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. 2025 Jul 17;10(29):32112–32121. doi: 10.1021/acsomega.5c03826

Investigating Thermoelectric Properties of GeTe Alloys with Multi Element Doping: Insights from High-Entropy Engineering

Yifan Sun †, Ken Kurosaki †, Tetsuya Imamura ‡, Ryusuke Torata ‡, Yuji Ohishi ‡, Dulyawich Palaporn §, Theeranuch Nachaithong §, Supree Pinitsoontorn §,∥, Jintara Padchasri ⊥, Pinit Kidkhunthod ⊥, Methus Suwannaruang #,∇, Sora-at Tanusilp ○,*
PMCID: PMC12311643  PMID: 40757290

Abstract

High-entropy engineering provides an effective strategy to enhance thermoelectric properties through increased lattice disorder induced by multielement doping. Recent multielement doping of GeTe-based alloys have significantly improved their thermoelectric performance, yet the vast compositional space of high-entropy GeTe makes identifying optimal compositions challenging. In this work, we investigate high-entropy GeTe alloys derived from the state-of-the-art Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te system by partially adding Au to increase elemental and structural complexity without significantly degrading the electrical properties. Transmission electron microscopy confirms the presence of nanoscale lattice distortions and stacking faults that promote mass and strain fluctuations by enhancing phonon scattering. The Ge0.59Au0.02Ag0.11Sb0.13Pb0.12Bi0.01Te composition exhibits an ultralow lattice thermal conductivity of 0.22 W m–1 K–1 and achieves a maximum zT of 2.0 at 780 K. These findings demonstrate the effectiveness of high-entropy doping in tuning thermoelectric performance and advancing the development of next-generation GeTe-based thermoelectric materials.

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1. Introduction

Thermoelectric devices are used to convert heat from sources like solar radiation, automotive exhaust, and industrial processes into electricity or to power cooling systems, but their low efficiency has limited broader use. , Enhancing the efficiency of these devices depends on improving a material’s dimensionless figure of merit (zT), which is determined by the Seebeck coefficient (S), electrical conductivity (σ), absolute temperature (T), and total thermal conductivity (κ), which is evaluated from the electronic (κelec) and lattice (κlat) contributions. , While increasing carrier density can enhance electrical conductivity, it typically reduces the Seebeck coefficient and increases electronic thermal conductivity, thereby limiting zT. Therefore, advanced techniques are required to overcome this trade-off and achieve higher thermoelectric performance. −

Among various thermoelectric material groups, GeTe-based alloys represent a family of p-type semiconductors with high carrier concentrations due to Ge vacancies, , which can be controlled to optimize thermoelectric performance through counter-doping or alloying with elements such as Sb, , In, and Bi. − Recent advancements in GeTe-based thermoelectrics have demonstrated various strategies to enhance performance. For instance, the incorporation of interstitial Cu has been shown to simultaneously improve carrier mobility and increase phonon scattering, resulting in a high zT of ∼2.3 at 623 K. Broader reviews have also highlighted the potential of GeTe through optimization and device integration approaches. In n-type GeTe, thermoelectric performance has been further improved by decoupling carrier and phonon scattering via annealing-induced grain enlargement and vacancy cluster formation. Moreover, combining bandgap widening with minority carrier filtering has proven effective in suppressing the bipolar effect, achieving a zT of 0.45 at 723 K and extending the usable temperature range of n-type GeTe materials.

In recent years, “entropy engineering” has emerged as an effective approach to simultaneously enhance the electrical and thermal properties of thermoelectric materials by increasing disorder within the crystal lattice through element doping, alloying, and other techniques. − This approach has drawn significant attention to entropy-stabilized (GeTe) x (AgSbTe2)100‑x , commonly known as TAGS-x, for its favorable thermoelectric performance at intermediate temperatures, with zT values approaching 2.0 at 773 K. The AgSbTe2 component of TAGS has been extensively studied and alloyed at all three atomic sites using various elements for specific purposes. For example, Cu, Bi, and Pb have been doped at the Ag, Sb, and Te sites, respectively, to minimize lattice thermal conductivity, while Na, Mn, and Se have been introduced to enhance electrical properties. Additionally, numerous studies have focused on doping the Ge site of TAGS, taking advantage of Ge vacancies and the strong thermoelectric properties of AgSbTe2. −

While GeTe-based alloys typically exhibit a rhombohedral structure at low temperatures, Jiang et al., successfully prepared entropy-stabilized cubic-phase TAGS samples by introducing Ag, Sb, and Pb to Ge sites. This multielement alloying induced a rhombohedral-to-cubic phase transformation in GeTe at room temperature, which improved its crystal symmetry and electrical properties, while reducing its lattice thermal conductivity. − These combined effects enabled the Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te sample (referred to as S9) to achieve an average zT over 1.7 between 300 and 750 K, highlighting high-entropy GeTe as a promising material for thermoelectric applications.

In this work, we investigate the substitution of Ag, Au, Sb, Pb and Bi at the Ge site of Ge1–x Te, the best-performing composition reported in the literature. The rationale for partially substituting Ag with Au lies in its much larger mass difference from Ge, which enhances phonon scattering and reduces lattice thermal conductivity. In addition, the structural disorder introduced by Au as an additional dopant is expected to further lower lattice thermal conductivity. Simultaneously, the excellent electrical conductivity of Au can mitigate potential negative effects on the material’s electrical properties, leading to an improvement in its zT. Furthermore, the atomic similarity between Au and Ag is expected to make the substitution more feasible within the lattice. This work begins by examining the thermoelectric properties of the multi element-doped high-entropy GeTe sample. Subsequently, the oxidation states of Ge and Te, along with the substitution at the Ge site, are examined using the synchrotron-based X-ray absorption near edge structure (XANES) technique.

2. Experimental Section

The starting materials, Ge (4N, Kojundo Chemicals), Au (4N, Nilaco), Ag (5N, Kojundo Chemicals), Sb (3N, Kojundo Chemicals), Pb (4N, Nacalai Tesque), Bi (3N, Kojundo Chemicals), and Te (4N, Kojundo Chemicals), were mixed in stoichiometric amounts as shown in Table . The nominal compositions of the synthesized GeTe-based samples are presented in atomic percentage (atom %). In this study, the total cation content (Ge + Au + Ag + Sb + Pb + Bi) was intentionally fixed at 0.98 atom % to account for intrinsic Ge vacancies and doping effects, while the Te content was fixed at 1.00 atom %. In designing the compositions, Sb, Pb, and Bi were kept constant at 0.13, 0.12, and 0.01, respectively. As a result, the sum of Ge, Au, and Ag contents was maintained at 0.72 atom %, with their ratios varied to examine their relative contributions to thermoelectric properties. This compositional strategy aligns with previously reported high-performance GeTe-based thermoelectric systems. Although a fully systematic single-variable study would have provided more detailed insights, the number of required samples was impractically large. Thus, we selected representative compositions to balance experimental feasibility with the need to capture key thermoelectric trends.

1. Nominal Compositions of the Prepared GeTe Samples (in Atomic Percentage) .

sample nominal composition
No.1 Ge0.59Au0.02Ag0.11Sb0.13Pb0.12Bi0.01Te
No.2 Ge0.56Au0.05Ag0.11Sb0.13Pb0.12Bi0.01Te
No.3 Ge0.59Au0.11Ag0.02Sb0.13Pb0.12Bi0.01Te
No.4 Ge0.61Au0.11Sb0.13Pb0.12Bi0.01Te
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a

The total cation fraction (Ge + Au + Ag + Sb + Pb + Bi) is fixed at 0.98. Te content is fixed at 1.00, following standard formulation practices in GeTe-based thermoelectric materials.

A total of four samples were prepared, and each sample was cold-pressed into a pellet and loaded into an evacuated, sealed SiO2 glass tube. The tubes were placed into a resistance furnace, heated to 1273 K at a rate of 2 K/min, held for 10 h, and rapidly quenched in cold water. Subsequently, the SiO2 tubes were reheated to 953 K at a rate of 2 K/min, held for 96 h, and cooled naturally to room temperature. The obtained ingots were crushed into fine powders in an agate mortar and densified using spark plasma sintering (SPS) in an Ar atmosphere at 773 K for 10 min under an axial pressure of 40 MPa. The synthesis conditions, including heating treatment, annealing durations, and spark plasma sintering parameters, were adopted directly from the procedure reported to ensure consistency in processing.

Phase characterization was performed at 300 K by powder XRD (Ultima IV, Rigaku Co.) using Cu Kα radiation. Morphology and element distribution were investigated by using scanning electron microscopy (SEM, JEOL JSM-6500F) and energy dispersive X-ray spectroscopy (EDX, JEOL EX-2300BU). The sample’s high-temperature stability was examined via thermogravimetry/differential thermal analysis (TG-DTA, Bruker TG-DTA2000SA) under argon gas flow at 300–800 K. High-entropy crystallinity of the samples was investigated by a field-emission transmission electron microscope (FE-TEM, Thermofisher, Talos F200X G2). The Seebeck coefficient (S) and electrical conductivity (σ) were measured simultaneously in the range of 300–780 K using a commercial apparatus (ULVAC ZEM-3) under a reduced He atmosphere. The thermal conductivity (κ) was calculated from κ = α·d·C P, where α is the thermal diffusivity measured at 300–800 K using a laser flash apparatus (Netzsch LFA-467) in an Ar atmosphere. The sample density (d) was evaluated from its measured weight and dimensions. The specific heat capacity (C P) used in our thermal conductivity calculations was assumed to be equivalent to that reported for Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te. While our samples contain secondary impurity phases, previous studies on GeTe-based alloys have shown that their influence on heat capacity remains within a small margin, particularly when the impurity fraction is low. To ensure the repeatability of thermoelectric measurements, each parameter (Seebeck coefficient, electrical conductivity, and thermal conductivity) was measured three times at each temperature. The reported values represent the average results, with estimated measurement uncertainties incorporated into the data analysis. Based on the propagation of errors from individual measurements, the uncertainty in zT was estimated to be approximately ± 10%.

The Near Edge X-ray Absorption Fine Structure (XANES) measurement, performed at the Synchrotron Light Research Institute (SLRI) via the SUT-Nanotec-SLRI XAS beamline (BL5.2), was used to determine the sample’s oxidation states. The Ge K-edge (11,103 eV) was examined in transmission mode (TM-mode), while the Te L3-edge (4,341 eV) and Ag L3-edge (3,351 eV) were investigated using fluorescence mode (FL-mode). The X-ray beam dimensions were 13 mm by 1 mm in TM-mode and 20 mm by 1 mm in FL-mode. A double crystal monochromator (DCM) utilizing Si(111) for the Ge K-edge, Ge(220) for the Te L3-edge, and InSb(111) for the Ag L3-edge was used, achieving an energy resolution of 2 × 10–4 eV. Finally, the Athena program was utilized to analyze the XANES results after background subtraction in the pre-edge and postedge regions to determine the oxidation states of Ge, Te, and Ag. −

3. Results and Discussion

3.1. Sample Characterization

Figure a shows the powder XRD patterns of the high-entropy GeTe-based samples (No.1-No.4), compared with standard reference patterns of cubic GeTe (JCPDS Card No. 00-052-0849) and rhombohedral GeTe. The results revealed that the XRD patterns of all samples did not fully align with the reference patterns, indicating the presence of secondary phases. This discrepancy was likely caused by the incorporation of multiple dopant elements, including Au, Ag, Sb, Pb, and Bi, which induced complex structural changes. The overall shift in diffraction peaks to lower 2θ values reflected an increase in the lattice parameters, attributable to both the presence of larger atoms in the crystal lattice and the formation of structural defects. − Specifically, all alloying elements (Au, Ag, Sb, Pb, and Bi) possess larger atomic radii than Ge, thereby increasing the interplanar spacing of the unit cell. − In addition, lattice defects and compositional disorder can further disrupt atomic regularity, enhancing lattice expansion and contributing to the observed peak shifts. −

1.

1

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(a) Powder XRD patterns of the fabricated multielement doped high-entropy GeTe samples. (b) Detailed XRD pattern of sample No.1, with labeled peaks corresponding to GeTe-based compounds and impurity phases, which are likely formed as a result of complex multielement doping.

To further investigate the incorporation of all elements into the GeTe structure, the XRD pattern of sample No.1 was indexed using HighScore Plus analysis software based on the 2024 database, as shown in Figure b. Although various phases were detected, we focused our discussion primarily on the major identified peaks. Notably, despite introducing only a small amount of Bi into the GeTe matrix, a cubic (Ge,Bi)Te phase exhibited the highest peak intensity at 29.18°. This peak appeared at a lower angle compared to the main peak of standard cubic GeTe (29.5°) (see Figure a), which was attributed to lattice expansion resulting from the substitution of Ge atoms with larger dopant atoms, particularly Bi. Additional peaks observed at 25.2, 42.5, and 52.5° also match with Bicontaining cubic GeTe-based phases, further confirming Bi substitution in the lattice.

In addition to the primary phase, minor peaks corresponding to secondary cubic Te-based phases, such as (Ag,Sb,Pb)Te and (Ag,Pb)­Te, were observed at 27.78°, and an Au–Te phase was also detected (41.5, 43.2 and 57°). Furthermore, peaks corresponding to pure Ag appeared at 37.45 and 37.54°, indicating incomplete dissolution of Ag during heat treatment, as the processing temperature (1273 K) was only slightly above the melting point of Ag (1235 K). Overall, although sample No.1 did not achieve single-phase purity, its XRD results clearly indicated the synthesis of a predominantly cubic-phase GeTe alloy stabilized by high-entropy doping with multiple elements. Although a distinct rhombohedral GeTe phase was not observed in the XRD patterns, we cannot rule out the possibility of a minor rhombohedral fraction below the detection limit of XRD. This hypothesis is supported by DTA analysis shown in Figure , where a subtle endothermic peak near 700 K may correspond to the rhombohedral-to-cubic phase transition, as previously reported in GeTe systems.

3.

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Differential Thermal Analysis (DTA) curve of sample No.1 measured up to 800 K under argon gas flow, showing a subtle endothermic peak near 700 K that may indicate a minor phase transformation.

Interestingly, the current results show that the presence of Au leads to the formation of multiple secondary phases rather than a single-phase GeTe structure. This observation suggests that Au may reduce the solubility limitation of other dopant elements within the GeTe lattice, thereby promoting phase separation and the formation of compositionally complex phases. As a result, the XRD patterns do not align well with those of standard GeTe, supporting the conclusion that these materials are not pure GeTe but rather GeTe-based high-entropy compounds incorporating Ag, Sb, Pb, Bi, and Au. In the context of this study, samples No.1-No.4 are referred to as GeTe-based high-entropy alloys.

3.2. Thermoelectric Properties

Figure summarizes the measured thermoelectric properties of samples No.1-No.4 as functions of temperature, compared with those of GeTe and Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te (S9). Despite the presence of secondary impurity phases, the samples exhibited higher Seebeck coefficients, reduced thermal conductivity, and enhanced zT values compared to those of undoped GeTe. This indicates that the dominant high-entropy GeTe phase is the primary contributor to these improvements, and that the impurity phases do not significantly degrade the overall thermoelectric performance. To evaluate the thermoelectric performance of our high-entropy GeTe samples, we compared key properties with those of the S9 sample reported by Jiang et al. While S9 contained a relatively homogeneous cubic GeTe phase, our samples incorporated additional dopants, leading to impurity phases. Consequently, this comparison was not intended to imply equivalence in phase compositions but rather to provide insights on the effects of high-entropy engineering on the thermoelectric properties of GeTe.

2.

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Temperature dependence of of (a) S, (b) σ, (c) PF, (d) κ, (e) κlat, and (f) zT of samples No.1-No.4, together with those of pure GeTe and Ge0.61Ag0.11Sb0.13Pb0.12Bi0.01Te (S9). The data points represent the average values from three independent measurements.

In Figure a, the Seebeck coefficients for all samples were positive, indicating p-type conduction, and they increase with temperature, with sample No.2 exhibiting the highest value overall. At lower temperatures, our samples showed lower Seebeck coefficients compared to the S9 sample. This may have been attributed to a higher carrier concentration and reduced band convergence caused by compositional inhomogeneity and the introduction of Au, which modified the band structure and introduced impurity states. The sharp decrease in electrical conductivity (Figure b), accompanied by an increase in the Seebeck coefficient around 700 K, suggested possible changes in carrier concentration or energy filtering effects. , However, in the absence of Hall effect measurements, this interpretation remained speculative, and alternative explanations such as enhanced carrier scattering or structural transitions involving impurity phases cannot be ruled out.

As shown in Figure b, electrical conductivity decreased with increasing temperature, consistent with the reduction in carrier mobility at high temperatures. As expected, samples No.1-No.4, as well as S9, which incorporated multiple alloying elements, naturally exhibited much lower electrical conductivity compared to pure GeTe. This reduction may be due to both a decrease in carrier mobility caused by impurity and grain boundary scattering, and a possible reduction in carrier concentration. Further investigation, such as Hall effect measurements, would be required to quantitatively distinguish between these two effects. Additionally, the introduction of Au in our samples contributed to increased structural disorder and the formation of minor impurity phases, further reducing electrical conductivity relative to the more homogeneous S9 sample. These observations were consistent with the XRD and SEM-EDX results, which will be discussed in detail in the following section. The power factor in Figure c increased with temperature, showing a significant enhancement around 700 K, with sample No.1 outperforming the others, achieving a maximum value of 1.64 mW m–1 K–2 at 730 K. However, the power factor (PF) of all samples in this study was lower than that of pure GeTe and S9, primarily due to their lower Seebeck coefficient compared to S9 and lower electrical conductivity compared to pure GeTe.

The total thermal conductivity of samples No.1-No.4 remained relatively stable, ranging from 0.8 to 1.0 W m–1 K–1 across the measured temperature range, as shown in Figure d. These values were nearly 1 order of magnitude lower than those of undoped GeTe, primarily due to the reduced electronic thermal conductivity of our samples. Additionally, Figure e revealed that the lattice thermal conductivity of our cubic-GeTe samples was remarkably low and remained nearly constant, comparable to those of undoped rhombohedral GeTe, despite cubic phases generally exhibiting higher lattice thermal conductivity. This indicates that our multielement doping strategy effectively suppresses lattice thermal transport by significantly enhancing phonon scattering through mass and strain field fluctuations, as well as lattice distortions.

The ultralow lattice thermal conductivity observed in all samples can be attributed to several synergistic mechanisms. First, high-entropy doping with Au, Ag, Sb, Pb, and Bi introduced significant mass and strain field fluctuations, effectively scattering phonons. Second, structural disorder confirmed by high-resolution TEM and Inverse fast Fourier transformed (IFFT) images contributes to this effect. The irregular lattice orientation arose from (1) overlapping fringe patterns, likely Moiré effects from nanoscale precipitates or secondary phases embedded in the GeTe matrix, and (2) variation in d-spacing due to atomic size mismatch among the dopants. Additionally, a subtle phase transformation near 700 K, evidenced by DTA, may cause dynamic defect restructuring or interfacial scattering, further reducing phonon transport. Grain boundary scattering and secondary impurity phases, as revealed by SEM-EDX and XRD, also likely served as additional phonon scattering centers at high temperatures. These combined effects contributed to the consistently low lattice thermal conductivity values across the studied temperature range. The supporting microstructural evidence from TEM, IFFT, SEM, and EDX analyses will be discussed in detail in the following sections. Among all samples, sample No.1 achieves the lowest thermal conductivity at high temperatures, reaching an ultralow value of 0.22 W·m–1·K–1 at 780 K.

As shown in Figure f, the zT values increased with temperature for all prepared samples, indicating that they are promising thermoelectric materials at high temperatures due to the combination of high Seebeck coefficient, moderate electrical conductivity, and suppressed thermal transport. Among these, sample No.1 (Ge0.59Au0.02Ag0.11Sb0.13Pb0.12Bi0.01Te) stood out as the most promising, exhibiting the highest zT value of 2.0 at 780 K, which was significantly higher than that of pure GeTe while slightly lower than S9 due to the larger number of impurities. Overall, this study demonstrates that the thermoelectric properties of GeTe can be efficiently enhanced through the high-entropy approach, even with the incorporation of up to five alloying elements. However, while some impurity-related phase transformations may contribute to enhanced thermoelectric properties at specific temperatures, they also introduce compositional and structural variability. Therefore, minimizing uncontrolled impurity phases remains important for achieving reproducible and consistently high zT across the entire operating range. The high-entropy approach introduces controlled atomic-scale disorder that improves phonon scattering and reduces lattice thermal conductivity. However, the abrupt changes in Seebeck coefficient, electrical conductivity, and thermal conductivity observed around 700–750 K were likely associated with thermally activated transformations of secondary phases, which are not a direct result of the high-entropy design but rather a byproduct of multielement interactions during synthesis. While the thermoelectric properties reported here are based on measurements repeated three times at each temperature during a single heating cycle, this study did not include thermal cycling tests (i.e., repeated heating–cooling runs). As such, the long-term operational stability of the materials under cyclic thermal conditions remains to be investigated.

3.3. Structural Analysis

3.3.1. TG-DTA Results

As shown in Figure , the DTA data revealed a subtle endothermic peak near 700 K, which we interpreted as a potential phase transformation. Although a clear rhombohedral phase was not distinctly observed in the XRD patterns of sample No.1, the presence of a small amount of rhombohedral GeTe-based impurity cannot be ruled out. Therefore, the observed endothermic peak may be associated with a phase transition of this trace rhombohedral phase to the cubic phase at elevated temperatures. Notably, these phase transformations coincided with the temperatures at which the Seebeck coefficient increased and the lattice thermal conductivity decreased in Figure a–e, suggesting that they may arise from impurity-related phases introduced by the combined effects of multielement doping (Ag, Au, Sb, Pb, and Bi), which potentially contributed to the observed enhancement in the sample’s thermoelectric properties at elevated temperatures.

3.3.2. FE-TEM and EDX results

The SEM image and EDX mappings of sample No.1 are shown in Figure , with the detected atomic fractions of Ge, Te, and dopant elements (Au, Ag, Sb, Pb, and Bi) listed in Table . The EDX mappings revealed a distinct Ge-rich impurity phase embedded within the Ge–Te matrix, which may influence the sample’s thermoelectric properties, as discussed in the previous section. The observed Ge segregation may be attributed to the oxidation of Ge, forming GeO2, which can separate from the matrix during sintering. In support of this, the main peak of GeO2 appeared near 26.6° in the XRD pattern, which overlaps with peaks from other phases, making it difficult to distinguish definitively. Moreover, the absence of other secondary phases in the EDS mapping, despite their presence in the XRD results, can be explained by several factors: (1) Some secondary phases involve dopants like Au and Bi, which are present in very low concentrations, making them difficult to detect in EDS. (2) The quantity of secondary phases is small compared to the dominant GeTe-based matrix, as shown in the XRD analysis. (3) These phases may be finely dispersed or exist in very small regions that are not captured in the localized EDS mapping. (4) Signal overlap in EDS due to proximity of characteristic peaks makes it challenging to distinguish individual elements clearly. The composition of the matrix was qualitatively determined to be Ge0.45Au0.01Ag0.08Sb0.08Pb0.15Bi0.004Te, which was overall consistent with its original nominal composition, with minor deviations likely attributable to the inherent difficulties in quantitatively detecting small amounts of dopants using EDX, as well as the formation of the Ge-rich impurity. Despite the presence of the impurity phase, EDX results confirmed that the constituent elements were uniformly distributed within both the impurity phase and the Ge–Te matrix.

4.

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Low-magnification scanning electron microscopy (SEM) image and energy dispersive X-ray (EDX) mapping of sample No.1.

2. Elemental Composition (Atom %) of GeTe sample No.1 Evaluated by Energy-Dispersive X-ray Spectroscopy (EDX).
Ge Au Ag Sb Pb Bi Te
0.257 0.01 0.043 0.046 0.083 0.002 0.569
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The influence of high-entropy doping on the atomic orientation in sample No.1 was clearly visible in the TEM images in Figure a–c, where a disruption in the atomic arrangement was observed at the center of these images. To further analyze these areas, the inverse fast Fourier transform (IFFT) was applied to the selected regions in Figure a–c, and the transformed images are shown in Figure b,d, respectively. The observed irregular lattice orientation was possibly caused by two factors: (1) some overlapping fringe patterns could be attributed to Moiré effects induced by nanoscale precipitates or secondary particles embedded within the GeTe matrix. These patterns may arise due to the presence of multiple dopants (Au, Ag, Sb, Pb, Bi), leading to local phase segregation or compositional inhomogeneities at the nanoscale. (2) The variation in atomic sizes within the structure, resulting from the high-entropy doping at the Ge sites of GeTe, leads to differences in the d-spacing of the crystal planes, as shown in Figure b. This variation in d-spacing induces a misalignment in the atomic layers, leading to stacking faults and dislocations within the microstructure, as indicated by the arrows in Figure b,d. Such misalignment and the resulting defects are commonly observed in GeTe-based alloys and other similar materials. −

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(a) and (c) High-resolution transmission electron microscopy (TEM) images of sample No.1. (b) and (d) Inverse fast Fourier transformed (IFFT) images corresponding to the selected areas in (a) and (c), respectively, illustrating lattice distortions and stacking faults induced by high-entropy doping.

3.3.3. Synchrotron-Based XANES Analysis

To verify the substitution at the Ge site, synchrotron-based XANES analysis was conducted to examine the oxidation state variations of Ge, Te, and Ag in the fabricated samples. In Figure a, the absorption threshold energy (E 0), which corresponds to the first allowed transition in the absorption spectrum, was positioned very close to that of Ge2+, indicating the presence of Ge2+ as summarized in Table . The energy position at the maximum white line (WL) intensity (E max) corresponded to the 1s-4p dipole transition. Similar postedge features were observed, which correspond to transitions toward p-like partially filled states and multiple scattering resonances in Ge. , The variations in WL intensity among the samples may be attributed to alterations in the local coordination environment surrounding the Ge atoms, caused by the multielement doping. These changes can lead to either a downward or upward shift in the measured peak (or shoulder) position, as shown in Figure a. ,

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XANES spectra of the samples and reference materials at the (a) Ge K-edge, (b) Te L3-edge, and (c) Ag L3-edge. The insets highlight the energy regions around the white lines, indicating potential variations in oxidation states and local coordination environments.

3. XANES Parameters of No.1-No.4, Standard GeO2, and Ge2+.
sample E0 (±0.1 eV) Emax (eV) WL intensity oxidation
standard GeO2 11,107.2 11,112.1 1.468 +4
No.1 11,101.5 11,107.6 1.411 ∼ +2
No.2 11,102.2 11,109.0 1.461 ∼ +2
No.3 11,101.6 11,107.9 1.346 ∼ +2
No.4 11,100.4 11,107.3 1.396 ∼ +2
Ge2+ 11,101.7     +2
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a

E 0 and E max refer to the absorption threshold energy and the energy position at the XANES white line (WL) intensity maximum, respectively, both determined from the first derivative. The WL intensity (the first resonance after the edge) is expressed in units of the atomic edge jump of Ge.

For Te, its L3-edge XANES data reflect electron transitions from the 2p3/2 core level to unoccupied states in the 6s and 5d orbitals. In contrast to the spectra of TeO2 (Te4+) and H6TeO6 (Te6+), our measured Te L3-edge XANES spectra for high-entropy GeTe lacked the pre-edge peak detected around 4,348 eV due to orbital hybridization, as illustrated in Figure b. This absence of the pre-edge feature implies the lack of low-energy unoccupied states for electronic transitions in the GeTe samples. Consequently, this suggests that, while not definitive, Te is mostly likely in a reduced oxidation state of Te2–, contributing to the stabilization of the alloy. For Ag, the XANES spectra of samples No.1 and No.2 were comparable and appeared to align with the Ag foil reference sample, as shown in Figure c, suggesting an oxidization state of Ag0. Finally, while noise appeared around 3,345 eV in the XANES spectrum of sample No.3, likely due to its relatively low Ag content, the spectral pattern of Ag closely matched that of AgNO3, suggesting an oxidation state of Ag1+.

4. Conclusions

This study demonstrates the successful application of high-entropy doping to optimize the thermoelectric properties of GeTe-based alloys. By introducing multiple dopants (Au, Ag, Sb, Pb, and Bi) into the GeTe matrix, we achieved a high degree of atomic disorder, as evidenced by lattice distortions, stacking faults, and broadened XRD peaks. Synchrotron-based XANES analysis confirmed the substitution behavior and oxidation states of key elements, supporting the presence of Ge2+ and Te2– in the matrix. Sample No.1 (Ge0.59Au0.02Ag0.11Sb0.13Pb0.12Bi0.01Te) exhibited the best performance, with an ultralow lattice thermal conductivity of 0.22 W m–1 K–1 and a peak zT of 2.0 at 780 K. The observed performance improvement was attributed to enhanced phonon scattering and the effects of high-entropy-induced disorder. The Seebeck coefficient showed a significant increase at elevated temperatures, particularly above 700 K. We speculate this enhancement is likely driven by a reduction in carrier concentration caused by band structure modifications and low-energy carrier filtering effects associated with impurity phases. Although impurity phases were observed, they did not significantly degrade thermoelectric performance and may contribute positively through subtle phase transformations. Together, these findings highlight the promise of high-entropy engineering as a strategy for developing advanced, high-performance thermoelectric materials based on GeTe.

Acknowledgments

This project is funded by National Research Council of Thailand (NRCT): Contract Number N42A660898.

The authors declare no competing financial interest.

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