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Page 4 of 21 Duparchy et al. Energy Mater. 2025, 5, 500134 https://dx.doi.org/10.20517/energymater.2025.51

Mg Si Sn Sb -based solid solutions. This decreased dopant efficiency is expected due to the lower
0.3-y
y
0.7
2-δ
density of Mg interstitials and the formation of compensating defects [47,48] originating from reduced amount
of Mg in the nominal composition, but we demonstrate here that the carrier concentration required for
optimal thermoelectric performance can be obtained. We furthermore show that the synthesized Mg-poor
material undergoes a “self-adjusting” single-phase synthesis, insensitive to the nominal starting
composition, leading to highly reproducible TE properties. Such phenomenon is explained by a loss of
Mg-Sn melt during the pressure-assisted sintering, driving the material to the well-defined Mg-poor state by
a slight adjustment of the Mg and Sn content of the samples.

Microstructural analysis of the samples shows a phase constitution comparable to that of Mg-rich samples
and high temperature property measurements confirm that it is possible to synthesize a highly doped
Mg-poor solid-solution, with good and reproducible TE properties, comparable to those of Mg-rich
solid-solutions. Furthermore, extracting the microscopic material parameters, such as the effective mass and
scattering constants, using a single-parabolic band analysis reveals similar results for Mg-poor and Mg-rich
samples. However, comparing the synthesized Mg-poor material with synthesized Mg-rich samples that are
fully Mg-depleted due to annealing shows a drastic performance reduction of the synthesized Mg-rich
samples and detrimental grain boundary scattering, while the synthesized Mg-poor samples show only very
limited grain boundary scattering. Besides being a possible solution towards improved material stability,
Mg-poor material leads to better reproducibility as there is less chance for Mg loss during synthesis.

EXPERIMENTAL
Material synthesis
N-type Mg Si Sn Sb (δ = 0.1, 0.05; x = 0.7; y = 0, 0.035, 0.05, 0.067) solid solutions were synthesized by
x
y
1-x-y
2-δ
mechanical alloying, using commercially available Mg turnings (Merck, purity 99%), Si (< 6 mm, chemPur,
purity 99.99%), Sn (< 71 µm, Merck, purity 99.99%) and Sb (5 mm, Alfa Aesar, purity > 99.5%). The
precursor elements were weighted according to the targeted nominal stoichiometry, then milled for 4 h
until homogeneous powders were obtained using a high energy ball mill (SPEX 8000D Shaker Mill) with
stainless steel balls. In order to avoid oxidation and contamination of the powders during synthesis, they
were handled in an argon glove box for the complete synthesis. The resulting powders were transferred into
a 12.7 mm diameter graphite die and sintered by a direct current sinter press (DSP 510 SE, Dr. Fritsch
GmbH) in vacuum (~10 bar) at a temperature of 973 K for 20 min under a uniaxial pressure of 66 MPa on
-5
the die with a heating rate of 1 K/s to obtain compacted pellets. Two Mg Si Sn Sb samples have been
0.7
1.95
0.233
0.067
sintered from the same powder. They are differentiated as Mg Si Sn Sb -I and Mg Si Sn Sb -II.
0.067
0.067
1.95
1.95
0.233
0.7
0.7
0.233
Mg Si 0.233 Sn Sb 0.067 -I was used for low-temperature measurement and Mg Si 0.233 Sn Sb 0.067 -II for Hall
0.7
1.95
1.95
0.7
measurement. Both samples have the same properties and microstructure.
Material characterization
The sample density was determined using Archimedes’ method with an uncertainty of around 5%. The
pellet’s microstructure and phase purity were characterized by Scanning Electron Microscopy (SEM) and
Energy Dispersive X-ray spectroscopy (EDS) using a Hitachi High Tech’s SU3900 SEM device. X-ray
diffraction (XRD) patterns of the pellets were obtained using a Bruker D8 device with secondary
monochromator, Co-K radiation (1.78897 Å) and a step size 0.01° in the 2θ range of 20°-80°. The elemental
α
composition of a single Mg Si Sn sample was determined using Inductively Coupled Plasma Atomic
0.7
1.95
0.3
Emission Spectroscopy (ICP-AES) [49-51] .
The functional homogeneity of the samples at room temperature was checked by spatial mapping of the
Seebeck coefficient using an in-house developed Potential and Seebeck microprobe (PSM) with a spatial