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Duparchy et al. Energy Mater. 2025, 5, 500134 https://dx.doi.org/10.20517/energymater.2025.51 Page 17 of 21

Figure 7. (A) Predicted temperature-dependent material parameter of Mg-poor doped and Mg-rich doped samples; and (B) SPB
prediction (line) and experimental data (symbols) for the figure of merit versus carrier concentration of Mg-poor and Mg-rich doped
samples at 700K. SPB: Single parabolic band.

concentration in the match between theory and model can be expected. Our findings combined with those
by Sankhla et al. suggest that, although Mg-loss (due to Mg sublimation from an initially Mg-rich
[57]
composition) can degrade the material properties strongly, combining an initial Mg-deficient compositions
with optimized doping can achieve properties similar to those of optimized Mg-rich materials.

CONCLUSIONS
We have demonstrated the successful and reproducible synthesis of single phase Mg-poor n-type Mg (Si,Sn)
2
TE materials, and discovered that the material undergoes a self-adjusting synthesis. In fact, it appears that
Mg-poor material synthesis is insensitive to the precise nominal composition, which makes the material
synthesis suitable for upscaled synthesis. Up to now, Mg-rich compositions were typically employed, first to
compensate for Mg loss during synthesis and second to achieve high carrier concentrations. We show here
that synthesized Mg-poor samples have highly reproducible and spatially homogeneous thermoelectric
properties, presumably due to avoiding loss of excess or loosely bound Mg and second that a sufficiently
high carrier concentration can be achieved, despite a reduced dopant efficiency. Moreover, analysis of the
transport properties reveals that optimal doping of synthesized Mg-poor solid solutions can achieve
transport properties with microscopic parameters comparable to those of synthesized Mg-rich
compositions. Furthermore, synthesized Mg-poor samples were compared to samples that were initially
synthesized Mg-rich, but experienced Mg-loss, showing that the performances’ degradation of the latter was
linked to increased grain boundary scattering while in this work we showed that, when the material is
synthesized Mg-poor, grain boundary effects are negligible and the material performances are good. Hence,
synthesized Mg-rich Mg-depleted and synthesized Mg-poor sample behave differently, leading to very
different properties and microscopic parameters. Overall, we show that Mg-poor Mg (Si,Sn) materials can
2
exhibit similar or even better TE properties and disprove key arguments for Mg-excess, paving the way for
Mg-poor materials, which could exhibit better chemical stability.

DECLARATIONS
Acknowledgements
de Boor, J. would like to acknowledge support and fruitful discussion with Prof. Ernst Bauer during his stay
at TU Vienna. The authors also would like to express their gratitude to Przemyslaw Blaschkewitz for his
help and assistance with the TE measurements and to Aryan Sankhla for provision of data of Mg-rich
samples.