Enhanced thermoelectric properties of the Zintl phase BaGa_2Sb_2 via doping with Na or K

Na- or K-doped samples of Ba_(1−x)(Na, K)xGa_2Sb_2 were prepared by ball-milling followed by hot-pressing. The topological analysis of the electron density of BaGa_2Sb_2 implies a polar covalent nature of the Sb–Ga bonds in which the Sb atoms receive the electrons transferred from Ba rather than the Ga atoms. Successful doping of BaGa_2Sb_2 with Na or K was confirmed with combined microprobe and X-ray diffraction analysis. Alkali metal doping of BaGa_2Sb_2 increased the p-type charge carrier concentration to almost the predicted optimum values (∼10^(20) h^+ cm^(−3)) needed to achieve high thermoelectric performance. With increasing alkali metal concentration, electronic transport was shifted from non-degenerate semiconducting behaviour observed for BaGa_2Sb_2 to degenerate one for Na- or K-doped compounds. Overall, the thermoelectric figure of merit, zT, values reached up to ∼0.65 at 750 K, considerably higher than the undoped sample (zT ∼ 0.1 at 600 K), and a slight improvement relative to previously reported Zn-doped samples (∼0.6 at 800 K)

Thermoelectric Enhancement in BaGa_2Sb_2 by Zn Doping

The Zintl phase BaGa_2Sb_2 has a unique crystal structure in which large tunnels formed by ethane-like dimeric [Sb_3Ga−GaSb_3] units are filled with Ba atoms. BaGa_2Sb_2 was obtained in high purity from ball-milling followed by hot pressing. It shows semiconducting behavior, in agreement with the valence precise Zintl counting and band structure calculations, with a band gap ∼0.4 eV. The thermal conductivity of BaGa_2Sb_2 is found to be relatively low (0.95 W/K m at 550 K), which is an inherent property of compounds with complex crystal structures. As BaGa_2Sb_2 has a low carrier concentration (∼2 × 10^18 h^+/cm^3) at room temperature, the charge carrier tuning was performed by substituting trivalent Ga with divalent Zn. Zn-doped samples display heavily doped p-type semiconducting behavior with carrier concentrations in the range (5−8) × 10^19 h^+/cm^3. Correspondingly, the zT values were increased by a factor of 6 by doping compared to the undoped sample, reaching a value of ∼0.6 at 800 K. Zn-doped BaGa_2Sb_2 can thus be considered as a promising new thermoelectric material for intermediate-temperature applications

Thermoelectric properties of the Zintl phases Yb_5M_2Sb_6 (M = Al, Ga, In)

Zintl compounds with chemical formula Yb_5M_2Sb_6 (M = Al, Ga, and In) form one of two known A_5M_2Pn_6 structure types characterized by double chains of corner-linked MPn_4 tetrahedra bridged by Pn_2 dumbbells. High temperature electronic and thermal transport measurements were used to characterize the thermoelectric properties of Yb_5M_2Sb_6 compounds. All samples were found to exhibit similar high p-type carrier concentrations, low resistivity and low Seebeck coefficients in agreement with the band structure calculations. These results, combined with previous studies, suggest that Yb_5M_2Sb_6 compounds are semimetals (i.e., they lack an energy gap between the valence and conduction bands), in contrast to the semiconducting alkaline earth (Ca, Sr, Ba) and Eu based A_5M_2Sb_6 compounds. Yb_5M_2Sb_6 compounds have very low lattice thermal conductivity, comparable to other closely related A_5M_2Sb_6 and A_3MSb_3 phases. However, due to the semimetallic behaviour, the figure of merit of investigated samples remains low (zT < 0.15)

AFe2As2 (A = Ca, Sr, Ba, Eu) and SrFe_(2-x)TM_(x)As2 (TM = Mn, Co, Ni): crystal structure, charge doping, magnetism and superconductivity

The electronic structure and physical properties of the pnictide compound families $RE$OFeAs ($RE$ = La, Ce, Pr, Nd, Sm), $A$Fe$_{2}$As$_{2}$ ($A$ = Ca, Sr, Ba, Eu), LiFeAs and FeSe are quite similar. Here, we focus on the members of the $A$Fe$_{2}$As$_{2}$ family whose sample composition, quality and single crystal growth are better controllable compared to the other systems. Using first principles band structure calculations we focus on understanding the relationship between the crystal structure, charge doping and magnetism in $A$Fe$_{2}$As$_{2}$ systems. We will elaborate on the tetragonal to orthorhombic structural distortion along with the associated magnetic order and anisotropy, influence of doping on the $A$ site as well as on the Fe site, and the changes in the electronic structure as a function of pressure. Experimentally, we investigate the substitution of Fe in SrFe$_{2-x}TM_{x}$As$_{2}$ by other 3$d$ transition metals, $TM$ = Mn, Co, Ni. In contrast to a partial substitution of Fe by Co or Ni (electron doping) a corresponding Mn partial substitution does not lead to the supression of the antiferromagnetic order or the appearance of superconductivity. Most calculated properties agree well with the measured properties, but several of them are sensitive to the As $z$ position. For a microscopic understanding of the electronic structure of this new family of superconductors this structural feature related to the Fe-As interplay is crucial, but its correct ab initio treatment still remains an open question.Comment: 27 pages, single colum

High-pressure modification of BiI3

Structural and optical properties as well as chemical bonding of BiI3 at elevated pressures are investigated by means of refinements of X-ray powder diffraction data, measurements of the optical absorption, and calculations of the band structure involving bonding analysis in real space. The data evidence the onset of a phase transition from trigonal (hR24) BiI3 into PuBr3-type (oS16) BiI3 around 4.6 GPa. This high-pressure modification remains stable up to 40 GPa, the highest pressure of this study. The phase exhibits semiconducting properties with constantly decreasing band gap between 5 and 18 GPa. Above this pressure, the absorbance edge broadens significantly. Extrapolation of the determined band gap values implies a semiconductor to metal transition at approximately 35 GPa. The value is in accordance with subtle structural anomalies and the results of band structure calculations. Topological analysis of the computed electron density and the electron-localizability indicator reveal fingerprints for weak covalent Bi-I contributions in addition to dominating ionic interactions for both modifications. © 2019 by the authors. Licensee MDPI, Basel, Switzerland

On the twinning in ZnPd

The intermetallic compound ZnPd has demonstrated excellent catalytic properties in methanol steam reforming. While it is known that defects and microstructures influence the catalytic properties, little is known about the defects occurring in ZnPd. Due to recent advances in synthetic methods, coarse-grained ZnPd samples are accessible. This enables the detection and investigation of twinning in ZnPd by studying the twinned regions from the macroscopic scale by polarised light and electron backscattering diffraction (EBSD) down to the atomic scale by high-resolution transmission electron microscopy (HR-TEM). Twinning occurs in {101} and is coupled with a change in the c/a ratio in the vicinity of the twin boundary. Quantum chemical calculations result in only very small energy differences between the ideal and the twinned structure, explaining the experimentally observed thermal stability of the latter. The chemical bonding was investigated by the electron localizability indicator (ELI) and compared to the one in the ideal structure. The results confirm twinning along the {101} plane and demonstrate the high stability of the twin boundaries after formation

Synthesis of the intermetallic clathrate Na2Ba6Si46 by oxidation of Na2BaSi4 with HCl

A new preparation route to the intermetallic clathrate-I compound Na2Ba6Si46 is introduced, which allows one to make large amounts of product with standard laboratory equipment. The precursor Na2BaSi4 is oxidized with gaseous HCl at 673 K to Na2Ba6Si46, NaCl and BaCl2. Full-profile refinement of the crystal structure from the X-ray powder diffraction data revealed a composition close to Na2Ba6Si46 (Na1.94(1)Ba6.06(1)Si46, space group Pmbar 3n, a=10.281(1) Å). Differential scanning calorimetry showed an exothermic effect at 874 K, indicating that Na2Ba6Si46 is metastable. The product was additionally characterized by scanning electron microscopy. The electronic structure of Na2Ba6Si46 was investigated by a first-principles, all-electron full-potential method, predicting metallic conductivity. Na2Ba6Si46 obtained by oxidation with HCl shows Pauli paramagnetism; no bulk superconductivity was found down to 1.8 K in a magnetic field of 20 Oe