# X-ray study of a MgLi alloy with a reflection zone plate spectrometer on an electron microprobe

P. Jonnard, K. Le Guen, K. Hassebi, N. Rividi, O. Boudouma, R. Gauvin, N.

Brodusch

► To cite this version:

P. Jonnard, K. Le Guen, K. Hassebi, N. Rividi, O. Boudouma, et al., X-ray study of a MgLi alloy with a reflection zone plate spectrometer on an electron microprobe. X-Ray Spectrometry, 2025, 54 (5), pp.643-649. 10.1002/xrs.3501. hal-05227970

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Distributed under a Creative Commons Attribution 4.0 International License
X-ray study of a MgLi alloy with a reflection
zone plate spectrometer on an electron
microprobe

P. Jonnard¹, K. Le Guen¹, K. Hassebi¹, N. Rividi², O. Boudouma², R. Gauvin³, N. Brodusch³

(1) Laboratoire de Chimie Physique — Matière et Rayonnement, Faculté des Sciences et Ingénierie, Sorbonne Université, UMR CNRS, 4 place Jussieu, 75252 Paris Cedex 05, France

(2) Service Camparis, UMS 7154-CNRS, OSU Ecce Terra, Sorbonne Université, 4 place Jussieu, 75005 Paris, France

(3) Department of Mining and Materials Engineering, McGill University, 3610 University, Montreal, Canada H2T 2X1

A MgLi dilute alloy (6% wt. Li) has been studied by x-ray emission spectroscopy in the ultra-soft x-ray range, 40-60 eV. For this purpose, a high-resolution reflection zone plate spectrometer, working as a variable line grating spectrometer, was used on an electron probe microanalyser. With the experimental conditions, current and acceleration voltage of the incident electron beam, geometry, CCD detection, the Mg L emission band is easily observed. The Fermi edge of this band shifts toward the low photon energies with the Li content. The Li K emission band is hardly detectable despite the non-negligible lithium mass fraction in the alloy. This is explained by the reabsorption of the lithium emission by the magnesium atoms, the reabsorption effect being strong in the present geometrical conditions of the experiment.

Keywords : x-ray emission spectroscopy ; grating spectrometer ; electron probe microanalysis ; MgLi alloy; ...
## Introduction

Magnesium is attractive for the transportation industry owing to its low density. However, to improve its mechanical properties, it is mixed with other elements such as aluminium or lithium to form some dilute alloys. Therefore, studying the band structure of Mg alloys is as important in practical and fundamental perspectives. A powerful mean to probe the band structure of solids is to use x-ray emission spectroscopy and to consider probing emission bands, that is to say transitions between valence bands and core levels. Indeed, the involvement of the valence electrons makes the emission bands sensitive to the chemical environment of the emitting atom. In the case of magnesium, emission band can be obtained either in the soft x-ray range (Kβ emission around 1250 eV¹) or in the ultra-soft x-ray range, also called vacuum ultraviolet region (Mg L emission band below 50 eV¹). Thus, some Mg alloys have been studied since many years in both soft²⁻⁴ and ultra-soft⁵⁻⁷ x-ray ranges. However, the band structure of these alloys has not really been studied with electron probe microanalysers (EPMA) or scanning electron microscopes (SEM), which are readily available laboratory apparatus

Despite recent works on magnesium⁸,⁹, there exist only a small number of spectroscopic studies of MgLi alloys performed in the ultra-soft x-ray range. In all of them, generation of the Mg L and Li K emission bands was performed upon electron excitation. The generated x-ray emissions were subsequently dispersed by using grating spectrometers. In the oldest study¹⁰, Catterall and Trotter emphasized the variation of width of the Mg L and Li K emission bands as well as of a shift of the band edges with the alloy concentration. They explain these changes by electron transfer taking place from magnesium atoms to lithium atoms. These observations were confirmed by a series of studies spanning the whole range of alloy composition, from pure lithium to pure magnesium¹¹⁻¹³. In another series of papers¹⁴⁻¹⁸, Crisp considered also the whole composition range with some emphasis on the dilute alloys. By comparing spectra obtained upon different incident electron energies, that is to say by analysing different depths, he clearly demonstrated that reabsorption is critical: a large part of generated intensity of the lithium emission band cannot be detected owing to the strong absorption in the alloy.

To our knowledge, since 1983 there have been no further studies of this kind on MgLi alloys. This is why we think interesting to investigate a MgLi alloy, here with 6% wt. Li, with the modern spectrometers¹⁹⁻²³ now available for SEM, transmission electron microscopes and EPMA. With respect to the spectrometers used in the references cited above where acquisition required angular scanning, the current state spectrometers rely on variable line spacing gratings (or equivalent) which makes possible the observation of a wide spectral range at once.

## Experimental

### Sample

The sample is an ingot being a few millimetres wide in all the directions, figure 1, which mass composition is Mg₉₄Li₆. This corresponds to an atomic composition of 18.4% Li. The preparation of this sample has already been described some years ago, before being studied by x-ray emission⁴. At that time lithium detection was not available. After polishing its surface, the sample was first characterised on a SEM (Zeiss Supra 55).

We show in figure 1 the electron backscattered image of the sample obtained with 15 keV electrons. Tens-to-hundred micrometre-size grains are surrounded by lighter boundaries. With the exception of lithium, an x-ray mapping was obtained with an energy dispersive spectrometer (Bruker silicon drift detector) upon irradiation with 5 keV electrons. The Mg, O and C elemental maps are presented in figure 2, showing that there is no (or very few) carbon, that oxygen is present mainly in
the grain boundaries and that the Mg concentration is constant within the grains and also from grain to grain. As a reference a pure Mg sample was also analysed in the same conditions as the MgLi alloy.

<figure><img src="image_2.png" /><figcaption>Figure 1: High magnification backscattered electron image of the MgLi alloy obtained with 15 keV electrons. The inset in the red box is a zoom of a zone of the sample.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2: X-ray maps of magnesium (left), oxygen (centre) and carbon (right) obtained with 5 keV electrons on a same zone of the sample. The size of the images is the same as the one of the inset of figure 1.</figcaption></figure>

## X-ray emission

The apparatus used to obtain the Li K and Mg L emission bands has already been described²⁴ as well as its capabilities in the spectral range of the Li K emission²⁵. Here we recall its main characteristics and show the geometry of the experiment in figure 3. The electron column of an EPMA CAMECA SX100 produces an electron beam under a 1-20 kV acceleration voltage, with a beam current ranging from some tens to some hundreds of nA. The practical conditions when using the spectrometer dedicated to the Li K range are: 2-5 kV acceleration voltage; 50-200 nA current; 2-3 $10^{-6}$ mbar in the sample chamber. The electron beam impinges the sample at normal incidence in a spot being 1-20 µm wide. Let us note that the contrast of the electron image obtained on the EPMA was not good enough to distinguish the various grains observed in figures 1 and 2. However, comparing the sizes of the grains and of the electron beam, we expect the analysis to be performed on a single grain.
<figure><img src="image_4.png" /><figcaption>Figure 3: Geometry of the x-ray emission experiment. The black line is the path of the incident electrons; the red line is the path of the emitted photons; the dashed lines are the paths inside the sample. RZP is the reflection zone plate used to disperse the polychromatic x-rays generated in the sample upon the electron irradiation. The drawing is not to scale.</figcaption></figure>

Just after its polishing, the sample was introduced in the EPMA to prevent the formation of a superficial oxide and contaminated layer. The pressure in the analysis chamber was 10⁻⁶ mbar (10⁻⁴ Pa). A -135°C cold finger was also in operation to minimize contamination from the residual atmosphere during the experiment. The x-rays generated upon the electron irradiation were detected under a glancing angle of 40°.

Dispersion and detection of x-ray coming from MgLi sample were performed through a fixed-geometry spectrometer from Nano-Optics Berlin²⁶, equipped with a reflection zone plate (RZP) and an x-ray CCD camera from Greateyes. To minimize noise, necessary when looking at faint emissions such as the Li Kα emission band, the CCD is operated at -60°C and at a reading speed of 50 kHz. The RZP is working as a variable line spacing grating allowing to focus the different wavelengths on a flat field, so avoiding any scanning and thus limiting the acquisition time and consequently the possible irradiation damages. The spectral resolution estimated from the width of the Fermi edge of the Mg L emission band of the pure metal at 49 eV was 0.25 eV²⁴. The spectrometer works at the first diffraction order in the 40-120 eV spectral range, but high-order diffracted emissions can be detected and analysed.

## Results

The x-ray analysis was performed at 5 kV and 100 nA over a 5 µm wide beam spot. The spectra accumulation took place on the part of the samples presenting the weakest oxygen intensity. Time acquisition of 50 s was repeated on a same position of the alloy. It was observed that the intensity decreases slowly after 5 to 10 repetitions. In that case, analysed spot was shifted to another position on the sample surface. Figure 4 presents the spectrum of the MgLi alloy, over the whole spectral range covered by the RZP spectrometer, 40-120 eV. The spectrum, normalised to 1 s and 1 nA, results from the sum of 29 individual spectra obtained on three different zones of the sample. A background has been subtracted following the process previously described²⁴. The inset of Figure 4 present the partial sum of raw spectra prior background subtraction. In this particular case, the background process mainly plays on both ends of the spectrum.
<figure><img src="image_5.png" /><figcaption>Figure 4: Wide spectrum of the MgLi alloy obtained with the RZP spectrometer; in the inset, sum of 10 raw spectra before background subtraction and normalization.</figcaption></figure>

The prominent feature toward the low photon energies (below 50 eV) is the Mg L emission band. The Li K emission band is observed as a weak peak around 54 eV. Peaks resulting from oxygen diffracted at high orders are also observed at 105 eV (5th order) and 87 eV (6th order). The oxygen comes from the small number of oxidized magnesium atoms formed at the sample surface prior its introduction in the EPMA. It cannot be excluded that some part of an oxidized grain boundary, located on the edge or below the analysed zone, is impinged by the electron beam. The 7th (expected around 75 eV) and higher diffracted orders of the oxygen emission being not observed, we do not expect some interference of high-diffracted order emissions of oxygen in the spectral range of the Mg L and Li K emission bands (40 – 60 eV). On the wide spectrum of figure 4, it cannot be seen any high diffraction order of the C K emission, for example around 95 eV at the third order. Again, we do not expect any spectral interference originating from the carbon emission in the spectral range of interest.

Figure 5(a) presents the spectral range of interest encompassing the Mg L and Li K emission bands. A striking feature for MgLi is that Li K emission band is 45 times lower than the one of the Mg L emission band when considering the surface under the peaks. A simple ratio of the Mg and Li mass concentrations would lead to a ratio of 16. Electrons having an energy of 5 keV in magnesium produce Mg L<sub>2,3</sub> ionisations in a depth of 500 nm as calculated from Monte Carlo simulation<sup>27</sup>. The depth distribution of the ionisations is maximum around 170 nm from the surface. In the considered alloy, the ionisation depth should be of the same order of magnitude for both Mg L and Li K because they have almost the same ionisation energies. So, this cannot explain the large intensity difference of the two emission bands.
<figure><img src="image_6.png" /><figcaption>Figure 5: Spectra of the MgLi alloy and Mg metal (a) in the range of the Mg L and Li K emission bands, (b) in the vicinity of the Mg L<sub>2,3</sub> edge.</figcaption></figure>

To get an explanation, we turn to the variation of the attenuation coefficient and the
attenuation length (AL) as a function of the photon energy. The AL is the length after which the beam
intensity is attenuated by a factor 1/e. We show in figure 6 these two quantities, extracted from the
CXRO database²⁸, for both Mg metal and the MgLi alloy in the spectral range of interest. The density
of the alloy, 1.65 g.cm⁻³, is calculated from the linear variation of the densities between pure Mg and
Li as a function of the mass fraction. In this low energy range the database is very probably imprecise
(regarding the energy of the absorption edge and the absolute value of the AL), so that the figure is
here to show a qualitative trend and not to give quantitative values. First, it can be seen that the pure
metal and the alloy behave in the same way, as expected from the small Li content. The Li K absorption
jump is seen for MgLi at 55 eV. Then, it is observed that the effect of the Mg L absorption edge around
49 eV is tremendous. Owing the large jump of the attenuation coefficient, the AL is maximal, 750 nm
for MgLi, for photon having an energy just below the Mg L<sub>2,3</sub> edge. After this edge, *i.e.* in the range of
the Li K emission, the attenuation length is limited to around 25 nm, a decrease by a factor 30. This
makes quite small the number of emitting lithium atoms possibly detected, with respect to the number
of detected magnesium atoms. So, in addition to the small Li content within the studied sample, the
large reabsorption of the lithium emission in the sample combined to the very low lithium fluorescence
yield²⁹ explain the very low collected intensity of the Li K emission band.
<figure><img src="image_7.png" /><figcaption>Figure 6: Attenuation coefficients (red) and attenuation lengths (blue) for Mg metal (a) and the MgLi alloy (b) as deduced from the CXRO database²⁸ in the range of the Mg L and Li K emission bands.</figcaption></figure>

Another manifestation of the reabsorption effect is observed in the region of the Mg L<sub>2,3</sub> edge, figure 5(b). The Mg L emission band comes from two electron transitions, 3sd – 2p<sub>1/2</sub> and 3sd – 2p<sub>3/2</sub>, so we expect to observe two Fermi edges shifted by the 2p<sub>1/2-3/2</sub> spin-orbit splitting, 0.28 eV<sup>1</sup>. But, despite a sufficiently good spectral resolution, this is not observed since only one edge is present. However, when geometrical conditions of the analysis are chosen to minimize the reabsorption effect, then the two Mg L edges can be observed. This is the case for example of spectra presented by Crisp<sup>18</sup>, where probably the emitted photons were collected in the direction normal to the sample surface, making shorter the path toward the detector and where the electron beam was incident on the sample surface under a glancing angle, making shallower the analysed depth. The large reabsorption effect and poor knowledge of the attenuation coefficients in the considered spectral range lead us not attempting to perform elemental quantification from the measurement of the Mg L and Li K collected intensities.

In addition to the intensity decrease of the Mg L emission band when going from the metal to alloy, as mentioned above, we note a strong decrease of the feature, observed as a narrow peak close to the edge, figure 5(a). This feature is not related to the occupied density of states as the other parts of the emission band, but originates from many-body effects between electrons present in the valence band<sup>30-32</sup>. It was observed that this feature vanishes with the Li content increasing in the alloy. For example in the work of Catterall et al.<sup>10</sup>, for an atomic concentration of 20.9% Li, the maximum of the many-body feature is as high of the maximum of the emission band located around 48 eV, as it is the case for our alloy which concentration is 18.4%.

It is also interesting to compare the Mg L edge in the case of the alloy and the metal. This is shown in figure 5(b), where the spectra were normalised to their maximum at 48 eV. It is clearly observed that the position of the edge shifts toward the low photon energies upon alloying. In our case this shift is estimated to -0.06 eV by comparing the energies of the minima of the edge derivatives, or to 0.07 eV by measuring the energy difference at half maximum. This shift could arise owing to the charge transfer occurring between Mg and Li atoms following their electronegativity difference: Mg, 1.31; Li, 0.98. This small electronegativity difference and the small Li content explain the limited shift of the band edge. Crisp noted a -0.09 eV shift for the alloy with 15 at.% of Li. This shift and
concentration are close to the ones corresponding to the present sample, but it is difficult to know if
the difference with the values of Crisp are significant or not because the two experiments were not
performed with the same self-absorption conditions, whom are prominent close to the Mg L
absorption edge.

We show in figure 7 the Li K emission band of the alloy compared to the one of the pure metal obtained in our previous study²⁵. As the lithium emission is in the foot of the Mg L emission, we use the energy tail of the Mg L emission of the metal as the background in the range of the Li K emission of the MgLi alloy. The spectra are normalised with respect to their maximum. In this condition, it is observed that within the experimental uncertainty, both alloy and metal spectra have almost the shape in agreement with previous observations¹¹. However, it is clear that the intensity of the Li K emission band is very weak, despite the large acquisition time of 1450 s. Thus, the number of counts is quite low with large statistical variations making this emission not very usable.

<figure><img src="image_8.png" /><figcaption>Figure 7: Li K emission bands of the MgLi alloy and pure metal³³; the spectra are normalised to their maximum.</figcaption></figure>

Conclusion

With an EPMA equipped with a RZP high-resolution spectrometer, it is possible to measure in the same photon energy range both Mg L and Li K emission bands of a MgLi alloy. However, the geometrical conditions of the analysis in EPMA are not optimal and make strong the absorption of the lithium emission within the sample, leading to a very high Mg L / Li K intensity ratio and preventing quantitative analysis. We plan to change the geometry of the experiment to make the x-ray detection normal to the sample surface thus minimising the reabsorption effect. The Li K emission band is hardly usable owing to the very low collected intensity. This emphasizes the difficulty to work with a weak emission present in the foot of an intense emission band of a metal. However, from the observation of the shape and edge position of the Mg L emission and comparison with older x-ray spectroscopic studies, the Li content in the considered alloy is consistent with a concentration of 6 wt.% Li.

Acknowledgments

This work was made in the framework of the ANR funded project ANR-20-CE29-0022, SQLX
"Spectroscopy and quantification of lithium by x-ray microanalysis", https://anr.fr/Projet-ANR-20-
CE29-0022.
A CC-BY public copyright license has been applied by the authors to the present document and will be applied to all subsequent versions up to the Author Accepted Manuscript arising from this submission, in accordance with the grant's open access conditions.

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