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Duparchy et al. Energy Mater. 2025, 5, 500134 https://dx.doi.org/10.20517/energymater.2025.51 Page 7 of 21

Table 1. Sample density and lattice parameter determined by Rietveld refinement for Mg Si Sn 0.7 , Mg 1.95 Si Sn 0.7 , Mg 1.95 Si 0.233 Sn 0.7
0.3
1.9
0.3
Sb 0.067 , Mg 1.95 Si 0.250 Sn Sb 0.050 and Mg 1.95 Si 0.265 Sn Sb 0.035 samples
0.7
0.7
-3
Density (g/cm ) Lattice parameter (Å)
Mg Si Sn 0.7 3.10 6.636
1.9
0.3
Mg Si Sn 3.10 6.643
1.95 0.3 0.7
Mg 1.95 Si 0.233 Sn Sb 0.067 3.22 6.669
0.7
Mg 1.95 Si 0.250 Sn Sb 0.050 3.23 6.664
0.7
Mg 1.95 Si 0.265 Sn Sb 0.035 3.22 6.659
0.7
Mg 2.06 Si 0.385 Sn Sb 0.015 [34] 3.00 6.608
0.6

Figure 2. Low and high magnification BSE-SEM images of (A and C) Mg 1.95 Si Sn , (B and D) Mg 1.95 Si 0.233 Sn Sb 0.067 . The BSE-SEM
0.7
0.3
0.7
images show the microstructure of undoped and doped Mg-poor material. One can see that globular, sub-structured inclusions are
visible in both samples. Those inclusions are Si-rich Mg X precipitated. Some contrast is also visible in both matrixes. Similar contrast is
2
also visible in Mg-rich material as shown in Figure 2. Such contrast is due to slight Si:Sn variations. EDS mapping of the Si-rich regions
are given in Supplementary Figure 2. EDS: Energy dispersive X-ray spectroscopy; BSE-SEM: backscattered electron-scanning electron
spectroscopy.
done by SEM, we do not observe precipitated elemental Si or Sn for any of the samples as might be expected
due to the Mg-deficient nominal stoichiometry. Besides, we observe a relatively homogeneous matrix with
globular, sub-structured Si-rich Mg X inclusions distributed all over the sample [Figure 2D], which are not
2
detected in XRD. This is explained by their low area fraction and variable composition which leads to small
and smeared signals. Furthermore, such Si-rich Mg X regions are not due to unmixing but rather a
2
consequence of an incomplete reaction, as is typical for this material system when prepared by ball
milling [27,34,57] , which originate probably from the relatively slow dissolution of Si into Mg X during milling
2