# Epitaxial stabilisation of uranium silicide line compounds

L.M. Harding, E. Lawrence Bright, J. Laverock, D.T. Goddard, R. Springell

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L.M. Harding, E. Lawrence Bright, J. Laverock, D.T. Goddard, R. Springell. Epitaxial stabilisation of uranium silicide line compounds. Thin Solid Films, 2023, 768, pp.139690. 10.1016/j.tsf.2023.139690. hal-05080030

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Submitted on 22 May 2025

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# Epitaxial stabilisation of uranium silicide line compounds

L.M. Harding<sup>a,*</sup>, E. Lawrence Bright<sup>b</sup>, J. Laverock<sup>c</sup>, D.T. Goddard<sup>d</sup>, R. Springell<sup>a</sup>

## ARTICLE INFO

**Keywords:**

Uranium silicide

Epitaxial growth

x-ray diffraction

x-ray photoelectron spectroscopy

Advanced technology fuels

## ABSTRACT

Epitaxial single crystal thin films of U₃Si, U₃Si₅, α - USi₂, and USi₃, alongside poly-crystalline U₃Si₂ have all been synthesised using DC magnetron sputtering. These idealised samples provide the bases on which fundamental studies can be conducted for the understanding of advanced technology fuel (ATF) candidates: U₃Si, U₃Si₂, and U₃Si₅. The silicon-rich phases, USi₂ and USi₃ are of interest as intermediate oxidation products, forming as a result of the surface oxidation of the fuel candidates. Films were characterised using x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS), with XRD results indicating the stabilisation of [001]-oriented surfaces for all epitaxial phases with the exception of hexagonal U₃Si₅, which was found to be [100]-oriented. The XPS area analysis results from the U-4f and Si-2s core levels indicate that all phases are stoichiometric within error.

## 1. Introduction

Uranium silicide compounds are considered as advanced technology fuels (ATFs); a key concept in the drive to improve the safety and overall performance in the nuclear industry [1–3]. The 2011 Fukushima Daiichi accident highlighted the thermal limitations of the current UO₂ – Zr alloy system, and now, significant effort is being invested in researching advanced fuels with improved thermal conductivities. In particular, uranium silicide phases U₃Si, U₃Si₂, and U₃Si₅ have all been considered as replacement fuels for uranium dioxide, UO₂ in commercial light-water reactors [4], as dispersion fuels in research reactors [5], or as composite fuels with uranium mononitride, UN [6,7]. The remaining U–Si compounds: USi, α–USi₂, and USi₃ have attracted recent attention due to their apparent formation as oxidation products when the former phases have been exposed to elevated temperatures [8–10].

The uranium–silicon phase diagram consists of seven phases that exist as stoichiometric line compounds and several mixed phases, as noted in the phase diagrams provided by Okamoto et al. [11], and Middleburgh et al. [12]. The nature of these line compounds makes conducting analysis on these materials challenging, with the process required to produce single phases often resulting in the formation of multi-phased uranium silicides [13,14]. Various studies, both experimental and theoretical, have been conducted in order to characterise the structural and chemical nature of the uranium silicide family across a range of temperatures. Multiple studies [15–17] have investigated the three crystallographic structures (othrorhombic, tetragonal, cubic) of U₃Si, which changes as a function of temperature. Further work

concentrates on the tetragonal and hexagonal-defect type structure that the uranium disilicide, USi₂, exhibits [18–20]. Additional work concentrates on the density of states for both triuranium silicide U₃Si, and uranium trisilicide USi₃; phases which are situated at the very extremes of the U–Si binary phase diagram [16,17]. More recent work has investigated the synthesis of U₃Si₂ and U₃Si₅ single crystals through the tri-arc Czochralski method [21]. This study produced near phase-pure single crystals, with the U₃Si₂ phase having inclusions of U₃Si₅.

The studies described above were conducted on bulk samples, or were theoretically predicted. Both of these cases carry their own practical limitations, with bulk studies on nuclear fuel exhibiting the true complex nature of the system with inclusions of secondary phases or macroscopic defects, whilst computational models often include simplifications or assumptions for idealistic scenarios. In this regard, thin films can bridge the gap between the two extremes, providing idealised surfaces on which single parameter studies can be conducted. Previous studies conducted using uranium based thin films [22–26] have all shown that the variation of growth parameters allows the formation of idealised surfaces, where stoichiometry, grain size, and crystallographic orientation can all be controlled. Thus, utilising thin films for the investigation of epitaxially matched surfaces allows for a study into particular crystallographic phases and orientations of each uranium silicide.

This work shows that through the use of epitaxial lattice matching, single crystals of uranium silicide compounds can be formed. Here

<footnote><sup>a</sup> School of Physics, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK</footnote>

<footnote><sup>b</sup> European Synchrotron Radiation Facility, 71 Avenue des Martyrs, 38040 Grenoble, France</footnote>

<footnote><sup>c</sup> School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TS, UK</footnote>

<footnote><sup>d</sup> National Nuclear Laboratory, Preston Laboratory, Springfields, Preston, Lancashire PR4 0XJ, UK</footnote>

<footnote>* Corresponding author.
E-mail address: lottie.harding@bristol.ac.uk (L.M. Harding).</footnote>
we map the uranium silicide binary phase diagram as a function of uranium content, with the inclusion of U₃Si, U₃Si₅, α-USi₂, USi₃ epitaxial surfaces. The stabilisation of poly-crystalline U₃Si₂ is presented alongside these epitaxial phases.

## 2. Experimental methods

The thin films were grown using DC magnetron sputtering within a dedicated actinide deposition chamber at the University of Bristol. This system operates under ultra-high vacuum conditions with a base pressure of 10⁻⁸ Pa. In-situ reflection high-energy electron diffraction allows for the monitoring of sample crystallinity. This is enhanced with a substrate heater, which is calibrated at the sample position, using a pyrometer up to 1200 °C. The chamber uses 5.5 N argon (99.9995%) as its primary sputtering gas. A co-sputtering technique was used to form the uranium silicide films, where sputtering from depleted uranium and elemental silicon targets was performed simultaneously using a constant argon pressure of ρ<sub>Ar</sub> = 0.75 Pa. The uranium content was varied while the silicon content was kept constant, which allowed for the phase diagram to be mapped as a function of uranium content.

Single crystal substrates were chosen to epitaxially match to each of the U–Si compounds. Single crystal [001] CaF₂ was chosen as the substrate for all of the phases, with the exception of α-USi₂ where [001] MgO was also used. Substrates purchased from MTI corporation were single side polished to 1.5 – 3 Å rms roughness and had dimensions of 10×10×0.5 mm. U–Si phases grown on CaF₂ were deposited at 800 °C and capped with niobium, Nb. The capping layer acts as a chemical and physical barrier to unwanted surface contamination. The CaF₂ substrate does not contain oxygen, which decreases the likelihood of oxidising at the interface between the substrate and the deposited U–Si film. However, epitaxially matching to MgO substrates required the use of a copper, Cu, buffering layer. This prevented unwanted oxidation from the substrate, and allowed for Cu to epitaxially match to MgO in the [001]-direction with a 7 : 6 unit cell match [27]. The Cu buffer was deposited at 100 °C onto the MgO substrate, with the α-USi₂ film deposited at 500 °C. Samples grown on [001] MgO were capped with Cu which was deposited at ambient temperatures. The ability to subject these two substrates to high-temperatures enhanced the likelihood of stabilising U–Si phases.

All samples presented in this work were characterised using high-angle X-ray diffraction (XRD), X-ray reflectivity (XRR), and X-ray photoelectron spectroscopy (XPS). XRD and XRR measurements were performed using a Philips X'Pert Pro MPD diffractometer with a Cu–Kα source. The crystallinity of the compounds, and the nature of their epitaxial match were observed through a series of specular longitudinal 2θ–ω, rocking curve ω, and off-specular φ azimuthal scans about the surface normal.

XRR was used to probe the samples at low angles, investigating the sample thickness and interface roughness. An example of a uranium silicide calibration sample is presented in Fig. 1. This data was collected from a calibration sample of U₃Si₅, and had a deposition rate of 0.96 Ås⁻¹. The XRR data collected was modelled using Genx software [28]. The scattering length density (SLD), shown as a function of depth, of the U₃Si₅ calibration sample is presented as an insert in Fig. 1, and shows the variation in electron density for the materials present within the sample.

XPS measurements were performed at the Bristol NanoESCA Facility, which utilises a monochromatic Al K-ray source at 1486.6 eV. Photoelectrons are collected through a ScientaOmicron XPS ARGUS analyser, with an energy resolution of < 300 meV. The NanoESCA Facility XPS station has surface sputtering capabilities which utilises an Ar⁺ ion source, with a beam energy of 1.5 kV and a current density of 0.5 μA mm⁻². This was used to remove the protective capping layer upon each epitaxial thin film, allowing for the U–Si surface to be probed. Survey data from all samples was collected using a pass energy of 50 eV, with the measurements obtained from the U-4f, and Si-2s

<figure><img src="image_4.png" /><figcaption>Fig. 1. XRR data collected from U₃Si₅ calibration sample. Uranium and silicon were deposited simultaneously onto a Corning glass substrate, using a uranium deposition power of 19 W, forming U₃Si₅. Inset: the SLD for the system indicating the relative scattering density of each layer within the film as a distance from substrate.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 2. XRR scans and models for each U–Si phase stabilised in this work. Data sets have been offset for an easier visual comparison. U₃Si in red, U₃Si₂ in orange, U₃Si₅ in green, α-USi₂ in blue, and USi₃ in purple. Data points are indicated with open black circles.</figcaption></figure>

being collected using a pass energy of 6 eV. Spectra were not collected from the more intense Si-2p spectra, due to the overlap with the U-5d core level. This overlap of core levels makes extraction of individual components challenging. Therefore, for stoichiometry calculations, the ratio between metallic components within the Si-2s and U-4f spectra was used.

## 3. Results

### 3.1. Structural characterisation

Structural characterisation of each uranium silicide phase was conducted initially. Fig. 2 presents the XRR data collected from each U–Si phase synthesised for this work. The data sets have been offset to present the films as a function of increasing uranium content. The measured layer thicknesses for each U–Si sample are presented in Table 1. Despite the thicknesses of the U₃Si and U₃Si₂ samples being of the same order, the ability to resolve Keissig fringes within the U₃Si data, presented in red, is difficult. The lack of fringes present within
<figure><img src="image_6.png" /><figcaption>Fig. 3. (Left) Specular $2\theta - \omega$ XRD scans of $U_3Si$, $U_3Si_5$, $\alpha$-USi$_2$, and USi$_3$ shown in red, green, blue, and purple, respectively. $CaF_2$ and $MgO$ substrate peaks and features are indicated with black triangles and diamonds, respectively. (Middle) Rocking curves of the specular $U_3Si$ (002), $U_3Si_5$ (100), $\alpha$-USi$_2$ (004), and USi$_3$ (001). (Right) $\phi$ scans collected from the off-specular $U_3Si$ (206), $U_3Si_5$ (201), $\alpha$-USi$_2$ (3110), and USi$_3$ (113).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. From left to right: epitaxial matches between U–Si phase and corresponding substrate. Two potential epitaxial matches are presented for the $U_3Si$ and $CaF_2$ system. Lattice mismatches are indicated next to each system, with negative values representing a compression of the U–Si phase.</figcaption></figure>

the sample is attributed to a roughening effect between film interfaces. Additionally, only the [001] $CaF_2$ stabilised $\alpha$-USi$_2$ is presented in Fig. 2. The [001] $MgO$ system has been omitted due to the complexity of the data.

Fig. 3 presents the specular $2\theta - \omega$ high-angle, rocking curves ($\omega$ scans), and off-specular $\phi$ data collected from each epitaxial U–Si phase. The measurements presented have been offset in order to present the data with increasing uranium content. Fig. 4 shows the epitaxial matches between single crystal substrate and deposited U–Si phase for $U_3Si$, $U_3Si_5$, $\alpha$-USi$_2$, and USi$_3$. Two epitaxial match configurations are presented for the $CaF_2$ and $U_3Si$ system due to the multi-domains present within the sample. Each epitaxial match is presented with respect to the specular direction of the chosen substrate: [001] $CaF_2$ and [001] $MgO$. Lattice mismatches between substrate and deposited U–Si film are noted within Fig. 4, where negative values indicate a compression of the deposited U–Si phase. Since these interfaces are relatively sharp (seen in XRD and XRR data), interdiffusion is unlikely, and mechanical defects more likely, such as dislocation formation, which allows the relaxation of strains caused by such mismatched crystal lattices. The epitaxial matches were deduced from the off-specular $\phi$-data (Fig. 3), collected from each U–Si phase and corresponding substrate.

The specular (002) reflection of $U_3Si$ was measured and modelled in this work to be at a $2\theta$ position of $20.37^\circ$. Calculating the $c$ lattice parameter from this $2\theta$ value indicates an expansion in the specular direction by around 0.28% when compared to the standardised measurements provided by Kimmel et al. [29] for bulk $U_3Si$ material. Transverse scans were conducted on the (002) specular reflection of $U_3Si$ in order to probe the mosaicity of the sample. The FWHM of the $\omega$ data was modelled to be $0.79 \pm 0.01^\circ$. All of the measured FWHM values for substrates and corresponding epitaxial surfaces are presented in Table 1.

Lattice reflections with an in-plane component were measured using off-specular azimuthal $\phi$ scans about the sample surface normal. The $\phi$

<caption>Table 1</caption>

Table of layer thickness extracted from XRR data and FWHM collected from each rocking curve data set from each U–Si phase presented in this work.

<table><thead><tr><th>Sample</th><th>Material</th><th>Layer</th><th>Thickness (Å)</th><th>FWHM (°)</th></tr></thead><tbody><tr><td rowspan="3">U<sub>3</sub>Si</td><td>CaF<sub>2</sub> [001]</td><td>substrate</td><td>–</td><td>0.065 ± 0.001</td></tr><tr><td>U<sub>3</sub>Si [001]</td><td>film</td><td>245</td><td>0.79 ± 0.01</td></tr><tr><td>Nb (poly)</td><td>cap</td><td>123</td><td>–</td></tr><tr><td rowspan="3">U<sub>3</sub>Si<sub>2</sub></td><td>CaF<sub>2</sub> [001]</td><td>substrate</td><td>–</td><td>0.052 ± 0.002</td></tr><tr><td>U<sub>3</sub>Si<sub>2</sub> (poly)</td><td>film</td><td>224</td><td>–</td></tr><tr><td>Nb (poly)</td><td>cap</td><td>123</td><td>–</td></tr><tr><td rowspan="3">U<sub>3</sub>Si<sub>5</sub></td><td>CaF<sub>2</sub> [001]</td><td>substrate</td><td>–</td><td>0.099 ± 0.001</td></tr><tr><td>U<sub>3</sub>Si<sub>5</sub> [100]</td><td>film</td><td>210</td><td>1.51 ± 0.02</td></tr><tr><td>Nb (poly)</td><td>cap</td><td>163</td><td>–</td></tr><tr><td rowspan="4">α-USi<sub>2</sub></td><td>MgO [001]</td><td>substrate</td><td>–</td><td>0.099 ± 0.001</td></tr><tr><td>Cu [001]</td><td>buffer</td><td>–</td><td>3.26 ± 0.03</td></tr><tr><td>α-USi<sub>2</sub> [001]</td><td>film</td><td>–</td><td>3.75 ± 0.01</td></tr><tr><td>Cu (poly)</td><td>cap</td><td>–</td><td>–</td></tr><tr><td rowspan="3">α-USi<sub>2</sub></td><td>CaF<sub>2</sub> [001]</td><td>substrate</td><td>–</td><td>0.085 ± 0.001</td></tr><tr><td>α-USi<sub>2</sub> [001]</td><td>film (uncapped)</td><td>290</td><td>1.53 ± 0.01</td></tr><tr><td>CaF<sub>2</sub> [001]</td><td>substrate</td><td>–</td><td>0.054 ± 0.001</td></tr><tr><td rowspan="2">USi<sub>3</sub></td><td>USi<sub>3</sub> [001]</td><td>film</td><td>150</td><td>1.23 ± 0.01</td></tr><tr><td>Nb (poly)</td><td>cap</td><td>135</td><td>–</td></tr></tbody></table>

spectra collected from $U_3Si$, presented in red, shows the {206} family of off-specular reflections. There are a total of four peaks within a $\phi$ range of 0–180°. Theoretically, bulk [001]-oriented $U_3Si$ would exhibit two reflections with a periodicity of $\Delta\phi = 90^\circ$ within the same range. This suggests the epitaxial film of [001] $U_3Si$ has two domains. The intensity contrast suggests that the two domains have different populations, with the $CaF_2$ [100] $\parallel$ $U_3Si$ [110] domain dominant.

By decreasing the uranium content being deposited during sample synthesis, it was possible to stabilise a [100]-oriented $U_3Si_5$ film. The specular, $\omega$, and off-specular $\phi$ diffraction spectra collected from $U_3Si_5$ is presented in Fig. 3. The specular high-angle diffraction data shows
<table><caption>**Table 2**
Table of the lattice parameters and unit cells for each epitaxial uranium silicide, with reference to literature values.</caption><thead><tr><th>Sample</th><th>Phase</th><th>a (Å)</th><th>c (Å)</th><th>Unit Cell (Å<sup>3</sup>)</th><th>Ref.</th><th>Parameter change</th></tr></thead><tbody><tr><td rowspan="2">SN1513<br>Bulk</td><td>CaF<sub>2</sub>/U<sub>3</sub>Si</td><td>6.05 ± 0.01<br>6.033</td><td>8.71 ± 0.02<br>8.69</td><td>319.12 ± 1.27<br>316.29</td><td>This study<br>[29]</td><td>a = 0.35%<br>c = 0.23%<br>Vol. = 0.90%</td></tr><tr><td>CaF<sub>2</sub>/U<sub>3</sub>Si<sub>5</sub></td><td>3.89 ± 0.01<br>3.843</td><td>3.97 ± 0.05<br>4.069</td><td>52.02 ± 0.79<br>52.04</td><td>This study<br>[31]</td><td>a = 1.23%<br>c = 2.46%<br>Vol. = 0.04%</td></tr><tr><td>SN1379</td><td>MgO/α – USi<sub>2</sub></td><td>3.96 ± 0.04</td><td>13.97 ± 0.03</td><td>219.02 ± 4.08</td><td>This study</td><td>a = 0.96%<br>c = 1.26%<br>Vol. = 0.60%</td></tr><tr><td>SN1645</td><td>CaF<sub>2</sub>/α – USi<sub>2</sub></td><td>3.93 ± 0.03</td><td>13.98 ± 0.01</td><td>215.98 ± 0.33</td><td>This study</td><td>a = 0.20%<br>c = 1.24%<br>Vol. = 0.80%</td></tr><tr><td>Bulk</td><td>α – USi<sub>2</sub></td><td>3.922</td><td>14.154</td><td>217.718</td><td>[18]</td><td></td></tr><tr><td>SN1699</td><td>CaF<sub>2</sub>/USi<sub>3</sub></td><td>4.05 ± 0.01</td><td>–</td><td>66.54 ± 0.35</td><td>This study</td><td>a = 0.25%<br>Vol. = 1.3%</td></tr><tr><td>Bulk</td><td>USi<sub>3</sub></td><td>4.03</td><td>–</td><td>65.69</td><td>[32]</td><td></td></tr></tbody></table>

the (100) reflection from U<sub>3</sub>Si<sub>5</sub>, alongside the (002) of the CaF<sub>2</sub> substrate. The (100) U<sub>3</sub>Si<sub>5</sub> has a shouldered feature situated at a lower 2θ position, however, from observing the ω data from the specular (100) reflection, the shoulder component cannot be resolved or distinguished from the primary reflection. This shouldered region could be attributed to strain in the sample or periodic lattice distortion [30]. The FWHM modelled from the (100) ω data indicated a mosaic, Δω of 1.51 ± 0.02°.

The off-specular (201) plane of U<sub>3</sub>Si<sub>5</sub>, as shown in Fig. 3, has two reflections present within the φ range of 0–180°. Bulk crystals of hexagonal U<sub>3</sub>Si<sub>5</sub> would exhibit two (201)-type reflections for a complete 360° azimuthal rotation about the surface normal. This suggests that the [100]-oriented U<sub>3</sub>Si<sub>5</sub> epitaxial crystal presented in this work has two domains, rotated Δφ = 90° from one another.

The uranium disilicide compound was stabilised in the α-phase on two different substrates. The X-ray diffraction data presented in Fig. 3 indicates the α- USi<sub>2</sub> phase stabilised in the [001]-direction onto [001]-oriented copper, deposited on a [001] MgO single crystal substrate. α – USi<sub>2</sub> was also stabilised on [001]-oriented CaF<sub>2</sub> substrates, much like the uranium silicide phases presented in Fig. 3. However, this stabilisation was only possible when the surface of the α – USi<sub>2</sub> was not capped with a protective Nb capping layer. This raises questions with the mid-region of the uranium-silicide phase diagram, and how the behaviour of these binary compounds change under ambient oxidative conditions. The specular X-ray diffraction data collected from the CaF<sub>2</sub> stabilised α – USi<sub>2</sub> is presented in Fig. 5. Much like the α – USi<sub>2</sub> data presented in Fig. 3, the stabilised uranium disilicide is also [001]-oriented, with the (004) Bragg reflection situated at a 2θ position of 25.45°. This value is smaller when compared to the position of the [001] MgO stabilised position by 0.04%, suggesting a contraction in the overall specular lattice parameter. The calculated lattice parameters for both stabilised α – USi<sub>2</sub> films are presented in Table 2.

The rocking curve width, presented in Table 1, was collected from the specular (004) Bragg reflection of α- USi<sub>2</sub>. The FWHM of the data was 1.53 ± 0.01°, which is a significant improvement on the crystalline quality of the [001]-oriented MgO surface. The poor quality of the α- USi<sub>2</sub> deposited upon [001] MgO can be attributed to the epitaxial lattice matching of the Cu/MgO system, resulting in the Cu buffering layer having a mosaic of Δω = 3.26 ± 0.03°.

Further data collected from the {3110} family of off-specular reflections of the CaF<sub>2</sub>-stabilised α- USi<sub>2</sub> is presented in Fig. 6. The tetragonal nature of the α – USi<sub>2</sub> film, stabilised on both [001]-oriented CaF<sub>2</sub> and MgO was confirmed by the calculated lattice parameters. When compared to the crystallographic data provided by Sasa et al. [18] on bulk α – USi<sub>2</sub>, both stabilised α – USi<sub>2</sub> surfaces had a contraction in the specular c-axis lattice constant ( Table 2). The azimuthal spectra collected from the {3110} of α – USi<sub>2</sub> shows a higher average background. This increased background could be attributed to the lack of a protective capping layer, resulting in the formation of a structured oxide at the surface of the sample.

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** High-angle XRD data collected from the specular film of [001]-oriented α- USi<sub>2</sub>, stabilised directly onto [001]-oriented CaF<sub>2</sub>. The specular (004) reflection from the U-Si phase is indicated alongside the CaF<sub>2</sub> substrate. The (111) Bragg reflection from aluminium stems from the diffractometer sample stage.</figcaption></figure>

[001]-oriented USi<sub>3</sub> was stabilised upon [001]-oriented CaF<sub>2</sub> by further reducing the uranium content during sample synthesis. The high-angle specular data presented in Fig. 3 shows the presence of the (110) Bragg reflection stemming from the USi<sub>3</sub> phase. Using off-specular φ scans, two reflections from the {113} family of USi<sub>3</sub> were found (Fig. 3). This indicates the crystal structure of this epitaxial film is cubic, matching the bulk work conducted by K. Remschnig et al. [32] and L. Brixner et al. [33]. By comparing the structural parameters measured in this work to the literature, it was found that the epitaxial match of USi<sub>3</sub> onto [001]-oriented CaF<sub>2</sub> resulted in an expansion in both the specular lattice parameter and the unit cell (Table 2).

In addition to the epitaxial phases presented above, U<sub>3</sub>Si<sub>2</sub>, an ATF candidate, was stabilised in this work as a poly-crystalline surface upon [001]-oriented CaF<sub>2</sub> substrates. The spectra collected from the specular surface of U<sub>3</sub>Si<sub>2</sub> is presented in Fig. 7. Here, multiple reflections can be observed, from U<sub>3</sub>Si<sub>2</sub>, with additional Bragg reflections from the (002), and (004) of U<sub>3</sub>Si, alongside the (100) Bragg reflection of U<sub>3</sub>Si<sub>5</sub>.

## 3.2. Chemical characterisation

The uranium silicide phases presented in this work were chemically characterised using X-ray photoelectron spectroscopy. This technique allowed for the bonding to be probed as a function of uranium content, and therefore, extracting chemical information about each of the U-Si phases presented in this work. Spectra collected from the U-4f and Si-2s core level of each uranium silicide sample are presented in Fig. 8. Here, the data from each U-Si phase have been offset to show the spectra as
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Off-specular data collected from the {3110} family of reflections of α-USi₂, presented in blue, compared against the {135} family of reflections collected from CaF₂, indicated by the black data.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** XRD data collected from poly-crystalline U₃Si₂ stabilised on [001]-oriented CaF₂. Orange drop lines indicate Bragg reflections from U₃Si₂, magenta indicating the CaF₂ substrate, and black showing the Nb capping layer. Impurities of U₃Si (red crosses) and U₃Si₅ (green triangles) are also labelled. Inset shows the (002) reflection of U₃Si₂.</figcaption></figure>

a function of increasing uranium content, with U₃Si presented in red, and USi₃ presented in purple. Observing the U-4f<sub>7/2</sub> spectra for each phase, there is a clear indication of metallic states with the presence of an asymmetric component situated at a binding energy around 377 eV. This metallic behaviour is replicated in the data collected from the Si-2s.

Efforts were made to reduce overall surface contamination of each U–Si thin film. The XPS measurements were acquired under UHV conditions below 1 × 10<sup>-8</sup> Pa. However, there is a clear indication of a small but detectable oxygen contamination in both the U-4f<sub>7/2</sub> and Si-2s spectra, with the presence of photoemission peaks at higher binding energy positions when compared to the main metallic components. The positions of each photoemission peak is presented in Table 3. This suggests that there is oxygen bonding with both uranium and silicon within each uranium silicide phase presented in this work. Observing the separation between the photoemission peaks associated with oxygen bonding and comparing to the collated work provided by [34], it is not clear enough from the XPS data whether the oxide formed with uranium is of U(IV), U(V), or U(VI) valency.

The data collected from the U-4f of [001]-oriented USi₃, modelled with a purple line, has a photoemission peak present at a lower binding energy position (C₄) when compared to the main metallic component of uranium at 377 eV. A similar peak is also noted in the USi₃ Si-2s spectra (C₄). The positions of these components are presented in Table 3 and are highlighted with an asterisk. As this feature is present in both spectra, it could be suggested that this stems from differential

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Figure of XPS spectra collected from the surface of each uranium silicide phase presented in this work. Spectra acquired from the U-4f and Si-2s core levels.</figcaption></figure>

charging of the surface during data acquisition [35]. The probed region of the film surface may have become dissociated from conducting sites resulting in charging. From observing the values presented in Table 1, the film and cap thicknesses are similar. The niobium cap required removal via Ar⁺ sputtering prior to acquiring spectra from the uranium and silicon sites, however, caution was taken not to remove the U–Si film. This may further suggest that in addition to differential charging, the additional feature may be attributed to the Nb cap, with the Nb-2p<sub>1/2</sub> level being present adjacent to the metallic site within the U-4f spectra [36–38].

All spectra presented in this work were modelled using CasaXPS software [39]. Fitting of the U-4f<sub>7/2</sub> components presented in Fig. 8, was conducted by using a Shirley background and a Gaussian-Lorentzian (GL) product peak type, which allows for the Lorentzian weighting to be changed. Additional asymmetric exponential tail modifiers, τ, were applied to the lowest binding energy peaks within the U-4f level that exhibited an asymmetric feature on the higher binding energy side — this excludes component C₄ noted in the spectra collected from USi₃. All of these parameters are presented in Table 3. The Si-2s spectra, presented in Fig. 8, were modelled using a linear background alongside the GL product peak type for each component.

The fits for each U-4f<sub>7/2</sub> presented in Fig. 8, are comprised of up to three components: C₁, corresponding to lowest binding energy component and assigned an asymmetric weighting, alongside two symmetric components, C₂ and C₃. An additional component appears in the data presented for USi₃, C₄. The metallic component responsible for bonding with silicon in the U-4f<sub>7/2</sub> spectra is situated around 377 eV for each phase presented. Further observations of the metallic components situated in the U-4f<sub>7/2</sub> spectra for each U–Si phase, show that the peak associated with the U₃Si phase is the most narrow, with the U–Si phase with the highest silicon content, USi₃, exhibiting the broadest peak. The compounds containing the most amount of uranium appear to have a more pronounced asymmetry in the metallic component, situated around 377 eV.

Much like the U-4f spectra, the component used to model the metallic bonding site within the silicon is indicated with component (C₁) (Fig. 8). Additional components associated with silicon bonding with oxygen are assigned C₂ and C₃, with the component arising due to differential charging is denoted C₄. The metallic component within each Si-2s spectra is situated around 150 eV, with little deviation from this position as the silicon content changes within each uranium silicide phase. There is no significant broadening in the metallic component of the Si-2s, situated around 150 eV for any of the U–Si compounds presented (Table 3).

From assessing the normalised areas associated with each metallic state in both the U-4f<sub>7/2</sub> and Si-2s core levels, it was possible to
<table><caption>**Table 3**
Table of component parameters modelled for each U–Si phase during XPS.</caption><thead><tr><th>Phase</th><th>Component</th><th>Position (eV)</th><th>FWHM</th><th>% GL</th><th>τ</th><th>Normalised area</th><th>U:Si ratio</th></tr></thead><tbody><tr><td rowspan="5">U<sub>3</sub>Si</td><td>U-4f<sub>7/2</sub> C<sub>1</sub></td><td>377.3</td><td>0.6</td><td>15</td><td>0.35</td><td>0.62</td><td rowspan="5">3.2 ± 0.3</td></tr><tr><td>U-4f<sub>7/2</sub> C<sub>2</sub></td><td>381.4</td><td>2.5</td><td>30</td><td>-</td><td>0.20</td></tr><tr><td>U-4f<sub>7/2</sub> C<sub>3</sub></td><td>384.4</td><td>2.5</td><td>60</td><td>-</td><td>0.18</td></tr><tr><td>Si-2s C<sub>1</sub></td><td>150.1</td><td>1.6</td><td>60</td><td>-</td><td>0.78</td></tr><tr><td>Si-2s C<sub>2</sub></td><td>152.9</td><td>2.5</td><td>60</td><td>-</td><td>0.22</td></tr><tr><td rowspan="5">U<sub>3</sub>Si<sub>2</sub></td><td>U-4f<sub>7/2</sub> C<sub>1</sub></td><td>377.4</td><td>0.8</td><td>80</td><td>0.4</td><td>0.67</td><td rowspan="5">1.5 ± 0.2</td></tr><tr><td>U-4f<sub>7/2</sub> C<sub>2</sub></td><td>381.2</td><td>2.3</td><td>30</td><td>-</td><td>0.17</td></tr><tr><td>U-4f<sub>7/2</sub> C<sub>3</sub></td><td>383.6</td><td>2.5</td><td>20</td><td>-</td><td>0.16</td></tr><tr><td>Si-2s C<sub>1</sub></td><td>150.2</td><td>1.4</td><td>30</td><td>-</td><td>0.65</td></tr><tr><td>Si-2s C<sub>2</sub></td><td>151.7</td><td>2.5</td><td>60</td><td>-</td><td>0.23</td></tr><tr><td rowspan="5">U<sub>3</sub>Si<sub>5</sub></td><td>U-4f<sub>7/2</sub> C<sub>1</sub></td><td>377.4</td><td>0.9</td><td>70</td><td>0.5</td><td>0.84</td><td rowspan="5">0.6 ± 0.1</td></tr><tr><td>U-4f<sub>7/2</sub> C<sub>2</sub></td><td>381.1</td><td>2.5</td><td>30</td><td>-</td><td>0.16</td></tr><tr><td>Si-2s C<sub>1</sub></td><td>150.3</td><td>1.4</td><td>60</td><td>-</td><td>0.70</td></tr><tr><td>Si-2s C<sub>2</sub></td><td>152.1</td><td>2.5</td><td>60</td><td>-</td><td>0.22</td></tr><tr><td>Si-2s C<sub>3</sub></td><td>154.4</td><td>2.5</td><td>60</td><td>-</td><td>0.08</td></tr><tr><td rowspan="5">α-USi<sub>2</sub></td><td>U-4f<sub>7/2</sub> C<sub>1</sub></td><td>377.4</td><td>0.9</td><td>60</td><td>0.5</td><td>0.58</td><td rowspan="5">1.1 ± 0.1</td></tr><tr><td>U-4f<sub>7/2</sub> C<sub>2</sub></td><td>380.1</td><td>1.5</td><td>80</td><td>-</td><td>0.21</td></tr><tr><td>U-4f<sub>7/2</sub> C<sub>3</sub></td><td>380.9</td><td>2.5</td><td>80</td><td>-</td><td>0.21</td></tr><tr><td>Si-2s C<sub>1</sub></td><td>150.2</td><td>1.5</td><td>30</td><td>-</td><td>0.57</td></tr><tr><td>Si-2s C<sub>2</sub></td><td>151.9</td><td>2.4</td><td>30</td><td>-</td><td>0.20</td></tr><tr><td rowspan="5">USi<sub>3</sub></td><td>U-4f<sub>7/2</sub> C<sub>1</sub></td><td>377.3</td><td>1.6</td><td>30</td><td>0.8</td><td>0.58</td><td rowspan="5">0.33 ± 0.03</td></tr><tr><td>U-4f<sub>7/2</sub> C<sub>4</sub><sup>*</sup></td><td>375.0</td><td>1.7</td><td>30</td><td>-</td><td>0.42</td></tr><tr><td>Si-2s C<sub>1</sub></td><td>150.3</td><td>1.5</td><td>30</td><td>-</td><td>0.41</td></tr><tr><td>Si-2s C<sub>2</sub></td><td>151.7</td><td>2.5</td><td>30</td><td>-</td><td>0.22</td></tr><tr><td>Si-2s C<sub>3</sub></td><td>154.2</td><td>2.5</td><td>30</td><td>-</td><td>0.07</td></tr><tr><td>Si-2s C<sub>4</sub><sup>*</sup></td><td>148.1</td><td>2.1</td><td>30</td><td>-</td><td>0.30</td><td></td></tr></tbody></table>

calculate the chemical stoichiometry of each film presented in this work. The areas for each metallic component were normalised using the cross-sectional values provided by Yeh et al. [40]. Table 3 showcases the relative ratios between each state, with a calculated stoichiometry. With the exception of the α-USi₂ compound, each of the phases are considered stoichiometric within error. The U:Si ratio calculated for the USi₂ phase presented in this work represents that of a USi phase with a near 1: 1 weighting. This result presents an interesting puzzle for future investigations on the uranium silicide phases.

## 4. Discussion

The aim of this work was to showcase the ability to epitaxially match uranium silicide phases to single crystal substrates in order to stabilise the stoichiometric line compounds that exist within the bulk binary phase diagram of uranium and silicon. The work presented above clearly demonstrates that compounds from across the uranium–silicon binary phase diagram have been structurally and chemically characterised via the use of X-ray based techniques. Here, we have stabilised [001]-oriented U₃Si, α-USi₂, and USi₃, [100]-oriented U₃Si₅, alongside poly-crystalline U₃Si₂. This stabilisation has allowed a unique insight into the structural and chemical properties of uranium silicide phases that expand across the phase diagram. The work has contributed to the overall understanding of the binary phase diagram.

The initial structural characterisation probed the overall thickness of each uranium silicide surface through the use of X-ray reflectivity. This technique allowed for the thin film interfaces to be probed, and for an overall film thickness to be extracted. Here, the spectra collected from each phase was modelled using GenX software [28]. Though the film thickness has no significant weighting on the ability to structurally characterise these phases, it does show that the stabilisation of uranium silicide compounds as epitaxial thin films allows XRR to be used to investigate roughness and thickness features, which may prove useful in further fundamental investigations on these phases at the surface level.

High-angle XRD, was deployed to structurally characterise these epitaxial phases. From these studies, the overall phase, crystallographic direction, structure, crystal quality, and lattice constants were extracted. The phase with the highest uranium content, [001]-oriented

U₃Si surface was identified to have a tetragonal structure, with an out-of-plane (001) lattice parameter of 8.71 ± 0.02 Å. The in-plane (h00) and (0k0) lattice constants were both found to be 6.05 ± 0.01 Å. This particular crystal phase of U₃Si exhibits itself as a body-centred tetragonal structure, space group I4/mcm, a structure which is noted in the work conducted by S. Middleburgh et al. [15].

The U₃Si phase deposited upon [001]-oriented CaF₂ was found to have the best registry, with pristine α – USi₂ deposited upon MgO having the broadest mosaic of 3.75 ± 0.01°. The stabilisation of the α-phase of uranium disilicide has allowed for a unique insight into the synthesis of high-silicon content phases. This is due to the lack of studies conducted on the α-phase within the literature [18], when compared to those conducted on the β-phase [19,20]. There were minimal changes made to the lattice constants of each U–Si phase as a result of the epitaxial stabilisation. The most significant change was the in-plane (001) constant of U₃Si₅ which experiences a 2.46% increase when compared to the standardised crystallographic data provided by Brown et al. [31].

From the binary phase diagram of uranium and silicon, provided by H. Okamoto et al. [11] and S. Middleburgh et al. [12], the phases exist as fixed stoichiometric line compounds. It is the nature of these phases which has resulted in the formation of mixed phase uranium silicides in the bulk. Through the use of DC magentron sputtering, four of these line compounds have been stabilised as epitaxial thin films. One phase, U₃Si₂, was stabilised as a poly-crystalline surface with additional secondary phases of U₃Si and U₃Si₅. Yttria vanadate, YVO₄, and lithium fluoride, LiF, were both identified as substrates for the epitaxial stabilisation of U₃Si₂. The oxygen presence within YVO₄ may result in the oxidation of the deposited U–Si phase, with the low melting temperature (845 °C) of LiF preventing the stabilisation of U–Si phases if replicating the deposition conditions used in this study. The efforts made to isolate U₃Si₂ and stabilise an epitaxial single crystal were not successful. This highlights the difficulty of synthesising U₃Si₂ in the bulk, particularly for large-scale production.

While the structural characterisation performed in this work proved useful to identify the crystallographic nature of each U–Si film, the chemical characterisation allowed for the U:Si ratio i.e. the stoichiometry of each U–Si phase presented in this work to be extracted.
All phases presented were considered to be stoichiometric, and aligned with the binary phase diagram [11,15], with the exception of α – USi₂, which structurally was found to be a tetragonal [001]-oriented USi₂ phase, but chemically exhibited a U:Si ratio closer to 1 suggesting the formation of USi. The USi phase, where indicated on the binary phase diagram of having 50 mol.% content, has not been structurally identified within this work. Synthesis studies previously conducted on USi suggest that the formation is only possible in the presence of small amounts of oxygen [32]. Non-stoichiometry of U₃Si₂ was investigated in the work conducted by S.Middleburgh et al. Here it was concluded that the formation of U₃Si is promoted by the inclusion of small amounts of U being incorporated into U₃Si₂. Thus, showing that the line-compound nature of these uranium silicides are valid. However, the data presented in this work indicates that there could be room for incorporating varied uranium or silicon contents within each UₓSiᵧ phase, suggesting the existence of U–Si phases which have not been identified within the current literature. This behaviour is indicated more clearly in the higher silicon content uranium silicide phase (U₃Si₅, USi₂), where, in this work, both U₃Si₅ and uncapped α – USi₂ were synthesised using the same uranium deposition power. While this could be a result of the uncapped nature of the α-USi₂, it does showcase that the silicon-rich end of the phase diagram requires a deeper understanding.

Observing the metallic nature of each uranium silicide phase, it was noted that all of the phases probed in this work had the main U-4f photoemission peak situated around 377 eV. This peak was found to be more asymmetric for the uranium silicide phases with larger uranium contents. The metallic-like nature of the U–Si phases presented in this work aligns with the additional XPS work conducted by S. Krummacher et al. [41], who probed the oxidation of U₃Si, USi, USi₂, and USi₃, and the work conducted by T. Yan et al. [42], where the oxidation of U₃Si₁ was probed using XPS. There is limited chemical characterisation on these materials, making it difficult to make comparisons between the work presented here. However, with regards to the XPS data presented in this work, the spectra presented from [001]-oriented U₃Si₅ is, as far as the authors are aware, the first of its kind. The spectra collected from U₃Si₅ contributes to the understanding of the U–Si phases as a collective, showing that this phase also exhibits the metallic states within the U-4f and Si-2s spectra, much like the rest of the phases.

The oxygen contribution noted in both the U-4f and Si-2s spectra for each U–Si phase (Fig. 8), cannot be attributed to any stoichiometric uranium or silicon oxide. The positions of the U-4f7/2 components C2 and C3 for each U–Si phase presented in Table 3, suggests the presence of U(VI) valency, with the exception of α – USi₂ which suggests a U(V) valency. This does not align with the multiple oxidation studies [8–10,41–43], which suggest the formation of UO₂ as an oxidation product. The U(IV) state is positioned between a binding energy range of 379.9–380.4 eV, with a U-4f7/2 satellite separation energy range of 6.6–7.3 eV, as compiled by E. Ilton et al. [34]. It is evident that the data presented here is not clear enough to deduce whether the oxide product formed is U(IV), U(V), or U(VI) valent. Literature assessing the oxidation of uranium silicide phases claim the formation of a uranium silicate compound, USiO₄, as a result of the oxidation of U₃Si₂ [42,44]. This compound was not found to form in this work, however, a more extensive oxidation or dissolution study could be conducted to investigate this further for each epitaxial uranium silicide phase.

The oxidation of silicon sites within each U–Si phase is noted by the photoemission peaks located at higher binding energy positions when compared to the metallic state of the Si-2s core level around 150 eV. Though this level is relatively unrepresented in the literature, the work observing the oxidation of the Si-2p state exhibits a similar behaviour [45–47]. Upon exposure to oxidising environments, the Si-2p spectra shifts to a higher binding energy, with Si¹⁺, Si²⁺, Si³⁺, and Si⁴⁺ valencies existing at increasingly higher binding energies [47]. It cannot be concluded if the Si–O phase forming as a result of oxidation in this work is a stoichiometric phase.

## 5. Conclusions

Epitaxial [001] U₃Si, [100] U₃Si₅, [001] α – USi₂, and [001] USi₃ thin films, alongside poly crystalline U₃Si₂, have been deposited via DC magnetron sputtering. Through structural characterisation, the thin films were found to have good registry with the substrate material, and their epitaxial relationships were identified. The specular and off-specular lattice parameters were found to be comparable to bulk values. The structural work presented here showed no indication of the formation of a USi phase. Chemical characterisation indicated that all phases were stoichiometric within error. High uranium content phases were found to have a more asymmetric photoemission peak in the U-4f spectra, showing metallicity to increase with U content. Area ratios between U-4f and Si-2s for α – USi₂ indicated an almost 1:1 ratio, suggesting that chemically this phase is USi. The XPS data collected from the U-4f and Si-2s core levels showed the presence of oxygen bonds, however, there was no indication of the formation of stoichiometric U–O or Si–O phases.

This work has demonstrated the use of depositing actinide surfaces for the application of nuclear fuel investigations. We have shown that entire systems can be delicately mapped, allowing for the investigation of individual phases. These engineered surfaces form the basis of future investigations for each individual uranium silicide phase, bridging the gap between theory and bulk studies.

### CRediT authorship contribution statement

**L.M. Harding:** Conceptualization, Investigation, Methodology, Writing – original draft. **E. Lawrence Bright:** Methodology, Visualization, Writing – review & editing. **J. Laverock:** Investigation. **D.T. Goddard:** Supervision, Funding acquisition. **R. Springell:** Conceptualization, Methodology, Writing – review & editing, Supervision, Funding acquisition.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgements

This project was funded by a joint EPSRC-NNL ICASE award (EP/R511857/1). The authors acknowledge access to the Bristol No-noESCA Facility under the EPSRC Strategic Equipment Grant: EP/M000605/1 where XPS measurements were made, and access to the University of Bristol NNUF FaRMS Facility (EP/V035495/1) where sample synthesis was conducted. The authors also thank G.H. Lander for helpful discussions.

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