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Page 4 of 17 Kumar et al. Energy Mater. 2025, 5, 500109 https://dx.doi.org/10.20517/energymater.2025.22

Measurements
Since the BST exhibits anisotropic characteristics due to its layer structure, the thermoelectric properties
and X-ray diffraction (XRD) patterns of the sintered samples were measured along the Pa and
perpendicular (Pe) directions to the applied pressure direction of the hot press. The XRD patterns for the
Pa- and Pe-directions were obtained using the Cu-κα radiation (D8 advance, Bruker, Germany).
High-resolution transmission electron microscopy (HR-TEM, JEM-ARM200F, JEOL, Japan) coupled with
energy-dispersive X-ray (EDX) spectroscopy was used for the nanostructure analysis.

The temperature-dependent electrical resistivities (ρ = 1/σ) and S were simultaneously measured under a
helium atmosphere using a thermoelectric properties measurement system (ZEM-3, ULVAC-RIKO, Japan).
The Hall carrier concentrations (n ) were obtained by the relations of n = -1/(R e), where R (= ρ /H) is
H
H
H
xy
H
the Hall coefficient, e is the elementary charge, ρ is the Hall resistivity, and H is the applied magnetic field.
xy
The ρ was measured by the fore-probe contact method under sweeping magnetic fields from -1 T to 1 T
xy
using a physical property measurement system (PPMS Dynacool 14 T, Quantum Design, USA). The κ total
was obtained using κ = ρ λC , where ρ , λ, and C are sample density, thermal diffusivity, and specific heat,
p
s
p
s
respectively. The λ was measured using a laser flash method (LFA-457, NETZSCH, Germany). The densities
of the sintered samples were determined from their mass and dimensions. The C was measured using
p
differential scanning calorimetry (DSC, NEXTA DSC200, HITACHI, Japan).
RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the hot-pressedBST+HEA (x = 0, 0.1, 0.5, and 1.0 vol%) samples for the
x
Pa and Pe -directions. The XRD peaks of the BST+HEA samples are normalized to the highest (015) peaks
x
to compare the intensity of the (00l) peaks. The XRD patterns of the BST+HEA samples exhibit a
x
rhombohedral structure with the lattice parameters (a ~4.296 Å, c ~30.496 Å), without any obvious
impurities and significant changes in lattice parameters due to the HEA additions. The XRD peaks of the
HEA powder show the BCC structure with the lattice parameter (a ~3.37 Å). The XRD patterns and the
lattice parameters of the BST and HEA powder are consistent with previously reported results in the
literature. [9,11,12,24,37] The intensities of the (00l) peaks for the Pa-direction are higher than those of the
Pe-direction. This indicates that more BST layers are stacked along the Pa-direction due to the preferred
orientation of the layer structure of the bismuth telluride [9,11,12,38,39] . The XRD peaks of the HEA in the
composite samples (BST+HEA , x = 0.1, 0.5, and 1.0 vol%) are not detectable due to the small amount of
x
HEA additions. The size of the HEA nanoparticles is estimated to be 4.6 nm, calculated using the
Williamson-Hall relation based on the peak broadening of the XRD peaks .
[24]

Figure 2A and B shows the HR-TEM image of the HEA nanoparticles and the BST matrix in the x =
0.1 vol% sample, respectively. The HEA nanoparticles were identified using EDX measurements, as shown
in the inset of Figure 2A. The lattice spacing (~0.33 nm) of Figure 2B corresponds to the (009) plane of the
BST structure. The TEM image and the elemental mapping results in Figure 2C and Supplementary Figure 1
indicate that the nanometer-scale HEA nanoparticles are randomly and homogeneously distributed within
the micrometer-scale BST matrix without agglomeration.

Figure 3A presents the temperature-dependent electrical conductivities [σ(T)] of the hot-pressed
BST+HEA samples for the Pa- and Pe-directions. The obtained σ(T) values exhibit highly degenerate
x
semiconducting behavior for the Pa- and Pe-directions. The σ of the pristine BST (550 S cm for the
-1
Pa-direction, 770 S cm for the Pe-direction at room temperature) are comparable with the literature values
-1
-1
[11]
(500~700 S cm for the Pa-direction, 650~850 S cm for the Pe-direction at room temperature) . The σ of
-1
the Pa-direction of the BST+HEA samples is significantly enhanced by the additions of the HEA
x