Page 274 - Read Online

Duparchy et al. Energy Mater. 2025, 5, 500134 https://dx.doi.org/10.20517/energymater.2025.51 Page 11 of 21

mismatch in the coefficient of thermal expansion presents significant challenges when oxides are employed,
particularly under thermal cycling. Furthermore, elemental Mg, which drives material degradation is highly
reactive. Consequently, many oxides such as Al O that have a lower formation enthalpy than MgO might
3
2
[72]
not work as a coating in the long run, as demonstrated by Deshpande et al. . Therefore, in our work, we
propose the synthesis of Mg-poor materials to prevent direct Mg loss, addressing the degradation
[35]
[27]
[34]
mechanism described by Sankhla et al. , Duparchy et al. and Ghosh et al. .
The synthesis of Mg-poor Mg Si Sn has been successful, leading to the formation of a typical n-type
0.7
0.3
1.95
semiconductor. The microstructural analysis proves that no elemental Si or Sn remains; only Si-rich
secondary phases within the solid solution system Mg (Si,Sn) were formed [Figure 2]. Furthermore, the
2
synthesized Mg-poor sample do not show any Mg Si and Mg Sn unmixing. These findings contradict the
2
2
[63]
speculation that Mg deficiency initiates unmixing of the solid solution, raised by Yasseri et al. . ICP
analysis was conducted to determine the elemental composition of the undoped Mg Si Sn sample,
0.3
0.7
1.95
resulting in a composition of Mg Si Sn , close to Mg (Si,Sn), with a higher Mg and lower Sn content
0.307
0.693
2
2.001
than the nominal one. We propose that these differences between nominal and measured composition are
the consequence of a self-adjusting stoichiometric balance during the synthesis process, in particular, during
uniaxial hot pressing. Indeed, according to the Mg-Si-Sn phase diagram , a sample that contains more Sn
[73]
than the Sn-rich limit according to the miscibility gap [62,73] and is Mg deficient, is located in a three-phase
region, with elemental Si, Sn-rich Mg X and liquid Mg-Sn (for temperatures above ~210 °C) as coexisting
2
phases. According to the phase diagrams provided in Figure 9 from the work by Orenstein et al. ,
[73]
Mg Si Sn is not located inside the Sn-rich Mg X phase, but we note that our samples have been sintered
0.7
0.3
1.95
2
at 700 °C, i.e., larger than the temperatures at which the ternary isothermal phase diagrams are known and
point out that the miscibility gap closes with increasing temperature, and, secondly that Mg X samples have
2
been synthesized repeatedly as single-phase materials inside the thermodynamically predicted miscibility
gap, presumably due to stabilization by coherent interfaces [63,74] . It is therefore highly plausible that the
sintering step leads to the formation of Mg (Si,Sn) and Mg-Sn, with the latter being expelled during the
2
pressure-assisted sintering as it is liquid. We do not observe elemental Si (see Figure 2). As Mg Si is tighter
2
bound than Mg Sn, Si might replace Sn in the structure as long as there is X excess. According to the
2
suggested procedure, one would expect a sample with reduced Sn content (as observed) and at the lower
solubility limit of Mg in Mg 2(1+δ1) X. The very similar Seebeck data for the two undoped sample supports this.
The ICP results with a Mg content slightly larger than 2 is not a contradiction to the sample composition
corresponding to the lower solubility limit of Mg but a result of the limited accuracy of the measurement
and an indication that the phase width with respect to Mg is smaller than the accuracy of the ICP
measurement. For undoped materials, we are not aware of any report on the phase width, but for doped
Mg X a value of Δδ = 0.008 was reported, smaller than typical uncertainties of less than 1 at.% for ICP .
[64]
2
Mg
All three Mg-poor doped samples were examined by XRD and SEM/EDS [Figures 1 and 2], demonstrating
successful dopant incorporation, as no visible Sb-rich phase precipitated were detected. Furthermore,
Seebeck coefficient measurements [Figure 3] revealed differences in the Seebeck coefficient between
undoped Mg Si Sn and doped Mg Si 0.233 Sn Sb 0.067 (-350 µV/K vs. -80 µV/K). These observations
0.3
0.7
1.95
0.7
1.95
suggest that Sb was effectively incorporated on the Si/Sn site, with the doped sample exhibiting relatively
low local functional fluctuations. The thermoelectric properties of the Mg-poor material improve
significantly with doping, resulting in a high power factor and an overall figure of merit comparable to that
[57]
of Mg-rich material compounds [Figure 4F]. Also, samples Mg Si Sn Sb - I and II are very similar
0.233
1.95
0.067
0.7
microstructure- and property-wise, indicating a high reproducibility of the synthesis route. Indeed, while
the charge carrier concentration for Mg-rich samples might depend on the details of the sintering step
(temperature, duration) [31,57] due to Mg loss, using Mg-poor materials might lead to a very high