This work focuses on the synthesis and properties of quaternary ZnSnP2-yAsy chalcopyrite solid solutions. Full miscibility of the solid solution is achieved using ball milling followed by hot press sintering. The measured electrical conductivity increases substantially with As substitution from 0.03 S cm-1 for ZnSnP2 to 10.3 S cm-1 for ZnSnAs2 at 715 K. Band gaps calculated from the activation energies show a steady decrease with increasing As concentration from 1.4 eV for ZnSnP2 to 0.7 eV for ZnSnAs2. The Seebeck coefficient decreases significantly with As substitution from nearly 1000 µV K-1 for ZnSnP2 to -100 µV K-1 for ZnSnAs2 at 650 K. Thermal conductivity is decreased for the solid solutions due to alloy phonon scattering, compared to the end members with y = 0 and y = 2, with the y = 0.5 and y = 1.0 samples exhibiting the lowest values of 1.4 W m-1 K-1 at 825 K. Figure of merit values are increased for the undoped solid solutions at lower temperatures when compared to the end members due to the decreased thermal conductivity, with the y = 0.5 sample reaching zT = 1.6 × 10-3 and y = 1 reaching 2.1 × 10-3 at 700 K. The largest values of the figure of merit zT for the undoped series was found for y = 2 with zT = 2.8 × 10-3 at 700 K due to the increasing n-type Seebeck coefficient. Boltztrap calculations reveal that p-doping could yield zT values above unity at 800 K in case of ZnSnAs2, comparable with ZnSnP2.
During our group's continued exploration of group 14 chalcogenides, we discovered two new compounds, Sr8Ge4Se17 and Ba8Sn4Se17. Both compounds have an 8:4:17 stoichiometric ratio but adopt different centrosymmetric crystal structures. Sr8Ge4Se17 crystallizes in the triclinic P1Ì space group with a = 11.8429(18) Å, b = 12.172(3) Å, c = 13.624(3) Å, α = 114.472(5)0, ß = 97.396(5)0, γ = 107.040(5)0, and Z = 2. Ba8Sn4Se17 crystallizes in the monoclinic C2/c space group with a = 47.286(3) Å, b = 12.6294(5) Å, c = 25.7303(15) Å, ß = 104.585(5)0, and Z = 16. The unit cell of Ba8Sn4Se17 is approximately eight times larger than the unit cell of Sr8Ge4Se17, which is a consequence of the differently aligned tetrahedra, resulting in a quadrupled a axis, unchanged b axis, and doubled c axis. The lattice parameters and atomic coordinates were finalized via Rietveld refinements on data collected at a high-energy synchrotron beamline. Both compounds are semiconductors with band gaps in the visible region. Sr8Ge4Se17 and Ba8Sn4Se17 have optical band gaps of 1.88 and 1.93 eV, respectively. Both compounds have remarkably low thermal conductivities owing to their low symmetries and large unit cells. The minimum experimental thermal conductivity values of Sr8Ge4Se17 and Ba8Sn4Se17 are 0.45 W m-1 K-1 at 321 K and 0.31 W m-1 K-1 at 324 K, respectively.
The new compound Ba6Si2Se12 was synthesized, and the crystal structure and physical properties are reported here. Ba6Si2Se12 adopts a new structure type in the triclinic P1Ì space group with the lattice parameters a = 9.1822(7) Å, b = 12.2633(14) Å, c = 12.3636(18) Å, α = 109.277(3)°, ß = 104.734(2)°, and γ = 100.4067(16)°. Notably, the structure features disordered Se22- dumbbells that have also been observed in the germanium selenide with the analogous stoichiometry (Ba6Ge2Se12). Density functional theory calculations revealed that Ba6Si2Se12 is a semiconductor with a calculated band gap of 1.74 eV. UV/vis/NIR absorption spectra indicated that the experimental band gap of Ba6Si2Se12 is 1.89 eV. While exploring this compound's phase width, it was discovered that up to 75% of the Si could be substituted with Ge while retaining the structure type. Rietveld refinements were performed on the phase-pure samples of Ba6Si2-xGexSe12 (x = 0, 0.5, 1, and 1.5) using data collected at the Canadian Light Source's High Energy Wiggler Beamline. The cell parameters, Si/Ge occupancies, and disordered Se22- occupancies were studied. Raman spectra displayed the expected Si-Se and Ge-Se stretching modes from 215 cm-1 to 280 cm-1. The samples were also hot-pressed into pellets to determine their thermal conductivity values ranging from 0.5 to 0.4 W m-1 K-1 for the x = 0, 0.5, and 1.5 samples. The x = 1 sample stood out with a remarkably low thermal conductivity of 0.3 W m-1 K-1, consistent from room temperature up to 573 K.
Nanostructuring of thermoelectric (TE) materials leads to improved energy conversion performance; however, it requires a perfect fit between the nanoprecipitates' chemistry and crystal structure and those of the matrix. We synthesize bulk Bi2Te3 from molecular precursors and characterize their structure and chemistry using electron microscopy and analyze their TE transport properties in the range of 300-500 K. We find that synthesis from Bi2O3 + Na2TeO3 precursors results in n-type Bi2Te3 containing a high number density (Nv â¼ 2.45 × 1023 m-3) of Te-nanoprecipitates decorating the Bi2Te3 grain boundaries (GBs), which yield enhanced TE performance with a power factor (PF) of â¼19 µW cm-1 K-2 at 300 K. First-principles calculations validate the role of Te/Bi2Te3 interfaces in increasing the charge carrier concentration, density of states, and electrical conductivity. These optimized TE coefficients yield a promising TE figure of merit (zT) peak value of 1.30 at 450 K and an average zT of 1.14 from 300 to 500 K. This is one of the cutting-edge zT values recorded for n-type Bi2Te3 produced by chemical routes. We believe that this chemical synthesis strategy will be beneficial for future development of scalable n-type Bi2Te3 based devices.
Herein, the crystal structures and physical properties of two previously unreported barium seleno-germanates, Ba6Ge2Se12 and Ba7Ge2Se17, are presented. Ba6Ge2Se12 adopts the P21/c space group with a = 10.0903(2) Å, b = 9.3640(2) Å, c = 25.7643(5) Å, and ß = 90.303(1)°, whereas Ba7Ge2Se17 crystallizes in the Pnma space group with a = 12.652(1) Å, b = 20.069(2) Å, c = 12.3067(9) Å. Both structures feature polyatomic anion disorder: [Se2]2- in the case of Ba6Ge2Se12 and [GeSe5]4- in the case of Ba7Ge2Se17. The anion disorder is verified by comparing pair distribution functions of ordered and disordered models of the structures. These anions are split unevenly across two possible sets of atomic coordinates. The optical band gaps obtained from the powdered samples are found to be 1.75 and 1.51 eV for Ba6Ge2Se12 and Ba7Ge2Se17, respectively. Differential scanning calorimetry experiments indicate that the compounds are stable under the exclusion of air up to at least 673 K. The thermal diffusivity measurements revealed thermal conductivities reaching values as low as 0.33 W m-1 K-1 in both compounds at 573 K.
The aim of this study was to investigate the thermoelectric properties of hot-pressed Ba3Cu14-δTe12 as well as its stability with regards to Cu ion movement. For the latter, two single crystals were picked from pellets after they were measured up to 573 and 673 K, which showed no significant changes in the occupancies of any of the Cu sites. All investigated Ba3Cu14-δTe12 materials displayed low thermal conductivity values (<1 W m-1 K-1) and appropriate electrical conductivity values (300-600 Ω-1 cm-1). However, the thermopower values were comparably low (<+65 µV K-1), resulting in uncompetitive zT values, with the highest being achieved for Ba3Cu13.175Te12, namely zT = 0.12 at 570 K. In an attempt to decrease the thermal conductivity, and thereby enhance the figure of merit, a brief alloying study with Ag was undertaken. The incorporation of Ag, however, did not produce any significant improvements.
High-performance thermoelectric materials are currently being sought after to recycle waste heat. Copper chalcogenides in general are materials of great interest because of their naturally low thermal conductivity and readily modifiable electronic properties. The compounds Cu5Sn2Q7 were previously reported to have metal-like properties, which is not a desirable characteristic for thermoelectric materials. The aim of this study was to reduce the carrier concentration of these materials by Zn-doping, and then investigate the electronic and thermoelectric properties of the doped materials in comparison to the undoped ones. The compounds were synthesized using both the traditional solid-state tube method and ball-milling. The crystal structures were characterized using powder X-ray diffraction, which confirmed that all materials crystallize in the monoclinic system with the space group C2. With the partial substitution of zinc for copper atoms, the compounds exhibited an overall improvement in their thermoelectric properties. Figure of merit values were determined to be 0.20 for Cu4ZnSn2Se7 at 615 K and 0.05 for Cu4ZnSn2Te7 at 575 K.
We investigated the Sr2-xPbxGeSe4 series from 0 ≤ x ≤ 2 to study the impact of Pb on structure and properties. While the noncentrosymmetric (NCS) compounds γ-Sr2GeSe4 and α-Pb2GeSe4 have already been reported previously, the substitution variants Sr1.31Pb0.69GeSe4 (space group Ama2, a = 10.31220(1) Å, b = 10.39320(1) Å, c = 7.42140(1) Å) and Sr0.19Pb1.81GeSe4 (I4Ì 3d, a = 14.6177(3) Å) are introduced here for the first time. The experimentally determined optical band gaps decrease as predicted with increasing Pb content from γ-Sr2GeSe4 to Sr1.31Pb0.69GeSe4, Sr0.25Pb1.75GeSe4, and α-Pb2GeSe4 from 2.00, to 1.65, 1.45 and 1.42 eV, respectively. The nonlinear optical (NLO) properties of the orthorhombic compounds γ-Sr2GeSe4 and Sr1.3Pb0.7GeSe4 (approximated with the supercell "Sr3PbGe2Se8") were studied both theoretically, using first-principle calculations, and experimentally. The calculations found the effective nonlinear susceptibility, deff, of γ-Sr2GeSe4 and "Sr3PbGe2Se8" at the static limit to be 10.8 and 8.8 pm V-1, respectively. The experimental deff values of γ-Sr2GeSe4, Sr1.31Pb0.69GeSe4, Sr0.25Pb1.75GeSe4, and α-Pb2GeSe4 were 2.6, 2.3, 0.68, and 0.79 pm V-1, respectively.
In this work, polycrystalline n-type Mg2Si0.30Sn0.67Bi0.03 dispersed with x wt % ß-SiC nanoparticles (x = 0, 0.5, 1.0, 1.5, and 3.0) thermoelectric materials were fabricated by a solid-state reaction in a low-cost container, consolidated by hot-pressing. We obtained figure of merit values zT above 1.4 at 773 K along with enhanced mechanical properties by adding ß-SiC into an Mg2Si0.30Sn0.67Bi0.03 matrix. Incorporation of SiC nanoparticles has thusly simultaneously increased toughness and, depending on the SiC content, thermoelectric performance. The peak figure of merit was improved from zT = 1.33 for Mg2Si0.30Sn0.67Bi0.03 to 1.45 for Mg2Si0.30Sn0.67Bi0.03 with 3 wt % at 773 K.
BaCuSiTe3 was prepared from the elements in a solid-state reaction at 973 K, followed by slow cooling to room temperature. This telluride adopts a new, hitherto unknown structure type, crystallizing in the noncentrosymmetric space group Pc with a = 7.5824(1) Å, b = 8.8440(1) Å, c = 13.1289(2) Å, ß = 122.022(1)°, and V = 746.45(2) Å3 (Z = 4). The structure consists of a complex network of two-dimensionally connected CuTe4 tetrahedra and ethane-like Si2Te6 units with a Si-Si bond. This semiconducting material has an optical band gap of 1.65 eV and a low thermal conductivity of 0.50 W m-1 K-1 at 300 K. Calculations of its optical properties revealed a moderate birefringence of 0.23 and a second-order harmonic generation response of deff = 3.4 pm V-1 in the static limit.
The title materials have been reported earlier to be p-type thermoelectrics when x = 0.1 and y = 0. Here, we studied the properties after varying the Cu and the Se/Te concentrations. At first, materials with the same nominal Cu concentration, 5.9 Cu per formula unit, and different Se/Te ratios were prepared. The different thermoelectric properties indicated that the Se/Te ratio strongly affected the Cu deficiency, which is directly responsible for the charge carrier concentration. Single crystal structure data revealed the Cu amount to be less than 5.8 per formula unit when y = 0.4; therefore a sample of nominal composition "BaCu5.74Se0.46Te6.54" was also studied. This sample exhibited an electrical conductivity of 685 Ω-1 cm-1 at room temperature, which is almost three times larger than in case of "BaCu5.9SeTe6", in accord with the lower Cu amount causing a larger hole concentration. The larger mass fluctuation on the Se/Te site resulted in a lower lattice thermal conductivity, but the decreased Seebeck coefficient mitigated a performance increase in form of a higher figure-of-merit. In contrast to other Cu chalcogenides, the data are reproducible under the measurement conditions.
Mg2(Si,Sn)-based compounds have shown great promise for thermoelectric (TE) applications, as they are nontoxic and comprised abundantly available constituent elements. In this work, the crystal structures and TE properties of polycrystalline materials with nominal compositions Mg2Si0.35Sn0.65- xBi x ( x = 0, 0.015, 0.030, and 0.045) and Mg2Si ySn0.97- yBi0.03 ( y = 0.30, 0.325, and 0.35) have been investigated. The electrical conductivity, Seebeck coefficient, and thermal conductivity are strongly affected by the presence of Bi. Undoped samples showed higher values of Seebeck coefficients (below 600 K), lower electrical conductivity, and lower thermal conductivity (above 600 K) in comparison to the Bi-doped samples. Furthermore, the signs of Seebeck coefficients are all negative, confirming that n-type conduction is dominant in these materials. Electrical conductivity was enhanced by increasing the Bi content up to 3% on the Si/Sn site because of the increasing amount of electron donors, and the absolute value of Seebeck coefficient decreased. When the Bi content is greater than 3%, lower zT values were obtained at 773 K. Thermal conductivity values might decrease with increasing Sn alloying for Mg2Si ySn0.97- yBi0.03, as mass and strain fluctuation caused by alloying can effectively scatter phonons. However, a different behavior was observed in higher Sn content material, possibly because of the absence of Mg atoms at the interstitial site [Mgi, on (1/2, 1/2, 1/2)] and vacancies of Mg atoms at the (1/4, 1/4, 1/4) site, as confirmed by Rietveld refinements. Outstanding figure of merit values in excess of unity were achieved with all samples, culminating in zTmax = 1.35.
A new state-of-the-art thermoelectric material, Tl2Ag12Te7+δ, which possesses an extremely low thermal conductivity of about 0.25 W m-1 K-1 and a high figure-of-merit of up to 1.1 at 525 K, was obtained using a conventional solid-state reaction approach. Its subcell is a variant of the Zr2Fe12P7 type, but ultimately its structure was refined as a composite structure of a Tl2Ag12Te6 framework and a linear Te atom chain running along the c axis. The super-space group of the framework was determined to be P63(00γ) s with a = b = 11.438(1) Å, c = 4.6256(5) Å, and that of the Te chain substructure has the same a and b axes, but c = 3.212(1) Å, space group P6(00γ) s. The modulation leads to the formation of Te2 and Te3 fragments in this chain and a refined formula of Tl2Ag11.5Te7.4. The structure consists of a complex network of three-dimensionally connected AgTe4 tetrahedra forming channels filled with the Tl atoms. The electronic structures of four different models comprising different Te chains, Tl2Ag12Te7, Tl2Ag12Te7.33, and 2× Tl2Ag12Te7.5, were computed using the WIEN2k package. Depending on the Te content within the chain, the models are either semiconducting or metallic. Physical property measurements revealed semiconducting properties, with an ultralow thermal conductivity, and excellent thermoelectric properties at elevated temperatures.
A few thallium based layered chalcogenides of α-NaFeO2 structure-type are known for their excellent thermoelectric properties and interesting topological insulator nature. TlScQ2 belongs to this structural category. In the present work, we have studied the electronic structure, electrical and thermal transport properties and thermal stability of the title compounds within the temperature range 2-600 K. Density functional theory (DFT) predicts a metallic nature for TlScTe2 and a semiconducting nature for TlScSe2. DFT calculations also show significant lowering of energies of frontier bands upon inclusion of spin-orbit coupling contribution in the calculation. The electronic structure also shows the simultaneous occurrence of holes and electron pockets for the telluride. Experiments reveal that the telluride shows a semi-metallic behaviour whereas the selenide is a semiconductor. The thermoelectric properties for both the materials were also investigated. Both these materials possess very low thermal conductivity which is an attractive feature for thermoelectrics. However, they lack thermal stability and decompose upon warming above room temperature, as evidenced from high temperature powder X-ray diffraction and thermal analysis.
Considering the effect of CO2 emission together with the depletion of fossil fuel resources on future generations, industries in particular the transportation sector are in deep need of a viable solution to follow the environmental regulation to limit the CO2 emission. Thermoelectrics may be a practical choice for recovering the waste heat, provided their conversion energy can be improved. Here, the high temperature thermoelectric properties of high purity Bi doped Mg2(Si,Sn) are presented. The samples Mg2Si1-x-ySnxBiy with x(Sn) ≥ 0.6 and y(Bi) ≥ 0.03 exhibited electrical conductivities and Seebeck coefficients of approximately 1000 Ω-1 cm-1 and -200 µV K-1 at 773 K, respectively, attributable to a combination of band convergence and microstructure engineering through ball mill processing. In addition to the high electrical conductivity and Seebeck coefficient, the thermal conductivity of the solid solutions reached values below 2.5 W m-1 K-1 due to highly efficient phonon scattering from mass fluctuation and grain boundary effects. These properties combined for zT values of 1.4 at 773 K with an average zT of 0.9 between 400 and 773 K. The transport properties were both highly reproducible across several measurement systems and were stable with thermal cycling.
These quaternary chalcogenides are isostructural, crystallizing in a unique structure type comprising localized Cu clusters and Te(2)(2-) dumbbells. With less than six Cu atoms per formula unit, these materials are p-type narrow-gap semiconductors, according to the balanced formula Ba(2+)(Cu(+))6Q(2-)(Te(2)(2-))3 with Q = S, Se. Encouraged by the outstanding thermoelectric performance of Cu(2-x)Se and the low thermal conductivity of cold-pressed BaCu(5.7)Se(0.6)Te(6.4), we determined the thermoelectric properties of hot-pressed pellets of BaCu(5.9)STe(6) and BaCu(5.9)SeTe(6). Both materials exhibit a high Seebeck coefficient and a low electrical conductivity, combined with very low thermal conductivity below 1 W m(-1) K(-1). Compared to the sulfide-telluride, the selenide-telluride exhibits higher electrical and thermal conductivity and comparable Seebeck coefficient, resulting in superior figure-of-merit values zT, exceeding 0.8 at relatively low temperatures, namely, around 600 K.
Composites made of Higher Manganese Silicide (HMS)-based compound MnSi1.75Ge0.02 and multi-walled carbon nanotubes (MWCNTs) were prepared by an easy and effective method including mechanical milling under mild conditions and reactive spark plasma sintering. SEM compositional mappings show a homogeneous dispersion of MWCNTs in the HMS matrix. Electronic and thermal transport properties were measured from room temperature to 875 K. While power factors are virtually unchanged by the addition of MWCNTs, the lattice thermal conductivity is significantly reduced by about 30%. As a consequence, the maximum figure of merit for the composites with 1 wt% MWCNTs is improved by about 20% compared to the MWCNT free HMS-based sample.
Due to increasing global energy concerns, alternative sustainable methods to create energy such as thermoelectric energy conversion have become increasingly important. Originally, research into thermoelectric materials was focused on tellurides of bismuth and lead because of the exemplary thermoelectric properties of Bi2Te3 and PbTe. These materials, however, contain toxic lead and tellurium, which is also scarce and thus expensive. A viable alternative material may exist in Mg2Si, which needs to be doped and alloyed in order to achieve reasonable thermoelectric efficiency. Doping is a major problem, as p-type doping has thus far not produced competitive efficiencies, and n-type doping is problematic because of the low solubility of the typical dopants Sb and Bi. This investigation shows experimentally that these dopants can indeed replace Si in the crystal lattice, and excess Sb and Bi atoms are present in the grain boundaries in the form of Mg3Sb2 and Mg3Bi2. As a consequence, the carrier concentration is lower than the formal Sb/Bi concentration suggests, and the thermal conductivity is significantly reduced. DFT calculations are in good agreement with the experimental data, including the band gap and the Seebeck coefficient. Overall, this results in competitive efficiencies despite the low carrier concentration. While ball-milling was previously shown to enhance the solubility of the dopants and thus the carrier concentration, this did not lead to enhanced thermoelectric properties.
We have synthesized and characterized new thallium chalcogenides of the general formula Tl2PbMQ4 (M = Zr, Hf; Q = S, Se) from the constituent elements via high-temperature reaction conditions. These sulfides and selenides crystallize in the monoclinic crystal system (space group C2/c). The unit cell parameters refined from single-crystal X-ray diffraction data for Tl2PbZrS4 are a = 15.455(4) Å, b = 8.214(2) Å, c = 6.751(2) Å, ß = 109.093(3)°, and V = 809.9(4) Å(3), with Z = 4. No corresponding tellurides were obtained from similar reaction conditions. The isostructural quaternary chalcogenides form a layered structure, composed of alternating metal and chalcogen layers. The latter are packed along the a axis as in the face-centered cubic packing (ABC), while the metal layers alternate between Tl layers and mixed Pb/Zr layers. All metal atoms are located in differently distorted Q6 octahedra, with the TlQ6 polyhedra being the least regular ones. Density functional theory based electronic structure calculations with inclusion of relativistic spin-orbit interactions predict (indirect) energy band gaps of 0.66 and 0.33 eV for Tl2PbZrS4 and Tl2PbHfSe4, respectively. Optical spectroscopy revealed significantly larger (direct) band gaps of 1.2 and 1.6 eV. The semiconducting character is in agreement with the charge-balanced formula (Tl(+))2Pb(2+)M(4+)(Q(2-))4. The electrical transport properties also show the semiconducting nature of these materials. For Tl2PbHfSe4, the Seebeck coefficient increases from +190 µV K(-1) at room temperature to +420 µV K(-1) at 520 K.
Chalcogens/chemistry , Models, Molecular , Quantum Theory , Thallium/chemistry , Crystallography, X-Ray , Electron Transport
