.5 mM H₂SO₄. In these conditions, the accumulation rates and the maximum amount of uranium observed in solution were more important in acidic media and almost non-existent in pure water. The consumption of H₂O₂ was significant in both cases notwithstanding a lower rate in H₂O (Fig. 4.b) agreeing with the results observed at 346 kHz (Fig. 1.b). In agreement with the previous experiments, these observations were also accompanied by the modification of the color of the suspension and conversion of UO₂ into uranyl peroxide which was confirmed by PXRD (Fig. 2.b), IR (Fig. S4.a) and Raman (Fig. S4.b))[55]. Curiously, we obtained studtite at 0.5 mM H₂SO₄ and meta-studtite in H₂O for 1047 kHz experiment. This difference may be explained by subtle distinction in the experimental drying conditions.[56]

<figure><img src="image_4.png" /><figcaption>Figure 4: (a.) Concentration of uranium accumulated in solution as a function of time during the dissolution of 200 mg UO₂ (3 mM) in H₂O or 0.5 mM H₂SO₄ (1047 kHz, 0.15 W.mL⁻¹, 20°C, Ar/(20%)O₂). (b.) Evolution of H₂O₂ concentration in the similar media.</figcaption></figure>
For both acoustic frequencies, the extremely low presence of U in pure water combined to the consumption of H₂O₂ and conversion into studtite suggest a different behavior between low and more acidic conditions. While concentrated H₂SO₄ conditions (5 mM) clearly involve the dissolution followed by reprecipitation of the neoformed phase, pure water conditions rather suggest a quick conversion occurring on surface and most likely attributed to a low dissolution rate of the oxide. Fig. 4.b shows two kinetic domains for H₂O₂ accumulation including an inhibition period in the 0-120 min range, followed by an almost linear accumulation rate after ca. 120 min (both rates remain lower than the one observed in the absence of UO₂ powder). Such a behaviour could be related to the conversion mechanism according to Clarens and al. studies suggesting that H₂O₂ oxidizes faster U(IV) than it dissolves UO₂.[48] The surface oxidation and conversion to a peroxide neophase being in competition with the dissolution reaction. The low dissolution rate expected for uranium oxide in such low acidic conditions in comparison to the precipitation kinetics of U(VI) peroxides may also contribute to this behaviour. Further experiments were also performed at 550 kHz leading to similar conclusions about U and H₂O₂ accumulation in solution, with a partial or total conversion of UO₂ into uranyl peroxide (Fig. S5).

**3.2. Unusual morphology of uranyl peroxides formed under ultrasound**

SEM investigations carried out on the recovered solids after sonication (346 kHz) evidenced different morphologies depending on the acidity of the system (Fig. 5). The solid sample obtained under the most acidic conditions (5 mM H₂SO₄, pH= 2.2) showed needle-like particles measuring ca. 5 µm long and agreeing with a dissolution/reprecipitation mechanism mentioned above (Fig. 5.c). These needle-like morphologies are typical of the formation of uranyl peroxide observed during the oxidative dissolution of UO₂ in the presence of H₂O₂ both on single crystals[57] and on sintered pellets.[53] Such a morphology agrees with the monoclinic (C2/c) and orthorhombic natural structure of studtite or (meta-)studtite, respectfully, composed of uranyl polyhedral chains forming elongated crystals.[11,12] Interestingly, when the pH of the sonication experiment increased (0.5 mM H₂SO₄ at pH= 3.2 and H₂O at pH= 5.8), the neo-formed phase is found to be composed of squared platelet similar to the one of the initial UO₂ (Fig. 5.a vs. 5.c). In agreement with PXRD measurements, these results demonstrate that the sonication of UO₂ at selected conditions allows a pseudomorphic transformation of UO₂ into (meta-)studtite. Surprisingly, the as-formed particles may be further drilled with a sub-micrometric hole in their exact centers (Fig. 5.c). It is worth noting that control experiments carried out with continuous addition of H₂O₂ reactant in the absence of ultrasound never led to such morphologies (Fig. 5.b). Sonochemical conditions generated at UO₂ interface appear different from the chemistry induced by H₂O₂ addition at silent conditions. A similar conclusion comes out from radiolysis experiments which are also different with sonochemistry pointed in this work.[53]
<figure><img src="image_5.png" /><figcaption>Figure 5: SEM images acquired on (a.) UO₂ reference synthesized by the oxalate route (800 °C, Ar/(4%)H₂), (b.) UO₂ after addition of H₂O₂ (12 mM, stirring) and (c.) the solid residues observed (0.2 W.mL⁻¹, 20°C, Ar/(20%)O₂) after 346 kHz sonication of UO₂ (3 mM, 200 mg, fired at 800°C) in H₂SO₄ (5 mM and 0.5 mM) and H₂O.</figcaption></figure>

Similar experiments were carried out with an uranium oxide sample exhibiting a lower refractory behaviour towards dissolution. The latter was prepared under similar conditions with a lower firing temperature (485 °C). Fig 6 presents SEM images corresponding to the sonolysis residue treated in pure water at 346 kHz and 1047 kHz with similar other conditions. In both cases, a similar behavior was observed for uranium and H₂O₂ with the observation of a partially converted studtite (80 wt.%) including a similar perforated platelet morphology. These holes were not found in the experiment carried out with UO₂ fired at 800 °C and sonicated at 346 kHz (Fig 5.c). Such a behavior suggests that refractoriness of the starting oxides impacts pseudomorphic transformation rate and progress. A summary of the main experiments carried out in this study is provided in the Table 3.
<figure><img src="image_6.png" /><figcaption>Figure 6. SEM images of the solid residue recovered after sonication of UO₂ fired at 485 °C in pure water (3 mM, 200 mg, 20 °C, Ar/(20%)O₂) at (a.) 346 kHz (0.2 W.mL⁻¹) and (b.) 1047 kHz (0.15 W.mL⁻¹)</figcaption></figure>

<table><caption>Table 3: Summary of the experiments as a function of the used oxide, ultrasonic frequency and medium (20°C, Ar/(20%)O₂)</caption><thead><tr><th>Oxide<br>Refractoriness<br>to dissolution</th><th>Frequency<br>(kHz)</th><th>Media</th><th>Conversion rate<br>after 6 h</th><th>Morphology</th><th>Center<br>Hole</th></tr></thead><tbody><tr><td rowspan="5">UO<sub>2</sub> - 485 °C</td><td>0</td><td>0.5 mM H<sub>2</sub>SO<sub>4</sub></td><td>Na</td><td>Random</td><td>-</td></tr><tr><td rowspan="2">346</td><td>H<sub>2</sub>O</td><td>Na</td><td>Pseudomorph</td><td>X</td></tr><tr><td>0.5 mM H<sub>2</sub>SO<sub>4</sub></td><td>> 98 % (12h)</td><td>DR + ε Pseudomorph</td><td>X</td></tr><tr><td>505</td><td>0.5 mM H<sub>2</sub>SO<sub>4</sub></td><td>Na</td><td>Pseudomorph + DR</td><td>X</td></tr><tr><td>1047</td><td>H<sub>2</sub>O</td><td>Na</td><td>Pseudomorph</td><td>X</td></tr><tr><td rowspan="7">UO<sub>2</sub> - 800 °C</td><td>0</td><td>H<sub>2</sub>O</td><td>Na</td><td>Random</td><td>-</td></tr><tr><td rowspan="2">20</td><td>0.5 mM H<sub>2</sub>SO<sub>4</sub></td><td>> 98 %</td><td>Random</td><td>-</td></tr><tr><td>0.5 mM H<sub>2</sub>SO<sub>4</sub></td><td>35 %</td><td>Pseudomorph + DR</td><td>-</td></tr><tr><td rowspan="3">346</td><td>H<sub>2</sub>O</td><td>77%</td><td>Pseudomorph</td><td>-</td></tr><tr><td>0.5 mM H<sub>2</sub>SO<sub>4</sub></td><td>88 %</td><td>Pseudomorph + DR</td><td>X</td></tr><tr><td>5 mM H<sub>2</sub>SO<sub>4</sub></td><td>100 %</td><td>Needles</td><td>X</td></tr><tr><td rowspan="2">1047</td><td>H<sub>2</sub>O</td><td>45 %</td><td>Pseudomorph</td><td>X</td></tr><tr><td>0.5 mM H<sub>2</sub>SO<sub>4</sub></td><td>Na</td><td>DR + ε Pseudomorph</td><td>X</td></tr></tbody></table>

DR: desert rose

Further AFM analyses performed on the UO₂ squared platelets fired at 485 °C (Fig 7.a) and the holed peroxide pseudomorphs demonstrated that the latter was covered with small needle-like particles (Fig 7.c). It was demonstrated that the presence of these needle-like crystals increased the local surface roughness when compared to initial UO₂ (Rₐ= from 4 to 43 nm and Rq= from 5 to 51 nm) (Fig 7.b vs. d). 3D reconstructions from SEM images of studtite obtained after sonolysis at 550 kHz of UO₂ for 7 h are shown in Fig 7.e illustrating a complete hole in the center of the plate and the high roughness caused by the formed peroxide crystals.
<figure><img src="image_7.png" /><figcaption>Figure 7: AFM images acquired on (a., b.) initial UO₂ and (c., d.) the corresponding solid residues (studtite) recovered after 505 kHz sonication in 0.5 mM H₂SO₄ (200 mg, 3 mM, 20°C, Ar/(20%)O₂, 7 h). (e.) SEM image and 3D reconstruction of a holed studtite pseudomorph.</figcaption></figure>

To better evaluate the effect of the acoustic frequency, further experiments were carried out in the 20 - 1047 kHz range under similar conditions (3 mM UO₂, 20°C, Ar/(20%)O₂, 0.5 mM H₂SO₄, **Fig. 8**). The holed platelets were only observed after sonication at 346, 505 and 1047 kHz (i.e., at high frequencies) contrasting with the experiments performed in the absence of ultrasound or at 20 kHz and 100 kHz (i.e., at low frequencies). Such a behaviour indicates that the formation of the hole is related to the acoustic cavitation phenomenon which only appears significant for high frequency ultrasound (≥ 346 kHz). Usually, low frequency ultrasound provides stronger physical effects (e.g., dispersion, erosion and fragmentation of particles) which are less important at high frequency ultrasound where chemical effects (including the formation of H₂O₂) can be dramatically enhanced. AFM, SEM and 3D reconstructions allowed further characterizations for this unprecedented morphology and confirmed that the central hole passes through the entire platelet (**Fig. 7**). Noteworthy, complementary experiments (**Fig. S6**) performed in dilute nitric acid showed very different shapes with a very bad conversion rate into metastudtite, possibly attributed to parasite NOₓ reactions arising from sonolytic decomposition of NO₃⁻ and HNO₃. [29,48,49]
<figure><img src="image_8.png" /><figcaption>Figure 8: SEM images acquired on the solid residues for UO₂ fired at 800 °C in 0.5 mM H₂SO₄ (200 mg, 3 mM, 20°C, Ar/(20%)O₂) after treatment at 20, 346, 505, 1047 kHz and in the absence of ultrasound.</figcaption></figure>

**Fig. 9** illustrates the time-dependent transformation of UO₂ surface morphology observed during its high-frequency sonication in 0.5 mM H₂SO₄. The typical smooth and flat squared platelet of initial UO₂ can be observed for the initial sample. An oxidation layer attributed to (meta-)stutdite rapidly forms on the surface of the samples after 1 h sonication and traces of alteration (orange arrows) can be observed in the center of some platelets. After 95 min, this alteration becomes more important and propagates through the first layers of the platelet in their center and a hole begins to form. Generally, the rest of the surface is poorly modified anymore, and we can notice the progressive drilling of the surface center, leading to a completely holed surface after 360 min sonication.

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<figure><img src="image_9.png" /><figcaption>Figure 9: SEM image of the solid residue evolution observed after 346 kHz sonication of UO₂ (200 mg, 3 mM) in 0.5 mM H₂SO₄ (0.2 W.mL⁻¹, 250 mL, 20°C, Ar/O₂).</figcaption></figure>

**3.3. Suggested conversion mechanism and pseudomorph origins**

Several consecutive reactions are controlling the conversion of the oxide into uranyl peroxide: (i) the oxidation of U(IV) into U(VI), (ii) the dissolution of uranium species associated with its transfer in the solution as an uranyl ion (Eq 4-7) and (iii) its precipitation into uranyl peroxide (Eq 13). These reactions are regulated by the available concentration of H₂O₂ and the acidity of the medium. Consumption of hydrogen peroxide was noticed for all the experiments with a higher rate in the most acidic conditions compared to water. Such an observation confirms a significant effect of the pH in the above discussed reactions. For an acidic environment, where the saturation index of the solution (SI) is the lowest, the dissolution is therefore more efficient so as the accumulation of uranium in solution (Fig. 1). As the dissolution kinetics are faster, the destruction of the starting oxide is more important leading to a complete dissolution followed by re-precipitation in agreement with the observation of dispersed needle-like particles. By contrast, in a less acidic environment (H₂SO₄ 0.5 mM and H₂O) the slower dissolution releases lower amounts of U in solution which immediately precipitate due to an excess of peroxides in the surroundings, which is in agreement with the evidenced absence of U in solution. Such a behavior would explain a surface re-precipitation phenomenon agreeing with SEM observations (after 1 h sonolysis) in
water and 0.5 mM H₂SO₄ (Fig. S7). Thermodynamic calculations also confirmed the decrease of uranium peroxide solubility with the rising pH.[48] Note also, some mechanisms have been proposed in the literature about the formation of peroxidic species on surface from oxo precursor or dissolution mechanism in acidic media.[26]

The formation of the peroxide pseudomorphs can be explained by a sono-capillary effect which would enable the circulation of the reactants in the volume of the initial UO₂ platelets. Such a mechanism is made possible by the foliated morphology of the oxides confirmed by SEM and TEM images (Fig. 10.a, c). Such morphology results from the firing conditions of the foliated U(IV) oxalate precursors going with the formation of very small coherent domains in the final oxide. Sono-capillary effect, which consist in an inrush of liquid at a capillary entrance resulting from bubble collapses driven by the action of ultrasound, may contribute to the circulation of the reactants (e.g. U, H₂O₂, H⁺) into small interstices acting as capillary channels. Tzanakis et al. described a similar phenomenon using a liquid metal melt to fill the preexisting grooves of an Al-Cu melt.[60] Results allowed to attribute this phenomenon to the implosion of cavitation bubbles at the opening of the grooves also acting as capillaries. Nikolai and Valeri reported the ultrasound-enhanced penetration of fluorescent liquid dyes into micro-cracks cavities that they attributed to ultrasonic capillary effect independently on the surface tension of the liquid and wettability of the cracks.[61] In our conditions, this effect can be strengthened by density differences between UO₂ (9.99 g.cm⁻³) and U(VI) peroxides (3.7 g.cm⁻³) that would also favor the formation of porosities in foliated UO₂.[62–64]

The resulting swelling of the platelet particles is clearly observed in Fig. 6 for UO₂ sonicated in pure water. Additional TEM images performed on a thin section (lamella) of a platelet sonicated in water (prepared using FIB-SEM) revealed the presence of contrasted sheets or layers differing with the homogeneous oxide observed before treatment. Such an observation confirms the volume conversion of UO₂ and suggest that it occurs by the pushing of reactants between the UO₂ sheets that convert into lower density peroxides (Fig. 10.d and Fig 11). In acidic environment, protons allow an easier leaching of the uranyl ions located in the foliated material as evidenced in Fig. 5. Sono-capillary effect would help in the circulation of the reactants and explain the observed pseudomorphic conversion of UO₂ into U(IV) peroxides. A complementary study (not presented here) also demonstrated that sulfate concentration is not significant towards the morphology conservation in agreement with the literature.[27] Note that for most of our experiments, UO₂ conversion yielded metastudtite rather than studtite. The predilection of one phase against the other is not yet very clear and most probably results from drying conditions.[65]
<figure><img src="image_10.png" /><figcaption>Figure 10: SEM images acquired on (a.) UO₂ reference synthesized by the oxalate route (485°C, Ar/(4%)H₂) and (b.) a solid residue observed after 346 kHz sonication of UO₂ (0.2 W·mL⁻¹, 250 mL, 20°C, Ar/O₂) in pure water. (c.) and (d.) TEM images acquired on thin sections of respectively (a.) and (b.) prepared by FIB-SEM. The green line indicates approximately the location of the thin section.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 11: TEM observation of a thin section (FIB-SEM) of a sonicated UO₂ platelet in pure water (346 kHz, 0.2 W·mL⁻¹, 250 mL, 20°C, Ar/O₂).</figcaption></figure>

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### 3.4. Hole formation

Concerning the hole, we only note its appearance for ultrasonic frequencies higher than 346 kHz (not observed for 20 and 100 kHz experiments). Solids observed for low frequency experiments (SEM) are indeed only remnants of $UO_2$ that has not been fully converted (Fig. S6) and are far less homogeneous than those obtained under high frequency ultrasound. Several parameters can influence this discrepancy: first, the important physical effects driven by low frequency ultrasound break up the platelets and avoid the morphological conservation.[36,40] Secondly, the deficiency in $H_2O_2$ formation for low frequency experiments does not allow a conversion as fast and complete as it is observed for high frequency experiments where $H_2O_2$ formation rate is higher. Third, the specific acoustic pressure and acoustic intensity (Table 4) estimated using Eq. 14 with the plane waves assumption, may provide a good penetration of reactants in the solid volume (sono-capillary effect) while preserving the morphology of the sample when compared to 20 and 100 kHz experiments.

$$I = \frac{p^2}{2\rho c} \qquad \text{Eq. 14}$$

Where, *I* is the sound intensity (W.m⁻²), *P* the sound pressure (Pa), *ρ* the density of water (1000 kg.m⁻³) and *c*, the speed of sound in the medium (1480 m.s⁻¹ in water).

Many hypotheses have been proposed to explain the formation of a hole in the center of the platelets including Kirkendall effect,[66] galvanic exchange,[66] mechanical traction,[67,68] etc. These hypotheses were not compelling regarding the sonication-dependent effects revealed in the studied system. The progressive formation of the holes noticed in Fig. 9 strengthens their relation with the physical effects of ultrasound. The clarification of the mechanism is still in progress but some observations pave the way for a relevant explanation.

<table><caption>Table 4: Comparison of various parameters affected by the applied acoustic frequency.</caption><thead><tr><th>f (kHz)</th><th>Acoustic intensity (W.m<sup>-2</sup>)</th><th>*Acoustic power P<sub>ac</sub> (W.mL<sup>-1</sup>)</th><th>Acoustic pressure P (Pa)</th><th>**Bubble resonant size R<sub>max</sub> (μm)</th></tr></thead><tbody><tr><td>20</td><td>1.7 x 10<sup>5</sup></td><td>0.35 ± 0.05</td><td>7.2 x 10<sup>5</sup></td><td>150</td></tr><tr><td>100</td><td>3.4 x 10<sup>3</sup></td><td>0.12 ± 0.05</td><td>1.0 x 10<sup>5</sup></td><td>20</td></tr><tr><td>346</td><td>1.9 x 10<sup>4</sup></td><td>0.20 ± 0.05</td><td>2.4 x 10<sup>5</sup></td><td>8</td></tr><tr><td>1047</td><td>1.5 x 10<sup>4</sup></td><td>0.15 ± 0.05</td><td>2.1 x 10<sup>5</sup></td><td>3</td></tr></tbody></table>

One hypothesis could be related to repeated impacts associated with the asymmetric implosion of cavitation bubbles, referred to as liquid micro-jets.[32,36,70] Usually, this effect is associated with low frequency ultrasound but examples of cavities produced by micro-jets in materials at high frequency have been put forward by recent studies.[71–73] In particular, Sugino and Yamamoto reported the effects of ultrasound ranging from 26 kHz to 3.6 MHz on starch material and proposed several important parameters to explain the pit formations

<footnote>* computed with calorimetric method; [43] **estimated using Minnaert's equation in agreement with literature[69]</footnote>
including the size ratio between the cavitation bubble and the surface, the object's physical
characteristics and the distance of the bubble from its surface.[72] In our case, as the
uranium layers are oxidized and become more vulnerable (see density above), they are more
likely to break under the mechanical pressure generated by these oriented jets which can go
up to 100 m/s and a wild range of depths and diameter holes.[70,71] Therefore, low
frequency ultrasound (20 and 100 kHz) provides bubbles that are too large (and violent)
compared to the size of the oxides (ca. 4-6 µm). Furthermore, holes are only observed for 4-
6 µm particles, even if there is a slight disparity in the size of the platelets in the starting
oxide (Fig S2.b). This indicates that only one specific size range of platelets with respect to
the bubbles can undergo this phenomenon, supporting our hypothesis. The main question
concerning this possibility is the location of the hole, which is always in the centre of the
substrate. Recent papers dedicated to the fundamental understanding of micro-jet
demonstrated that edge effects play a significant role on the orientation of the micro-jet, as
well as the implosion symmetry of the bubble as a function of the object/bubble size ratio,
the distance between them and the geometry of the surface.[74,75] However, given the
probabilities for this type of event to occur, it is very unlikely that holes are related to micro-
jets.

Another hypothesis is rather related to gradients of reactant concentrations in the samples
that would result from a progressive conversion of the oxide into peroxides. As seen
previously, high frequency ultrasound (346 and 1047 kHz) may provide a good penetration
of reactants in the solid volume with sono-capillary effect while preserving the solid integrity
(Table 3).[32,64] Once peroxide forms, protons are generated in accordance with Eq. 13
(step 1 on Fig. 12). Sono-capillary effect driven by the local acoustic pressure could then
push reactants (H⁺, H₂O₂) in the solid volume (step 2). Introduced reagents can then react
with UO₂ in the spaces according to Eq. 12 producing uranyl ions which are further pushed
towards the center of the platelet (step 3). Uranyl peroxide would then precipitate at UO₂
surface and participate in the opening of the grooves in the sample. In accordance with Eq.
13, the peroxide formation consumes H₂O₂ and releases protons that can be sonochemically-
pushed towards the center thereby giving a gradient of reactant concentrations (step 4). In
such conditions, the acidity of the platelet center would be stronger and very prone to
dissolution also involving a higher solubility of peroxides as discussed in the first part of this
study. Such conditions would favor a hole formation in the platelet center (step 5).

<figure><img src="image_12.png" /></figure>
<caption>Figure 12: proposed scheme depicting the formation of the platelet center hole.</caption>

4. Conclusion

This work describes the sonochemical conversion of powdered UO₂ into uranyl peroxides ((meta-)studtite) in dilute aqueous media saturated with Ar/(20%)O₂ at 20 °C. While strongly acidic conditions resulted in the precipitation of needle-like peroxide particles, milder pH conditions yielded peroxide pseudomorphs which formation depends on the applied acoustic frequency and the *in-situ* generation of H₂O₂. Pseudomorphic transformation was demonstrated to be driven by the acoustic cavitation phenomenon and suggested to result more precisely from sono-capillary effect. The additional formation of a unique hole in the centre of the platelets was suggested to result from a gradient of reactant concentration most probably contributing in an increase of acidity and peroxide solubility in the centre of the samples. The clarification of the formation mechanism appears complex and the influence of the medium composition, the considered solids and the applied acoustic frequency will be investigated in details. Complementary experiments involving other particle shapes in the micrometric range appear particularly relevant. Up to our knowledge, ultrasound-assisted pseudomorphic conversion is an uncovered topic auguring exciting opportunities in the next future.

Acknowledgement

We acknowledge CEA RCHIM/RTA (cross-cutting basic research program). We would like to thank Joseph Lautru (ICSM Marcoule), Renaud Podor (ICSM Marcoule) and Michael Odorico (ICSM Marcoule) for their help in SEM and AFM characterizations. We also acknowledge Martiane Cabié (CP2M Aix-Marseille university) for FIB-SEM investigations.
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