Unraveling the Origin of Glassy Thermal Transport in Medium-Entropy Semiconductors: From Nanoscale Phase Segregation to Atomic-Scale Lattice Distortion
Year: 2025
Type: article
Abstract: Thermoelectric (TE) materials can directly and reversibly convert heat into electricity, offering a promising pathway for reshaping the global energy landscape. The performance of thermoelectric materials is evaluated using the figure of merit, zT = S2σT/κ, which can be optimized by improving Seebeck coefficient (S) and electrical conductivity (σ) while suppressing the thermal conductivity (κ) at the operating temperature (T). We introduce a medium-entropy metal chalcogenide that exhibits phonon-glass electron-crystal behavior. The conventional perspective hypothesizes that the suppression of thermal transport arises solely from disordered atomic occupation, which enhances alloy scattering mechanisms for phonon transport [1]. However, emerging evidence suggests the presence of structural and chemical inhomogeneity at multiple length scales in entropy-engineered materials, a phenomenon that has rarely been investigated in detail. We investigate the medium-entropy metal chalcogenide at two distinct processing stages—water-quenched (WQ) and heat-treated (HT)—and systematically characterize the evolution of its structural features. X-ray diffraction (XRD) analysis confirms a single-phase nature with a globally average rock-salt structure in both WQ and HT samples. Similarly, time-of-flight secondary ion mass spectrometry (ToF-SIMS) reveals a three-dimensional homogeneous chemical distribution with an approximate 100 nm spatial resolution. In contrast, in the WQ sample, scanning/transmission electron microscopy (S/TEM) analysis (Fig. 1) identifies the formation of needle-shaped nanoprecipitates that grow endotaxially with respect to the matrix, with their {100} lattice planes aligning (Fig. 1b). As shown in Fig. 1c, energy-dispersive X-ray spectroscopy (EDS) further confirms that these nanoprecipitates are compositionally enriched in silver. Atomic-resolution high-angle annular dark-field (HAADF) imaging indicates that the nanoprecipitates form through the incorporation of silver atoms into tetrahedral interstitial sites (Fig. 1d). These nanoprecipitates can be indexed to be β-Ag2Te structure type with a space group of Fm-3m. This incorporation induces local lattice expansion, as revealed by geometric phase analysis (Fig. 1e). The resulting strain fields are known to enhance phonon softening. In addition to the needle-shaped nanoprecipitates, differential phase contrast (DPC) phase reconstruction (Fig. 1f) reveals the presence of individual silver atoms. These atoms diffuse out of the supersaturated matrix and aggregate into energetically favorable needle-shaped configurations. Heat treatment further drives the evolution of the nanostructure. Fig. 2-3 present 4D-STEM results for the HT sample, where Fig. 2 highlights three distinct classes of nanobeam diffraction patterns observed in the material [2]. The nanoscale phase segregation is indexed to the α-Ag2Te structure type with a space group of P2₁/c, while maintaining an endotaxial relationship with the matrix. The virtual dark-field (DF) image Fig. 3a reveals the spatial distribution of nanophase segregation, exhibiting a blob-like morphology rather than sharp interfaces, with sizes ranging from 10 to 50 nm. Significant lattice expansion (Fig. 3b) is evident at the boundary of the segregated phases, inducing strain fluctuations (∼5%) with a periodicity of approximately 10-20 nm. Additionally, small lattice rotations (∼1-2°) are observed at phase interfaces. These nanoscale strain fluctuations strongly enhance phonon scattering and softening, leading to suppressed lattice thermal conductivity. Beyond phase segregation, lattice distortion within the matrix further contributes to phonon softening. Multislice electron ptychography (Fig. 3c-e) reconstructs the phase image of the matrix, where atomic displacement mapping reveals nearly isotropic displacements with magnitudes on the order of 10 pm. The identified nanophase segregation and atomic displacements uncover the hidden structural complexity within entropy-engineered thermoelectrics, which were previously considered homogeneous single-phase materials. These findings challenge conventional explanations for low lattice thermal conductivity, providing new insights into engineering the phonon transport [3]. Nanostructure of water-quenched medium-entropy semiconductor. (a) Bright-field (BF) image. (b) Selected area electron diffraction (SAED) pattern along the [001] zone axis. (c) High angle annular dark-field (HAADF) image and corresponding energy dispersive spectroscopy (EDS) map. (d) Atomic-resolution HAADF image on the endotaxial precipitates with simulated HAADF image as an inset. (e) Vertical deformation in the area from (d), revealed by the geometric phase analysis (GPA). (f) Differential phase contrast (DPC) reconstructed phase image of endotaxial precipitates and interstitial point defects. The dashed lines highlight the individual interstitial point defects (4D STEM data collected with DECTRIS ARINA detector operated at 30 kHz in a JEOL ARM200CF). Convergent beam electron diffraction (CBED) patterns from matrix phase and nanophase segregation extracted from 4D STEM dataset acquired using Gatan K3 detector at JEOL ARM300. Nano- and atomic structure of heat-treated AgMnSbPbTe4. (a) Virtual dark-field (DF) images highlighting the spatial distribution of nanophase segregation and the corresponding virtual DF detectors. (b) Strain maps (εxx, εyy, εxy, and εrot) on the same area shown in (a). (c) Multislice electron ptychography reconstructed phase on the matrix phase of AgMnSbPbTe4. (d) Atomic displacement map based on reconstructed phase image. (e) Atomic displacement for each atomic column compared to reference lattice. (4D STEM data for electron ptychography collected with DECTRIS ARINA detector operated at 60 kHz in a JEOL ARM200CF).
Source: Microscopy and Microanalysis
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W4412910565