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This paper reports the electrical transmission properties of single-crystal PbSe samples prepared via Bi-flux method based on a stoichiometric ratio of Pb:Se:Bi = 1:1: x (x = 2.5, 3, 3.5, 4). Results show that all samples adopt an NaCl structure with the space group $$Fm\overline{3 }m$$ , with no cracks or micropores; the Bi atom substitutes for Pb consistently. All samples prepared by this method demonstrate n-type conduction and carrier concentrations varying from 2.48 and 4.21 × 1018 cm−3, which far exceeds that of the polycrystalline sample. Transmission electron microscopy (TEM) and corresponding area electron diffraction (SAED) further verify that the prepared sample has a typical single-crystal structure. Electronic band structure calculations indicate that the band gap of Pb32Se32 is 0.43 eV. After replacing a Pb by Bi atoms, the band gap is invariable and a resonant energy level is introduced into the band gap, which enhances the Seebeck coefficient. The power factor of the sample x = 3.5 reaches 3.6 × 10−3 W m−1 K−2 at 550 K, which is approximately 50% higher than that of the polycrystalline sample (1.8 × 10−3 W m−1 K−2) and can be attributed to the introduction of resonant states in the energy band for Bi replacing Pb.

In this work, the phase transitions, the elastic properties, and the electronic properties of Sr8Al16Sn30 under different pressures are investigated by first-principle calculations based on density functional theory. The E–V curves show that the type-VIII is the stable ground state phase, while the type-I is considered to be metastable at the lowest energy. Ignoring the temperature effect, type-VIII clathrate is always more stable than type-I, and the indication of phase transitions from type-I to type-VIII structure is not observed under increasing hydrostatic pressure. The obtained elastic constant values indicating that two structures under 0 and 3 GPa are stable, and not stable under 7 GPa. The dependences of the elastic constants cij, the elastic modulus, the Poisson ratio, the brittle and ductile behavior, and the elastic anisotropy on pressure in two structures are further analyzed. The electronic structures of type-I and type-VIII clathrates significantly change under increased pressure. The type-I clathrate with a narrow gap turns into the metal, and for type-VIII the band gap decreases in 3 GPa and the band gap increases in 7 GPa. This show that pressure tuning plays an important role in improving material properties.

In order to execute assembly of TE modules, it is necessary to synthesize high-performance n-type SnSe. In this study, Pb element as a dopant to synthesize SnSe single crystal successfully via a facile Sn-flux route. For the larger atomic radius of Pb, the SnSe antisite defects are restrained and hence induce more Se vacancy (VSe) in the larger content Pb-doped SnSe, which makes the system n-type conduction. For the transition of majority carriers from hole to electron, the carrier concentration decreased from 4.2 × 1017cm−3 in p-type SnSe to 3.3 × 1017cm−3 in p-type Sn0.85+5Pb0.15Se and then increased to 4.1 × 1017cm−3 in n-type Sn0.1+5Pb0.9Se. A maximum power factor of 1.2 μW cm−1·K−1is obtained at room temperature. The density functional theory shows that due to the stronger metallicity of Pb, the electrons become delocalized around Sn atom, suggesting a much higher degree of electron delocalization in the Pb doped SnSe, thus it can facilitate n-type conduction.

At present, the researchers pay much more attention on thin films and ceramics of Bi6Fe2Ti3O18 with different element doping, not nanomateials. Therefore in this work, the single-crystalline Bi6Fe2-xCoxTi3O18 (BFCTO) (x = 0–0.5) nanocrystals were synthesized successfully by the hydrothermal method. As the Co ions doping concentration increase, the main morphology of BFCTO nanocrystals changed from nanoplate to nanosheet. Meanwhile, doping Co ions in the BFTO can efficiently reduce the band gap of products. As the Co ions doping concentration increase to x = 0.5, the corresponding bandgap is shorten from 2.576 to 2.201eV. Besides, the BFCTO-0.5 sample exhibited optimal ultraviolet (UV)-light-driven photocatalytic activity for decomposition of rhodamine B (RhB). At room temperature, upon Co doping, the samples have spontaneous magnetic moments, indicating the ferromagnetic nature. The hysteresis loop of BFCTO-0.2 and BFCTO-0.1 nanocrystals obtained the maximum remnant magnetization 2Mr of 0.27emu/g and the maximum coercive field 2Hc of 1600Oe, respectively. The BFCTO-0.5 nanocrystal performed the best ferromagnetism with a saturated magnetization Ms of 2.15emu/g. All the results indicate the BFCTO nanocrystals have potential applications in data storage, sensors and nanodevices.

In this study, α-Cu2Se single crystal with space group C2/c was prepared by the Bi-flux method according to the stoichiometric ratio of Cu2Se(1+x)Bi3 (x = 0, 0.01, 0.02, 0.03). All samples involve hole conduction and carrier concentration increases with the increase of Se content. The valence of Bi atom is +3 in the sample, and the solubility limit of Bi atom is approximately 0.012% in Cu2Se. The crystal cell volume of all samples is larger than that of standard α-Cu2Se for the larger ion radius of Bi3+ than Cu+. In addition, the crystal cell volume of the samples decreases with the increase of Se content because of more Cu vacancies caused by excessive Se. The transmission electron microscopy image and the corresponding area electron diffraction pattern show that the prepared samples have a complete crystal structure. Eventually, the single crystal α-Cu2Se1.01 sample obtains the highest power factor of 1.53 × 10−3 W m−1 K−2 at 690 K. Thermogravimetric-differential thermal analysis shows that no weight loss occurred below the melting point. After three continuous thermal cycles, the value of power factor drops down to ∼0.04 × 10−3 W m−1 K−2 at 660 K. In comparison with polycrystalline sample prepared by hot pressing, the thermal stability is improved obviously.

Single crystalline PbTe samples are prepared by the Pb-flux method according to the stoichiometric ratio of Pb1+xTe (x = 1.5, 2.0, 2.5, and 3.0). Experimental results show that all the samples exhibit n-type conductivity. The order of magnitude for carrier concentration is 1019, which is one order of magnitude higher than that in pure n-type PbTe sample (2.56 × 1018 cm−3). The highest power factor reaches 2.07 × 10−3 W m−1 K−2 at 420 K for the sample with x = 1.5, which is an approximately 30% enhancement compared with the polycrystalline sample (1.59 × 10−3 W m−1 K−2). In a theoretical study on the PbTe system, Te vacancy (VTe) is preferentially formed under the Pb-rich condition. To further understand the effect of VTe on the electrical performance of PbTe thermoelectric material, the electronic structures of PbTe with VTe are determined by first principle calculations. Theoretical calculations reveal that VTe can modify the electronic density of states by introducing resonant level, which can increase the number of carrier pockets. Consequently, carrier concentration has increased remarkably. In this study, a Pb-flux method for synthetizing single crystalline PbTe thermoelectric material is convenient and low cost, and it has potential for mass production in commercial applications.

The coexistence of luminescent and magnetic properties in nanocomposites has attracted the interest of many researchers to study nanocomposites due to their unique properties. In this study, we present a convenient synthesis of Bi6Fe1.9Co0.1Ti3O18/SiO2/NaGdF4:Yb3+,Er3+ (BFCTO/SiO2/NGF) core/shell/satellite nanocomposites with precise control over the thickness of the SiO2 shell. By adjusting the amount of tetraethyl orthosilicate (TEOS), we obtained BFCTO/SiO2/NGF samples with different SiO2 thicknesses (0 nm, 8 nm, 13 nm, 18 nm, 25 nm and 36 nm). On account of the SiO2 spacer effectively reducing the absorption intensity around the green and red regions, the optimal upconversion luminescence intensity of BFCTO/SiO2/NGF was observed at a separation distance of 36 nm. Moreover, at room temperature, the hysteresis loop of BFCTO/∼13 nm SiO2/NGF obtained the maximum remnant magnetization (2Mr) of 0.13 emu g−1. Also, the BFCTO/∼36 nm SiO2/NGF sample reached the maximum coercive field (2Hc) of 1200 Oe. Besides, the BFCTO/∼8 nm SiO2/NGF sample had the best ferromagnetism with a saturated magnetization Ms of 0.63 emu g−1.

In this study, a nano-layered Cu2Se high-performance material is successfully grown using a NaCl-flux method based on the stoichiometric ratios of Cu2Se(NaCl)x (x = 1.5, 2, 2.5, 3, and 3.5). X-ray diffraction results show that samples possess an α-phase structure for 1.5 ≤ x ≤ 2.5, whereas samples consist of a mixture of β- and α-phase structures for x ≥ 3. Electron probe microanalysis results present that almost no Na or Cl atoms were found in all the samples. Meanwhile, the actual composition is close to the nominal composition of Cu2Se when 1.5 ≤ x ≤ 2.5, whereas it deviates from the nominal composition when x ≥ 3. The valence values of Cu and Se atoms are +1 and −2 in the sample, respectively. Transmission electron microscopy results prove that the crystals prepared using the NaCl-flux method have a good quality. The field emission scanning electron microscopy image shows that the samples prepared using the NaCl-flux method have a stacking nano-layered structure with an average thickness of approximately 110 nm. These thin lamellar crystals have several grain boundaries, which can effectively scatter the phonon and thus result in ultra-low lattice thermal conductivity. Cu2Se(NaCl)2.5 exhibits the best thermoelectric performance among all the samples because it has the lowest thermal conductivity. The maximum ZT value reaches 1.42 at 700 K, which is much higher than those prepared using fast melt-quenching (∼0.88 at 700 K), spark plasma sintering (∼0.8 at 700 K) and high-temperature and high-pressure synthesis (∼1.13 at 700 K).

In this paper, Cd-doped β-Zn4Sb3 crystal with space group R 3 ¯ c was prepared using the NaCl-flux method according to the stoichiometric ratio of Zn4.1Sb3(NaCl)7Cdx (x ​= ​0, 0.05, 0.1, 0.15). Electron probe microanalysis results showed that Cd and Na atoms were introduced into samples. The transmission electron microscopy and the corresponding area electron diffraction showed that the β-Zn4Sb3 samples prepared by NaCl flux method were of high quality and density. X-ray photoelectron spectroscopy results showed that the valence states of Zn, Cd, and Na were +2, +2, and +1, respectively. All the samples were p-type semiconductors with carrier concentration ranges from 18.61 ​× ​1019 to 9.04 ​× ​1019 ​cm−3. The thermal conductivity (κ) of the sample decreased with the increase of Cd content. Eventually, the maximum dimensionless figure of merit(ZT) value obtained 1.51 ​at 690 ​K for the β-Zn4.1Sb3(NaCl)7Cd0.1 sample, which is higher than that of the Bi-doped β-Zn4Sb3 samples(∼ 1.09 ​at 673 ​K) and Te-doped β-Zn4Sb3 samples(∼ 1.08 ​at 680 ​K). The findings provide a feasible insight toward understanding the effect of the Cd atom on the crystal structure and thermoelectric properties of β-Zn4Sb3.

In PbTe system, Pb vacancies (VPb) are preferentially formed compared with other intrinsic point defects (anti-sites TePb and interstitials Tei) under the Te-rich condition. In order to explore the effect of VPb on electrical transport properties of PbTe thermoelectric material, the electronic structures of PbTe with VPb are determined by the first principle calculations. Theoretical calculations reveal that the VPb can modify the band structure near the Fermi level, which can increase the number of carrier pockets thus increase the carrier concentration. Based on the calculated results, single crystalline PbTe samples with VPb have been prepared by Te-flux method according to the stoichiometric ratio of PbTe1+x (x = 2.0, 2.5, 3.0 and 3.5). The experimental results show that carrier concentration is optimized significantly by tuning Te-flux content, and it varies from 2.94 × 1018 to 8.01 × 1018 cm−3. The highest electrical conductivity reaches 3.1 × 104 S m−1 at 300 K for x = 3.5. A large Seebeck coefficient of 562 μV/K is obtained at 480 K for x = 2.0. Due to the synergistic effect of the Seebeck coefficient and the electrical conductivity, the highest power factor of 3.17 × 10−3 W m−1 K−2 is achieved at 450 K for x = 3.0.

In this study, n-type Bi-doped single-crystal PbTe thermoelectric materials were prepared by melting and slow cooling method according to the stoichiometric ratio of Pb:Bi:Te = 1−x:x:1 (x = 0, 0.1, 0.15, 0.2, 0.25). The X-ray diffraction patterns of Pb1-xBixTe samples show that all main diffraction peaks are well matched with the PbTe matrix, which has a face-centered cubic structure with the space group F m 3 ¯ m . Electron probe microanalysis reveals that Pb content decreases gradually, and Te content remains invariant basically with the increase of Bi content, indicating that Bi atoms are more likely to replace Pb atoms. Thermal analysis shows that the prepared samples possess relatively high thermal stability. Simultaneously, transmission electron microscopy and selected area electron diffraction pattern indicate that the prepared samples have typical single-crystal structures with good mechanical properties. Moreover, the electrical conductivity of the prepared samples improved significantly compared with that of the pure sample, and the maximum ZT value of 0.84 was obtained at 600 K by the sample with x = 0.2.

SnSe is a promising thermoelectric material with a record high dimensionless figure of merit ZT at high temperature ∼923 K. However, the ZT values for low-Temperature Pnma phase SnSe are just 0.1–0.9. Here, we use First-principle combine with Boltzmann transport theory methods to study the effect of tensile and compressible strain on the thermoelectric transport properties. The power factor of SnSe with −4% strain have a large boost along b and c directions of 7.7 and 3.9 μW cm−1 K−2, respectively, which are 2.5 and 2 times as large as those pristine SnSe. The charge density distributions reveal that the overlap of wave function has significant change due to the changed bond lengths and bond angles under different strain, which lead to the change of band gap and band dispersion. Our work provides a new effective strategy to enhance the thermoelectric properties of materials.

Single crystalline tin selenide (SnSe) has attracted considerable attention due to its record high figure of merit value in medium temperature range. However, their large band gap restricts the electron transport. Herein, we tailor the carrier concentration by offering a variability of the Sn/Se ratio to fine-tune the electrical transport properties. Density functional theory demonstrated that the collaborative optimization of the Seebeck coefficient and the electrical conductivity through optimizing carrier concentration can improve significantly the power factor, and deviation from stoichiometry is an effective strategy to modulate the carrier concentration because anti-site defects SnSe lower the Fermi level and increases the number of carrier pockets in SnSe. Meanwhile, experimental observations confirm that the carrier concentration and electrical conductivity are changed with varied Sn/Se ratio. Through this work, a viable design principle for optimized carrier concentration for achieving high thermoelectric performance is provided.

Construction of heterostructure is an efficient approach to improve the photocatalytic performance of catalysts. Herein, the Bi6Fe1.5Co0.5Ti3O18 (BFCTO) nanoplates are successfully loaded on g-C3N4 nanosheets by a facile chemical process for the first time. The photocatalytic activity of all g-C3N4/BFCTO catalysts are measured by the degradation of Rhodamine B (RhB) under visible-light irradiation. The results display that g-C3N4/BFCTO heterojunctions exhibit greatly improved photocatalytic activity compared with corresponding bare g-C3N4 and BFCTO catalysts. Based on the experimental results of photocurrent responses, EIS and ESR, it is clearly demonstrated that the improved activity is attributed to the formation of Z-scheme photocatalyst. The g-C3N4/BFCTO-2 heterojunction showed the optimal photocatalytic activity with the photodegradation efficiency of 84.9% within 20 min. We hope that the work can provide a new insight for exploring Bin+1Fen−3Ti3O3n+3-based Z-scheme heterojunction for pollutant degradation and environment restoration.

Metallic cobalt-based sulfide, selenide and phosphide will undergo superficial transformation into insufficient activity cobalt oxides/(oxy)hydroxides during the oxygen evolution reaction (OER) process, which limits the efficiencies of electrical-conversion applications. It is necessary to activate the inert oxidation layer on the surface of metallic compounds for highly efficient OER catalysts. Taking cobalt sulfide as an example, we designed a facile strategy to introduce oxygen vacancy (Vo) in the inert oxidation layer CoOx coating on metallic Co9S8 by in-situ derived Zn doped Co9S8. First-principles calculation and experimental analysis verify that introduced Vo leads to lesser electron filling of the anti-bonding states by tailoring the positions of d-band center, which can facilitate the fast kinetics of OER. Meanwhile, internal metallic Zn-Co9S8 can accelerate the electron transfer process. Benefiting from the advantages of geometric construction and electronic regulation, fabricated Zn doped CoOx/Co9S8 nanotube arrays exhibit remarkable electrocatalytic activity with a low overpotential of ~ 256 mV to achieve the current density of 10 mA cm−2, Tafel slope drops as low as ~ 44 mV dec−1, accompanied by good long term stability toward OER conditions. The present work provides valuable insights into the design of other catalysts for OER and beyond.

Single crystal samples β-Zn4Sb3 have been prepared by using Bi-Sn mixed-flux method. The obtained crystals exhibit p-type conduction behavior with carrier concentration varying from 4.40 × 1019 to 18.12 × 1019 cm−3 as carrier mobility changes from 25.8 to 61.5 cm2 V−1 s−1 at room temperature. Electrical transport properties of the samples were optimized by Bi-Sn co-doped, which brought by Bi-Sn mixed-flux. And the maximal power factor of 1.45 × 10−3 W m−1 K−2 is achieved at 510 K for the sample with Bi flux content x = 0.5. Consequently, the oxidation resistance of the sample was determined by exploring the effects of heat treatment in air on electrical transport properties and thermal stability, which the single crystalline β-Zn4Sb3 still possess an excellent oxidation resistance and thermal stability after the heat treatment process.

In this study, we report the effect of Pb doping on both thermal stability and electrical transport properties of the single crystalline β-Zn 4 Sb 3 ，prepared based on the initial stoichiometric ratios of Zn 4- x Sb 3 Pb x Sn 3 ( x = 0, 0.2, 0.4, 0.6 and 0.8). All samples possess a metallic luster surface and hardly defects and pores. The TG-DSC results show that the Pb doping samples exhibit an excellent thermal stability. Electrical transport properties of the samples were optimized by Pb doping. Among all samples exhibit p -type conduction with carrier concentrations varying from 4.88 × 10 19 to 14.29 × 10 19 cm −3 , as carrier mobility changes from 31.1 to 66.4 cm 2 V −1 s −1 at room temperature. With the increase of Pb initial content, Seebeck coefficient increases and electrical conductivity decreases. The sample with Pb initial content x = 0.6 exhibits an excellent electrical properties, and obtains maximum power factor of 1.69 × 10 −3 W m −1 K −2 at 390 K.

In this study, Ga/Sn co-doped single-crystal β-Zn4Sb3 were prepared by a Sn-flux method based on stoichiometric ratios of Zn4−x Sb3Ga x Sn3 (x = 0, 0.25, 0.5, 0.6 and 0.75). The effect of Ga/Sn co-doping on the thermal stability and electrical transport properties of the obtained samples were investigated. All the prepared samples exhibit p-type conduction, and carrier concentration varies from 4.71 × 1019 cm−3 to 10.44 × 1019 cm−3, while carrier mobility changes from 34.2 cm2 V−1 s−1 to 68.9 cm2 V−1 s−1 at room temperature. Structure analysis indicates that all samples are β-Zn4Sb3 with space group $$ R\bar{3}c$$ . Thermal analysis results show that the Ga/Sn co-doped samples possess an excellent thermal stability. The results of crystal compositions indicate that both Ga and Sn atoms tend to replace Zn atoms, and the electrical transport properties of the samples were optimized by co-doping Ga and Sn. Meanwhile, the calculated values of the carrier effective mass, band gap and relaxation time agree with the result obtained from a band structure calculation. Consequently, the sample with Ga initial content x = 0.5 possesses excellent electrical properties, which obtains a maximal power factor of 1.56 × 10−3 W m−1 K−2 at 450 K.

β-Zn4Sb3 is a promising p-type thermoelectric material for utilization in moderate temperatures. This study prepares a group of single-crystalline β-Zn4Sb3 samples using the Zn-Sn mixed-flux method based on the stoichiometric ratios of Zn4+x Sb3Sn y . The effect of Zn-to-Sn proportion in the flux on the structure and electrical transport properties is investigated. All samples are strip-shaped single crystals of different sizes. The actual Zn content of the present samples is improved (>3.9) compared with that of the samples prepared through the Sn flux method. Larger lattice parameters are also obtained. The carrier concentration of all the samples is in the order of over 1019 cm−3. With increasing Sn rate in the flux, this carrier concentration decreases, whereas mobility is significantly enhanced. The electrical conductivity and Seebeck coefficients of all the samples exhibit a behavior that of a degenerate semiconductor transport. Electrical conductivity initially increases and then decreases as the Sn ratio in the flux increases. The electrical conductivity of the x:y = 5:1 sample reaches 6.45 × 104 S m−1 at 300 K. Benefitting from the electrical conductivity and Seebeck coefficient, the flux proportion of the x:y = 7:1 sample finally achieves the highest power factor value of 1.4 × 10−3 W m−1 K−2 at 598 K.

In this work, W6+ ions doped Bi2MoO6:20%Yb3+, 2%Er3+ samples were successfully synthesized by a facile and modified hydrothermal method. The main morphology of Bi2MoO6 samples were hollow microsphere structure assembled by nanosheets. The particle size was slightly increased as enhancing W6+ ions doping concentration to 5%, then decreased as the doping concentration increased. Besides, doping W6+ ions can increase the band gap of products from 2.65 ​eV to 2.71eV. The upconversion emission intensity of all samples was strongly influenced by the W6+ ions doping concentration. The luminescent intensity enhanced with the doping concentration increasing up to 3%, then decreased when the W6+ ions concentration further increased. Moreover, the intensity of red emission increased by a large margin than that of green-light region. Besides, the lifetime of 4F9/2 states of Er3+ ions for Bi2MoO6:20%Yb3+, 2%Er3+, 3%W6+ (τ655 ​= ​583.64 ​μs) had significant longer lifetime relative to the Bi2MoO6:20%Yb3+, 2%Er3+ (τ655 ​= ​70.29 ​μs).

In this study, β-Zn4Sb3 was prepared by using the NaCl-flux method according to the stoichiometric ratio of Zn4+xSb3(NaCl)7 (x = 0.1, 0.2, 0.3, 0.4). An effective carrier concentration regulation is realized by changing the content of Zn and the doping of Na. X-ray powder diffraction results indicate that the crystal samples prepared via the NaCl-flux method were all hexagonal rhombic crystal structures, and the space group was R 3 ¯ c . Electron probe microanalysis results showed that Na atoms were introduced into the Zn4Sb3 structure successfully. The transmission electron microscope and the corresponding selected area electron diffraction confirmed that the prepared samples had good crystal quality. X-ray photoelectron spectroscopy results show that the valence states of Zn, Sb, and Na were +2, −2 and −3, and +1, respectively. All samples exhibit P-type conduction behavior, and the carrier concentration of the material increases because of the increment in the Zn content of the raw material. The carrier concentration ranges from 1.86 × 1020cm−3 to 2.45 × 1020cm−3. Moreover, the metal Zn phase increases the carrier concentration. Thus, that the sample has higher electrical conductivity, and the power factor of the material is optimized. The thermal conductivity of the sample increases because of the appearance of metal Zn phase. The sample with the final nominal composition of β-Zn4.1Sb3(NaCl)7 exhibited the best thermoelectric performance. The maximum ZT value of 1.4 is obtained at 673 K because of its high power factor and low thermal conductivity. This value is higher than those of β-Zn4Sb3 prepared by ball milling (∼1.21 at 673 K) and hot pressing (∼1.09 at 673 K).

In this study, the structural and electronic structural properties of Ba8Ga16Sn30−xGex (0≤x≤30) are determined by the first-principle method on the basis of density functional theory. Consistent with experimental findings, calculated results reveal that Ge atoms preferentially occupy the 2a and 24g sites in these compounds. As the content of Ge in Ge-substituted clathrate is increased, the lattice parameter is decreased, and the structural stability is enhanced. The bandgaps of the compound at 1≤x≤10 are smaller than those of Ba8Ga16Sn30. By contrast, the bandgaps of the compound at x>10 are larger than those of Ba8Ga16Sn30. The substitution of Ge for Sn affects p-type conductivity but not n-type conductivity. As Ge content increases, the whole conduction band moves to the direction of high energy, and the density of states of valence-band top decreases. The calculated potential energy versus displacement of Ba indicates that the vibration energy of this atom increases as cage size decreases. Because Ge substitution also affects clathrate structural symmetry, the distance of Ba atom deviation from the center of the cage initially increases and subsequently decreases as the Ge content increases.

This study prepared a Zn-rich single crystal β-Zn 4 Sb 3 using a Sn flux method based on the stoichiometric ratios of Zn 4.4 Sb 3 Sn 3 . The oxidation resistance of the sample was determined by exploring the effects of heat treatment in air on electrical transport properties and thermal stability, which is of practical importance in the application of the material at high temperatures. Results showed that the prepared sample possessed high electrical transport properties, with a high power factor of 1.67×10 −3 W m −1 K −2 at 563 K. The heat treatment in air weakened the electrical conductivity of the single crystalline β-Zn 4 Sb 3 , whereas the Seebeck coefficients were rarely independent of the annealing condition. Eventually, the power factor obtained after the first heating at 523 K for 4 h became comparable to that of the as-prepared sample, although it decreased after the second heating at 573 K for 5 h. Nevertheless, the single crystalline β-Zn 4 Sb 3 still possessed a good thermal stability after the heat treatment process.

In this study, p-type single crystalline β-Zn4Sb3 thermoelectric materials with space group R\( {\bar{3}} \)c were successfully prepared using the α-Sn flux method based on the stoichiometric ratios of Zn4+xSb3Sn3 (x = 0, 0.2, 0.4, 0.6). All the crystal sizes were more than 5 mm. The prepared samples had high density and no microcracks. The powder x-ray diffraction patterns showed that all the samples were single phase. All single-crystal samples possessed good electrical transport performance. Compared with polycrystalline β-Zn4Sb3, the carrier mobility of the single crystals were significantly improved. Moreover, excess Zn improved the Seebeck coefficient of the material. The electrical conductivities of the samples were comparable with those obtained by the β-Sn flux method, whereas the Seebeck coefficient declined on the whole and the intrinsic conduction temperature decreased. The power factor of the sample with x = 0.2 is 0.82 × 10−3 W m−1 K−2 at 635 K, which exhibited the best electrical transport performance.

Sn-based clathrates possess excellent thermoelectric properties ascribed to their higher Seebeck coefficient and lower thermal conductivity. Guest atoms significantly modulate the thermoelectric properties of Sn-based calculates because of their diverse atomic radius and interactions with framework atoms. Thus, we explored the electronic structure of I-M8Ga16Sn30 (M = Ba, Sr, Yb) by first-principles calculation. Results revealed significant differences between Yb8Ga16Sn30 and M8Ga16Sn30 (M = Ba, Sr,). In particular, the Yb-filled compound substitution possesses lowest formation energy and the off-center distance of the Yb atom is the largest compared with the other structures. I-M8Ga16Sn30 (M = Ba, Sr, Yb) is an indirect band gap semiconductor, and the enhanced hybridization effect between the guest and framework atoms' orbits exists because the Yb f orbit results in a decrease in band gap. Ba- and Sr-filled clathrates have similar valence bands but slightly different conduction bands; however, Yb8Ga16Sn30 possess the spiculate density of states near the Fermi level that reveals excellent thermoelectric properties.

Single-crystal samples of Sr-filled Ge-based type I clathrate have been prepared by the Sn-flux method, and their thermoelectric properties investigated. The obtained samples exhibited n-type conduction with carrier concentration varying from 2.8 × 1019/cm3 to 6.8 × 1019/cm3 as the carrier mobility changed from 23.9 cm2/V-s to 15.1 cm2/V-s at room temperature. Structural analysis indicated that all samples were type I clathrate in space group $$ pm\bar{\textit{3}}n $$ . The total content of group IV (Ge + Sn) atoms in the crystalline structure increased with increasing x value (where x defines the atomic ratio of starting elements, Sr:Ga:Ge:Sn = 8:16:x:20), reaching a maximum value of 31.76 at.% for the sample with x = 30; consequently, the lattice parameters increased. The melting points for all samples were approximately 1012 K, being considerably lower than that of single-crystal Sr8Ga16Ge30 prepared by other methods. The electrical conductivity increased while the absolute value of α increased gradually with increasing temperature; the maximum value of α reached 193 μV/K at 750 K for the sample with x = 24. The sample with x = 30 exhibited lower lattice thermal conductivity of 0.80 W/m-K. As a result, among all the Sn-flux samples, single-crystal Sr7.92Ga15.04Sn0.35Ge30.69 had the largest ZT value of 1.0 at about 750 K.

In this study, type VIII Ba8Ga16Sn30 single-crystal clathrates with different stoichiometric ratios were prepared using the a-Sn self-flux method. The structures of the samples were also investigated. Results indicate that n-/p-type single-crystal clathrates can be obtained by adjusting the initial content of Ga. Samples were determined to be n-type when the initial Ga content was X = 12, 13, and 14; and p-type when X = 16 and 18. The samples exhibited high melting points and large lattice parameters, as well as increasing Ga content. The electrical conductivities of the samples were lower than those prepared using the Sn self-flux method because of the low carrier concentration. The elemental composition of the sample was near the theoretical value when the Ga content was X = 14 and 16, which also yielded the highest Seebeck values of −523 μV/K and 331 V/K, respectively. Finally, the sample with X = 14 and 18 exhibited the highest ZT values of 0.82 and 0.46, respectively, at a temperature of 480 K in n-/p-type.

In this work, the type-VIII Sn-based single-crystal clathrates with different stoichiometric ratios were prepared, and the structure of the samples was investigated. The results indicate that Eu atoms tend to replace Ba atoms. The introduced Eu atoms lead to a decrease in the materials’ melting point and the concentration of carriers, whilst the carrier mobility is increased. Despite the above, there was no obvious change on the electrical conductivity, and the lattice constant of samples were decreased. The Seebeck coefficient of the samples was found to be negative and its absolute value decreased with increasing Eu. The ZT values were found to be almost 0.46 at 550 k when the Eu initial ratio was equal to 0.25.

Cu2Se is a promising “phonon liquid–electron crystal” thermoelectric material with excellent thermoelectric performance. In this work, Cd-doped Cu2–x SeCd x (x = 0, 0.0075, 0.01, and 0.02) samples were prepared using NaCl flux method. The solubility of Cd in Cu2Se at room temperature was less than 6%, and a second phase of CdSe was found in the samples with large initial Cd content (x = 0.01 and 0.02). Field-emission scanning electron microscopic image showed that the arranged lamellae formed a large-scale layered structure with an average thickness of approximately 100 nm. Transmission electron microscopy demonstrated that doping of Cd atoms did not destroy the crystal integrity of Cu2Se. A small amount of Cd in Cu2Se could reduce the electrical and thermal conductivities of the material, thus significantly enhancing its thermoelectric performance. With the increase in Cd content in the sample, the carrier concentration decreased and the mobility increased gradually. Thermogravimetric differential thermal analysis showed that no weight loss occurred below the melting point. Excessive Cd doping led to the emergence of the second phase of CdSe in the sample, thus significantly increasing the thermal conductivity of the material. A maximum ZT value of 1.67 at 700 K was obtained in the Cu1.9925SeCd0.0075 sample.

In this study, PbSe single crystal with the grain size of 10 mm was prepared using Pb-flux method combined with melt slow cooling process, and its structure and thermoelectric transportation properties were systematically studied. X-Ray powder diffraction results indicated that all prepared samples has a NaCl-type crystal structure with a space group of F m 3 ¯ m . Transmission electron microscopy and the corresponding selected area electron diffraction results illustrated that the sample prepared by this method was a single crystal. Thermal analysis showed insignificant weight loss in the PbSe sample below the melting point, thereby indicating that PbSe has good thermal stability. X-Ray photoelectron spectroscopy results demonstrated that the valences of Pb and Se were +2 and −2, respectively. Given that the PbSe sample had a high mobility, the material has a high electrical conductivity near room temperature. Thus, the ZT value near room temperature reaches 1.25, which indicates that single crystal PbSe obtained by Pb flux method has potential application in room-temperature power generation.

This work is a joint experimental and theoretical study on the structural properties and thermodynamic stability of Cu-doped Ba8Ga16Sn30 single-crystalline clathrates. Different synthesis temperatures were adopted to prepare type-I and type-VIII Ba8Ga16–xCuxSn30 (x = 0, 1, 2) by the Sn-flux method. Experimental results revealed that the reaction transformation temperature from type-VIII to type-I clathrates is approximately 773 K, and Cu doping can increase the critical preparation temperature of type-I phase. The Cu-doped samples possess high conductivity resulting from the significant enhancement of carrier mobility. Theoretical calculation revealed that the density of states (DOS) near the Fermi level increased for Cu doping, which may lead to reduction in structural stability.

In and Sn codoped β-Zn4Sb3 single crystals were prepared by a Sn-flux method according to the formula Zn4.4Sb3Sn3Inx(x = 0–0.5). The thermal weight loss is suppressed completely until the melting point of the single crystals. All crystals exhibit p-type conduction. The carrier mobility of the single crystals is increased, compared to a β-Zn4Sb3 polycrystalline sample. All samples possess relatively high electrical conductivity, reaching 6.3 × 104 S/m for the sample with x = 0.18. The Seebeck coefficient is enhanced on increasing of the total content of In and Sn. The sample with x = 0.5 exhibits excellent electrical properties, and shows a maximal power factor of 1.53 × 10−3 W/m/K2 at 603 K.

In this work, β-Zn4Sb3 single crystals with space group R 3 ¯ c were prepared by Sn-flux method according to the stoichiometric ratios of Zn4+x(0–0.8)Sb3Sn3. thermogravimetric-differential thermal analysis (TG-DTA) shows that there has no weight loss untill temperature reaches to melting point of 821 K. All crystals behave characteristic of p-type conduction and the carrier densities are independent on the initial composition. Comparing with β-Zn4Sb3 polycrystalline sample, the carrier mobility of the present single crystals are increased. The crystals with X>0 possess higher electrical conductivity, and the highest power factor of 1.18×10−3 Wm−1 K−2 is obtained at 585 K for the sample with X=0.8.

The intermetallic clathrate Ba 8 Ga 16 Sn 30 with type-VIII structure is one of the promising thermoelectric materials. We report on significant effects of Cu doping on both the crystal growth and thermoelectric properties of type-VIII clathrate Ba 8 Ga 16 Sn 30 . By using the Ga flux and Sn flux, respectively, the carrier type could be tuned as p - and n -types. The p -type single crystals were grown to approximately 6 mm in diameter, which is twice as large as that grown without Cu. The crystals were characterized by electron-probe microanalysis and powder X-ray diffraction and used for the measurements of the electrical resistivity, Seebeck coefficient, and thermal conductivity. The values of the dimensionless figure of merit Z T of the p - and n -type crystals reached 0.88 and 1.45 at around 500 K, respectively, the latter of which is the highest among all substituted systems of type-VIII Ba 8 Ga 16 Sn 30 . This enhancement of Z T is discussed in relation with the charge balance and modification of the band structure caused by Cu doping.