Improved thermoelectric properties in ceramic composites based on Ca3Co4O9 and Na2Ca2Nb4O13

The oxide materials Ca3Co4O9 and Na2Ca2Nb4O13 were combined in a new ceramic composite with promising synergistic thermoelectric properties. Both compounds show a plate-like crystal shape and similar aspect ratios but the matrix material Ca3Co4O9 with lateral sizes of less than 500 nm is about two orders of magnitude smaller. Uniaxial pressing of the mixed compound powders was used to produce porous ceramics after conventional sintering. Reactions between both compounds and their compositions were thoroughly investigated. In comparison to pure Ca3Co4O9, mixing with low amounts of Na2Ca2Nb4O13 proved to be beneficial for the overall thermoelectric properties. A maximum figure-of-merit of zT = 0.32 at 1073 K and therefore an improvement of about 19% was achieved by the ceramic composites

Anomalous Raman Modes in Tellurides

Two broad bands are usually found in the Raman spectrum of many Te-based chalcogenides, which include binary compounds, like ZnTe, CdTe, HgTe, GaTe, GeTe, SnTe, PbTe, GeTe2, As2Te3, Sb2Te3, Bi2Te3, NiTe2, IrTe2, TiTe2, as well as ternary compounds, like GaGeTe, SnSb2Te4, SnBi2Te4, and GeSb2Te5. Many different explanations have been proposed in the literature for the origin of these two anomalous broad bands in tellurides, usually located between 119 and 145 cm-1. They have been attributed to the own sample, to oxidation, to the folding of Brillouin-edge modes onto the zone center, to the existence of a double resonance, like that of graphene, or to the formation of Te precipitates. In this paper, we provide arguments to demonstrate that such bands correspond to clusters or precipitates of trigonal Te in form of nanosize or microsize grains or layers that are segregated either inside or at the surface of the samples. Several mechanisms for Te segregation are discussed and sample heating caused by excessive laser power during Raman scattering measurements is emphasized. Finally, we show that anomalous Raman modes related to Se precipitates also occur in selenides, thus providing a general vision for a better characterization of selenides and tellurides by means of Raman scattering measurements and for a better understanding of chalcogenides in general.Comment: 45 pages, 8 figure

Substituted Ge–Sb–Te materials: structure, element distribution and thermoelectric properties

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Large Exchange Bias, High Dielectric Constant, and Outstanding Ionic Conductivity in a Single Phase Spin Glass

The multigram synthesis of K [Fe S ] starting from K S and FeS is presented, and its electronic and magnetic properties are investigated. The title compound obtains a defect variant of the K[Fe Se ] structure type. Dielectric and impedance measurements indicate a dielectric constant of 1120 at 1 kHz and an outstanding ionic conductivity of 24.37 mS cm at 295 K, which is in the range of the highest reported value for potential solid state electrolytes for potassium ion batteries. The Seebeck coeffcient of the n type conductor amounts to amp; 8722;60 amp; 956;V K at 973 K. The mismatch of the measured electrical resistivity and the predicted metal like band structure by periodic quantum chemical calculations indicates Mott insulating behavior. Magnetometry demonstrates temperature dependent, large exchange biasfields of 35 mT, as a consequence of the coexistence of spin glass and antiferromagnetic orderings due to the iron vacancies in the lattice. In addition, the decreasing training effects of 34 in the exchange bias are identified at temperatures lower than 20 K. These results demonstrate the critical role of iron vacancies in tuning the electronic and magnetic properties and a multifunctional material from abundant and accessible element

Structural and electrical study of the topological insulator SnBi2Te4 at high pressures

We report high-pressure X-ray diffraction and electrical measurements of the topological insulator SnBi2Te4 at room temperature. The pressure dependence of the structural properties of the most stable phase of SnBi2Te4 at ambient conditions (trigonal phase) have been experimentally determined and compared with results of our ab initio calculations. Furthermore, a comparison of SnBi2Te4 with the parent compound Bi2Te3 shows that the central TeSnTe trilayer, which substitutes the Te layer at the center of the TeBiTeBiTe layers of Bi2Te3, plays a minor role in the compression of SnBi2Te4. Similar to Bi2Te3, our resistance measurements and electronic band structure simulations in SnBi2Te4 at high pressure suggest that this compound exhibits a pressure-induced electronic topological transition or Lifshitz transition between 3.5 and 5.0 GPa. (C) 2016 Published by Elsevier B.V.We thank Dr. Philipp Urban for preparing the sample. This work has been performed under financial support from Spanish MINECO under projects MAT2013-46649-C4-2-P, MAT2015-71070-REDC and CTQ2015-67755-C2-1-R and from Spanish Ministerio de Educacion, Cultura y Deporte as part of "Programa Campus de Excelencia Internacional/Programa de Valoracion y Recursos Conjuntos de I + D + i VLC/CAMPUS" through projects SP20140701 and SP20140871. One of the experiments were performed at MSPD-BL04 beamline at ALBA Synchrotron with the collaboration of ALBA staff. J.A.S. thanks "Juan de la Cierva" fellowship program for funding. A. A.-C. and J.S.-B. are also grateful to Spanish MINECO for the FPI (BES-2013-066112) and Ramon y Cajal (RyC-2010-06276) fellowships. We acknowledge Diamond Light Source for time on beamline I15 under Proposal EE9102.Vilaplana Cerda, RI.; Sans Tresserras, JÁ.; Manjón Herrera, FJ.; Andrada-Chacón, A.; Sánchez-Benitez, J.; Popescu, C.; Gomis, O.... (2016). Structural and electrical study of the topological insulator SnBi2Te4 at high pressures. Journal of Alloys and Compounds. 685:962-970. https://doi.org/10.1016/j.jallcom.2016.06.170S96297068