Resonant bonding, multiband thermoelectric transport and native defects in n-type BaBiTe_(3-x)Se_x (x = 0, 0.05 and 0.1)

The unique crystal structure of BaBiTe_3 containing Te···Te resonant bonds and its narrow band gap motivated the systematic study of the thermoelectric transport properties of BaBiTe_(3–x)Se_x (x = 0, 0.05, and 0.1) presented here. This study gives insight in the chemical bonding and thermoelectric transport properties of BaBiTe_3. The study shows that the presence of Te···Te resonant bonds in BaBiTe_3 is best described as a linear combination of interdigitating (Te^(1–))_2 side groups and infinite Te_n chains. Rietveld X-ray structure refinements and extrinsic defect calculations reveal that the substitution of Te by Se occurs preferentially on the Te4 and Te5 sites, which are not involved in Te···Te bonding. This work strongly suggests that both multiband effects and native defects play an important role in the transport properties of BaBiTe_(3–x)Se_x (x = 0, 0.05, and 0.1). The carrier concentration of BaBiTe_3 can be tuned via Se substitution (BaBiTe_(3–x)Se_x with x = 0, 0.05, and 0.1) to values near those needed to optimize the thermoelectric performance. The thermal conductivity of BaBiTe_(3–x)Se_x (x = 0, 0.05, and 0.1) is found to be remarkably low (ca. 0.4 Wm^(–1)K^(–1) at 600 K), reaching values close to the glass limit of BaBiSe_3 (0.34 W m^(–1) K^(–1)) and BaBiTe_3 (0.28 W m^(–1) K^(–1)). Calculations of the defect formation energies in BaBiTe_3 suggest the presence of native Bi_(Ba)^(+1) and Te_(Bi)^(+1) antisite defects, which are low in energy and likely responsible for the native n-type conduction and the high carrier concentration (ca. 10^(20) cm^(–3)) found for all samples. The analyses of the electronic structure of BaBiTe_3 and of the optical absorption spectra of BaBiTe_(3–x)Se_x (x = 0, 0.05, 0.1, and 3) strongly suggest the presence of multiple electron pockets in the conduction band (CB) in all samples. These analyses also provide a possible explanation for the two optical transitions observed for BaBiTe_3. High-temperature optical absorption measurements and thermoelectric transport analyses indicate that bands higher in the conduction band converge with the conduction band minimum (CBM) with increasing temperature and contribute to the thermoelectric transport properties of BaBiTe_3 and BaBiTe_(2.95)Se_(0.05). This multiband contribution can account for the ∼50% higher zT_(max) of BaBiTe_3 and BaBiTe_(2.95)Se_(0.05) (∼0.4 at 617 K) compared to BaBiTe_(2.9)Se_(0.1) (∼0.2 at 617 K), for which no such contribution was found. The increase in the band offset between the CBM and bands higher in the conduction band with respect to the selenium content is one possible explanation for the absence of multiband effects in the thermoelectric transport properties of BaBiTe_(2.9)Se_(0.1)

Roadmap on energy harvesting materials

Ambient energy harvesting has great potential to contribute to sustainable development and address growing environmental challenges. Converting waste energy from energy-intensive processes and systems (e.g. combustion engines and furnaces) is crucial to reducing their environmental impact and achieving net-zero emissions. Compact energy harvesters will also be key to powering the exponentially growing smart devices ecosystem that is part of the Internet of Things, thus enabling futuristic applications that can improve our quality of life (e.g. smart homes, smart cities, smart manufacturing, and smart healthcare). To achieve these goals, innovative materials are needed to efficiently convert ambient energy into electricity through various physical mechanisms, such as the photovoltaic effect, thermoelectricity, piezoelectricity, triboelectricity, and radiofrequency wireless power transfer. By bringing together the perspectives of experts in various types of energy harvesting materials, this Roadmap provides extensive insights into recent advances and present challenges in the field. Additionally, the Roadmap analyses the key performance metrics of these technologies in relation to their ultimate energy conversion limits. Building on these insights, the Roadmap outlines promising directions for future research to fully harness the potential of energy harvesting materials for green energy anytime, anywhere

Molybdenum, Tungsten, and Aluminium Substitution for Enhancement of the Thermoelectric Performance of Higher Manganese Silicides

An easy and efficient process involving ball milling under soft conditions and spark plasma sintering was used to synthesize higher manganese silicide (HMS)-based compounds, for example MnSi1.75Ge0.02, with different molybdenum, tungsten, and aluminium substitution. The x-ray diffraction patterns of the samples after sintering showed the main phase to be HMS with the presence of some side products. Molybdenum substitution enlarges the unit cells more than tungsten substitution, owing to its greater solubility in the HMS structure, whereas substitution with aluminium did not substantially alter the cell parameters. The electrical resistivity of HMS-based compounds was reduced by andlt;10% by this substitution, because of increased carrier concentrations. Changes of the Seebeck coefficient were insignificant after molybdenum and aluminium substitution whereas tungsten substitution slightly reduced the thermopower of the base material by approximately 8% over the whole temperature range; this was ascribed to reduced carrier mobility as a result of enhanced scattering. Substitution with any combination of two of these elements resulted in no crucial modification of the electrical properties of the base material. Large decreases of lattice thermal conductivity were observed, because of enhanced phonon scattering, with the highest reduction up to 25% for molybdenum substitution; this resulted in a 20% decrease of total thermal conductivity, which contributed to improvement of the figure of merit ZT of the HMS-based materials. The maximum ZT value was approximately 0.40 for the material with 2 at.% molybdenum substitution at the Mn sites. © 2015, The Minerals, Metals and Materials Society

Crystal and electronic structures of two new iron selenides Ba<inf>4</inf>Fe<inf>3</inf>Se<inf>10</inf> and BaFe<inf>2</inf>Se<inf>4</inf>

The new ternary selenides, Ba4Fe3Se10 and BaFe2Se4, were synthesized from a reaction of appropriate amounts of elements at high temperature in a silica sealed tube, and their structures were resolved using X-ray single crystal diffraction. BaFe2Se4 crystallizes in the tetragonal space group I4/m with a=8.008(9) Å and c=5.483(3) Å as cell parameters. It is a new compound with a structure isotypical to the sulfide BaFe2S4 which belongs to the infinitely adaptive structures series Ba1+xFe2S4. The second compound, Ba4Fe3Se10, crystallizes in the monoclinic space group P21/n with a=8.8593(1) Å, b=8.8073(1) Å, c=12.2724(1) Å and β=109.037(6)° as cell parameters. It exhibits an original structure with a new type of iron selenide polyhedra. These data were consistent with the powder X-ray diffraction and TEM analyses. Their electronic structures point towards metallicity and electronic correlations for both selenides.2015 Published by Elsevier Inc

Crystal and electronic structures of two new iron selenides Ba<inf>4</inf>Fe<inf>3</inf>Se<inf>10</inf> and BaFe<inf>2</inf>Se<inf>4</inf>

International audienceThe new ternary selenides, Ba4Fe3Se10 and BaFe2Se4, were synthesized from a reaction of appropriate amounts of elements at high temperature in a silica sealed tube, and their structures were resolved using X-ray single crystal diffraction. BaFe2Se4 crystallizes in the tetragonal space group I4/m with a=8.008(9) Å and c=5.483(3) Å as cell parameters. It is a new compound with a structure isotypical to the sulfide BaFe2S4 which belongs to the infinitely adaptive structures series Ba1+xFe2S4. The second compound, Ba4Fe3Se10, crystallizes in the monoclinic space group P21/n with a=8.8593(1) Å, b=8.8073(1) Å, c=12.2724(1) Å and β=109.037(6)° as cell parameters. It exhibits an original structure with a new type of iron selenide polyhedra. These data were consistent with the powder X-ray diffraction and TEM analyses. Their electronic structures point towards metallicity and electronic correlations for both selenides.2015 Published by Elsevier Inc