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Critical phenomena are one of the most captivating areas of modern physics, whereas the relevant experimental and theoretical studies are still very challenging. Particularly, the underlying mechanism behind the anomalous thermoelectric properties during critical phase transitions remains elusive, i.e., the current theoretical models for critical electrical transports are either qualitative or solely focused on a specific transport parameter. Herein, we develop a quantitative theory to model the electrical transports during critical phase transitions by incorporating both the band broadening effect and carrier-soft TO phonon interactions. It is found that the band-broadening effect contributes an additional term to Seebeck coefficient, while the carrier—soft TO phonon interactions greatly affects both electrical resistivity and Seebeck coefficient. The universality and validity of our model are well confirmed by experimental data. Furthermore, the features of critical phase transitions are effectively tuned. For example, alloying S in Cu2Se can reduce the phase transition temperature but increase the phase transition parameter b. The maximum thermoelectric figure of merit zT is pushed to a high value of 1.3 at the critical point (377 K), which is at least twice as large as those of normal static phases. This work not only provides a clear picture of the critical electrical transports but also presents new guidelines for future studies in this exciting area. The authors develop a quantitative theory to model and tune the electrical transports during critical phase transitions by incorporating both the band broadening effect and carrier-soft TO phonon interactions. [ABSTRACT FROM AUTHOR]

Ge-Sb-Te compounds (GST), the well-known phase-change materials, are considered to be promising thermoelectric (TE) materials due to their decent thermoelectric performance. While Ge2Sb2Te5 and GeSb2Te4 have been extensively studied, the TE performance of GeSb4Te7 has not been well explored. Reducing the excessive carrier concentration is crucial to improving TE performance for GeSb4Te7. In this work, we synthesize a series of Se-alloyed GeSb4Te7 compounds and systematically investigate their structures and transport properties. Raman analysis reveals that Se alloying introduces a new vibrational mode of GeSe2, enhancing the interatomic interaction forces within the layers and leading to the reduction of carrier concentration. Additionally, Se alloying also increases the effective mass and thus improves the Seebeck coefficient of GeSb4Te7. The decrease in carrier concentration reduces the carrier thermal conductivity, depressing the total thermal conductivity. Finally, a maximum zT value of 0.77 and an average zT value of 0.48 (300–750 K) have been obtained in GeSb4Te5.5Se1.5. This work investigates the Raman vibration modes and the TE performance in Se-alloyed GeSb4Te7 sheddinglight on the performance optimization of other GST materials. [ABSTRACT FROM AUTHOR]

Diamondlike compound Cu2SnS3 has attracted great attention recently due to its excellent thermoelectric performance and the features of being eco-friendly and low cost. However, the underlying mechanism for its good electrical transports under high carrier concentration range is still not clear. In this work, we synthesized a series of Cu2+xSn1−xS3 (x = 0–0.08) samples. These nonstoichiometric Cu2+xSn1−xS3 samples are polymorph simultaneously crystalizing in the monoclinic, tetragonal, and cubic structures. The characterization on the electrical transports and the analysis on the band structure reveal that the large density-of-states effective mass, high Seebeck coefficient, and high mobility under a high carrier concentration in the Cu2+xSn1−xS3 system are originated from the existence of multiple bands near the edge of the valence band. Likewise, the polymorphic structure has little influence on the electrical transports. A maximum power factor of 12.6 μW cm−1 K−2 at 700 K has been obtained for the nonstoichiometric Cu2.08Sn0.92S3. Combining the low lattice thermal conductivity of these samples, Cu2.08Sn0.92S3 shows a peak dimensionless figure of merit of 0.5 at 700 K. This work is helpful for guiding the future optimization of thermoelectric performance on Cu2SnS3. [ABSTRACT FROM AUTHOR]

As the pseudo-binary alloys between GeTe and Sb2Te3, GeSbTe-based compounds are promising thermoelectric materials. Although Ge2Sb2Te5 and GeSb2Te4 have widely been studied, the thermoelectric properties of GeSb4Te7 have not been well understood yet. In this work, we design a series of GeSb4-xBixTe7 solid solutions and systematically study the variation in the crystal structure, electrical, and thermal transport properties. Alloying Bi effectively reduces the carrier concentration and, thus, enhances the Seebeck coefficient. Meanwhile, the thermal conductivity is greatly reduced via large mass fluctuations. A maximum zT of 0.69 at 550 K and an average zT value of 0.52 between 300 and 700 K have been achieved for GeSb1.5Bi2.5Te7. These findings will promote the understanding and development of GeSbTe-based thermoelectric materials. [ABSTRACT FROM AUTHOR]

Coupling of a phase transition to electron and phonon transports provides extra degree of freedom to improve the thermoelectric performance, while the pertinent experimental and theoretical studies are still rare. Particularly, the impaction of chemical compositions and phase transition characters on the abnormal thermoelectric properties across phase transitions are largely unclear. Herein, by varying the Cu content x from 1.75 to 2.10, we systemically investigate the crystal structural evolution, phase transition features, and especially the thermoelectric properties during the phase transition for Cu x Se. It is found that the addition of over-stoichiometry Cu in Cu x Se could alter the phase transition characters and suppress the formation of Cu vacancies. The critical scatterings of phonons and electrons during phase transitions strongly enhance the Seebeck coefficient and diminish the thermal conductivity, leading to an ultrahigh dimensionless thermoelectric figure of merit of ∼1.38 at 397 K in Cu2.10Se. With the decreasing Cu content, the critical electron and phonon scattering behaviors are mitigated, and the corresponding thermoelectric performances are reduced. This work offers inspirations for understanding and tuning the thermoelectric transport properties during phase transitions. [ABSTRACT FROM AUTHOR]

Thermoelectric materials (TMs) can uniquely convert waste heat into electricity, which provides a potential solution for the global energy crisis that is increasingly severe. Bulk Cu 2 Se, with ionic conductivity of Cu ions, exhibits a significant enhancement of its thermoelectric figure of merit zT by a factor of ~3 near its structural transition around 400 K. Here, we show a systematic study of the electronic structure of Cu 2 Se and its temperature evolution using high-resolution angle-resolved photoemission spectroscopy. Upon heating across the structural transition, the electronic states near the corner of the Brillouin zone gradually disappear, while the bands near the centre of Brillouin zone shift abruptly towards high binding energies and develop an energy gap. Interestingly, the observed band reconstruction well reproduces the temperature evolution of the Seebeck coefficient of Cu 2 Se, providing an electronic origin for the drastic enhancement of the thermoelectric performance near 400 K. The current results not only bridge among structural phase transition, electronic structures and thermoelectric properties in a condensed matter system, but also provide valuable insights into the search and design of new generation of thermoelectric materials. [ABSTRACT FROM AUTHOR]

Thermoelectric materials require not only high performance to maximize the energy conversion efficiency but also good mechanical properties to guarantee machinability and reliable operation. It is usually hard to embrace both at once. Herein, we demonstrated the entropy engineering as a promising avenue to realize both exceptional thermoelectric performance and robust mechanical properties in multicomponent alloys Cu 2- y Ag y Te 1-2 x S x Se x. Entropy engineering by mixing multiple elements stabilizes the high-symmetry hexagonal structure, extends the solubility limit of Ag, and concurrently lessens the phase transition numbers. Furthermore, with co-alloying of S/Se and Ag in Cu 2 Te, the carrier concentration is largely reduced while the effective mass is enhanced, yielding higher Seebeck coefficient and power factor. Owing to the strong phonon scattering by lattice disorder, the thermal conductivity is decreased by one order of magnitude, reaching 0.29 W m−1 K−1 at room temperature, which is even lower than the amorphous limit. A state-of-the-art peak zT of 1.4 and average zT of 0.74 are achieved in Cu 1.9 Ag 0.1 Te 0.6 S 0.2 Se 0.2. Moreover, the mechanical properties are significantly improved in virtue of the entropy engineering strengthening effect, making it more promising for thermoelectric applications. Record-high average zT of 0.74 and remarkably improved compressive strength were achieved in multicomponent alloys Cu 2- y Ag y Te 1-2 x S x Se x through entropy engineering [Display omitted]. [ABSTRACT FROM AUTHOR]