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Cu2Se is a promising thermoelectric (TE) material due to its low cost, Earth abundance, and high thermoelectric properties. However, the biggest problem of Cu2Se is its unstable chemical properties. In particular, under the action of an electric field or gradient temperature field, the chemical potential of copper ions inside the material increases. When the external field is strong enough, the chemical potential of copper ions at the negative end of the material reaches the chemical potential of elemental copper. Under these conditions, copper ions must precipitate out, causing Cu2Se to be unstable, and making it unsuitable for use in applications. In this study, we prepared Cu2−xMnxSe (x = 0, 0.02, 0.04 and 0.06) series bulk materials by vacuum melting–annealing and sintered by spark plasma sintering (SPS). We investigated the effects of Mn doping on the composition, microstructure, band structure, scattering mechanism, thermoelectric properties, and stability of Cu2Se. The results show that Mn doping can adjust the carrier concentration, promote the stabilization of the β-phase structure and improve the electrical properties of Cu2Se. When x = 0.06, the highest power factor (PF) value of Cu1.94Mn0.06Se at 873 K was 1.62 mW m−1 K−2. The results of carrier scattering mechanism analysis based on the conductivity ratio method show that the sample doped with Mn and pure Cu2Se had the characteristics of ionization impurity scattering, and the scattering factor was 3/2. However, the deterioration in thermal conductivity was large, and a superior zT value needs to be obtained. The cyclic test results of high-temperature thermoelectric properties show that Mn doping can hinder Cu+ migration and improve its thermoelectric stability, which preliminarily verifies the feasibility of using the stable zirconia mechanism to improve the thermoelectric stability of Cu2Se. [ABSTRACT FROM AUTHOR]

• Many locally nano-domains exist in the high pressured NaCl-type SnTe binary compounds. • The domain phases are the CuPt and CuAu-I-type variants evolved from the NaCl-type matrix phase. • The formations of the domain phase are related to locally off-stoichiometric of SnTe and strong inner-stress in the lattices. • The domain phase can inhibit the phonon relaxation time by orders of magnitude over a wide phonon frequency range. • The domain phase benefits to a wide frequency phonon scattering network and thus a low lattice thermal conductivity. The phonon transport is intensively related to microstructure defects. Herein, a unique high-pressure sintering technology (GPa-level) was applied to modify the microstructure of pristine SnTe. There are many nano-domains exist in the high pressured NaCl-type SnTe binary compounds. The fast Fourier transform images show the domain phases have two sets of reflections with comparable intensity and exhibit doublet periodicities of special lattice planes, which demonstrates the domain phases are the CuPt and CuAu-I-type variants evolved from the NaCl-type matrix phase. An in-depth investigation indicates the constructions of the domain phase are related to locally off-stoichiometric of SnTe and the introduced strong inner-stress during high pressure sintering process. Additionally, a calculation of the phonon transport discloses the nano-domains restrict the phonon relaxation time by orders of magnitude over a wide phonon frequency range. As a result, a wide frequency phonon scattering network can be established to lower the lattice thermal conductivity. [ABSTRACT FROM AUTHOR]

The resonance effect has been used as a theoretical strategy in enhancing the thermoelectric performance. However, its origin and mechanisms are still ambiguous, which complicates the rapid screening of resonance dopants and thereby hinders the improvement of the thermoelectric performance. In this work, we attempt to disclose the mechanism of resonance effect and propose a straightforward method to screen the resonance dopants suitable for SnTe. Hence, two triggering conditions of achieve the resonance effect are proposed. One is the energy correlation between impurity and host states. The other is the position correlation between impurity state level and Fermi level. Based on the two conditions, a variety of elements (including Y, Ru, In, Sb, La, Gd, Lu, Os, Tl and Bi) are selected as the potential resonance dopants suitable for SnTe. Among these elements, we herein re-study the effect of Bi doping on the thermoelectric performance of SnTe. Consequently, a maximum PF of ∼32.46μWcm−1K−2 and the optimal ZT of ∼1.23 were achieved in Sn 0 · 94 Bi 0 · 04 Te at 873 K due to a high Seebeck coefficient and low thermal conductivity after Bi doping. • The mechanism of RE was disclosed and two triggering conditions were proposed. • A straightforward method for selecting effective resonance dopants was put forward based on the two conditions. • The elements Y, Ru, In, Sb, La, Gd, Lu, Os, Tl and Bi were screened as effective resonance dopants for SnTe. • The power factors after Bi doping were significantly improved due to Bi induced RE (from ∼20.86 to ∼34.46 μWcm−1K−2). • The value of ZT was enhanced over ∼100% than pristine SnTe to ∼1.23 at 873 K via Bi doping. [ABSTRACT FROM AUTHOR]

Defect equation: 2 S b S n ∙ + V S n ' ' = 3 S n S n ×. • Fine-turning the carrier concentration via charge-balanced compensation-doping. • Preserving the composition of primary phase in presence of secondary phase. • Optimized density of states enhances the Seebeck coefficient. • Sn vacancies strengthen phonon scattering and reduce lattice thermal conductivity. • A state-of-the-art figure-of-merit of ~1.1 is achieved at 873 K for Sn 0.88 Sb 0.08 Te. A synergy of fine-tuning the carrier concentration and implementing scatter of heat-carrying phonons is vital to the thermoelectric study of narrow bandgap semiconductors. In this work, we adopted a synergistic charge-balanced compensation-doping approach derived from the interplay between heterovalent Sb dopants and Sn vacancies in narrow band gap semiconductor SnTe, a lead-free alternative to the classic thermoelectric material PbTe. Specifically, we designed a composition series of Sn 1- x Sb 2 x /3 Te (0 ≤ x ≤ 0.20 in steps of 0.02), in which every three Sn2+ are substituted by two Sb3+ and one Sn vacancy. To the first order, such chemical composition does not contribute net charge carriers in the context of electron counting and also ensures that the composition of the primary phase is not altered by the formation of secondary phase Sb 2 Te 3 , which greatly simplifies the data analysis. Interestingly, we found that the measured carrier concentration gradually reduced with increasing Sb content and in turn increased the Seebeck coefficient. Meanwhile, the coexistence of Sb dopants and Sn vacancies effectively scattered heat-carrying phonons at elevated temperatures. Consequently, a peak ZT value of ~1.1 at 873K was achieved in the x = 0.12 sample. These results offer a new avenue for thermoelectric study of narrow bandgap semiconductors. [ABSTRACT FROM AUTHOR]

[Display omitted] • Realizing a phase transition from orthorhombic to rhombohedral and then to cubic. • The enhanced crystal symmetry and off-stoichiometric doping manipulate Ge vacancy. • Achieving a delicate balance between carrier concentration and carrier mobility. • The introduction of multiple phonon scattering reduces the thermal conductivity. • The maximum zT value of 0.62 at 573 K is achieved in cubic GeSeTe 0.15 (InSnTe 2) 0.2. In thermoelectrics, the native point defect enables the delicate balance between carrier concentration and carrier mobility. The proper native point defect is prerequisite for the carrier concentration optimization. However, the carrier mobility is inevitable to be affected in case of the carrier mean free path approaches to the size of point defects. Herein, we reported the stepwise Ge vacancy manipulation as a point defect engineering to realize the zT enhancement in cubic GeSe. The pristine orthorhombic GeSe has the ultralow zT of 0.05 due to the inferior carrier concentration arising from the low Ge vacancy concentration, as well as the low carrier mean free path. Hence, we adopted InSnTe 2 alloying to stabilize the cubic phase at ambient conditions and significantly increase the Ge vacancy concentration due to the generation of Ge precipitates, thereby increasing the carrier concentration by several orders of magnitude but damaging the carrier mobility. Subsequently, the moderate Te doping was adopted to make the Ge-precipitations to be dissolved into the matrix, thus mitigating the strong carrier scattering deriving from the Ge vacancy and enhancing the carrier mobility to some extent. Combined with the restrained thermal conductivity originating from the multiscale defects and the reduced sound velocity, the maximum zT of 0.62 at 573 K was achieved in cubic GeSeTe 0.15 (InSnTe 2) 0.2. These results demonstrated that the stepwise point defect manipulation is an effective strategy for enhancing the figure of merit in thermoelectric materials with intrinsic low carrier mobility. [ABSTRACT FROM AUTHOR]

It work provides a simple and effective doping strategy to obtain high-temperature cycle service stability of Cu 2 Se-based thermoelectric materials while maintaining their intrinsic excellent thermoelectric properties. Cu 2-x Cd x Se (x = 0, 0.02, 0.04, and 0.06) block materials were synthesized by vacuum melting annealing and spark plasma sintering (SPS). The effects of Cd doping on the composition, microstructure, band structure, and thermoelectric properties of Cu 2 Se were investigated. The development of Cd doping on the thermoelectric stability of Cu 2 Se was studied by four high-temperature cycles. The results show that Cd doping can reduce Cu + migration and improve its thermoelectric stability. The dissolution of Cd in Cu 2 Se is limited, and a second phase of CdSe is found in samples with high Cd content (x = 0.04 and 0.06). The second phase CdSe and element segregation (copper-rich phase) in the x = 0.06 sample enhance phonon scattering and reduce lattice thermal conductivity (κ l). Therefore, the maximum ZT value of the Cu 1 · 94 Cd 0 · 06 Se sample at 873 K (about 1.37) is about 1.02 times higher than that of the undoped sample (about 1.34). The research will accelerate the practical application of liquid thermoelectric materials and provide the possibility of Cu 2 Se series thermoelectric materials in the use of waste heat power generation in the middle-temperature area, which has great application prospects. • There is a second CdSe phase and the segregation of elements (copper-rich phase), enhancing the phonon scattering and reducing κ l. • The Cu 1 · 94 Cd 0 · 06 Se sample is about 1.02 times higher at 873 K (about 1.37) than that of the undoped sample (about 1.34). • The Cd impurity sample and pure phase Cu 2 Se have the exact the scattering characteristic of ionized impurities. • The high-temperature cycle stability is better than that of Cu 2 Se. [ABSTRACT FROM AUTHOR]

Herein the effect of Co element doping on the thermoelectric properties of n-type Bi 2 (Te 0.97 Se 0.03) 3 alloys fabricated by a melting synthesis technique followed by spark plasma sintering was reported. The micro-morphology and composition of the specimens was characterized by means of X-ray diffraction and scanning electron microscopy. The electrical and thermal properties of (Bi 1- x Co x) 2 (Te 0.97 Se 0.03) 3 with x = 0%, 2%, 4% and 8% were measured from 298K to 473K. We found that (i) the micro-scale secondary phase CoTe 2 was embedded into the matrix due to the low solubility of Co element; (ii) Co doping generally improved the effective mass due to the scattering by local magnetic moments carried by Co ion and practically retained the carrier concentration, thereby yielding the higher Seebeck coefficient; (iii)both the electronic and lattice thermal conductivity decreased simultaneously by introducing the hierarchical defects (point defects and the micro-scale defects). Specifically, the x = 8% sample obtained a minimum thermal conductivity ~1.26 Wm−1K−1 at 398K. This results in a maximum ZT value of ~0.77, approximately ~20% higher than that of pristine Bi 2 (Te 0.97 Se 0.03) 3. • Co doping introduced the micro scale secondary phase CoTe 2 due to the low solubility of Co element in Bi 2 (Te 0.97 Se 0.03) 3. • Co doping enhanced the effective mass, thereby leading to the higher seebeck coefficient. • Acoustic phonon scattering was the dominant carrier scattering mechanism in all samples. • The lattice thermal conductivity of all samples was significantly lowered due to the hierarchical defects. • The maximum ZT value corresponding to about 20% higher as compared to pristine sample via Co doping. [ABSTRACT FROM AUTHOR]

Abstract We herein report the effects of Sm doping on the thermoelectric performance of (Bi 0.2 Sb 0.8) 2 (Te 0.97 Se 0.03) 3 compounds prepared by a growth-from-the-melt spark-plasma-sintering procedure. In conjunction with micro-morphology and compositional analyses, the electrical conductivity, Seebeck coefficient, and thermal conductivity of (Bi 0.2- x Sm x Sb 0.8) 2 (Te 0.97 Se 0.03) 3 samples were measured as a function of temperature (298–473 K) and the Sm doping ratio (x = 0%, 0.2%, 0.4%, and 0.8%). We found that (i) there was no discernible crystal lattice symmetry change upon Sm doping; (ii) Sm doping generally increased the electric conductivity and practically retained the Seebeck coefficient, thereby yielding higher power factors; and (iii) specially, a minimum thermal conductivity ∼ 0.92 Wm−1K−1 was attained in the x = 0.4% sample at 423 K, resulting in a state-of-the-art ZT value ∼ 1.22, which is ∼25% higher than the maximum ZT value of pristine sample. Contrary to the customary view that Sm dopant adopts a trivalent state Sm3+, the variations of lattice constant pointed toward a lower valence state of Sm. The results demonstrated the efficacy and deepened our understanding of rare earth doping in thermoelectric study of Bi 2 Te 3 -based materials. Highlights • It is inferred that Sm ions adopt a valence state lower than 3 + in the host matrix. • Sm-doping improved the electric conductivity, practically retained the Seebeck coefficient. • Acoustic phonon scattering is the dominant carrier scattering mechanism in all samples. • The thermal conductivities of the x = 0.2% and 0.4% samples were significantly lowered. • The maximum ZT value was enhanced by about 25% via Sm doping. [ABSTRACT FROM AUTHOR]