Search

Thermoelectric effects are one of the promising ways to utilize waste heat. Novel approaches have appeared in recent decades aiming to enhance thermoelectric conversion. The theory of energy filtering of free carriers by inclusions is among the latest developed methods. Although the basic idea is clear, experimental evidence of this phenomenon is rare. Based on this concept, we searched suitable systems with stable structures showing energy filtering. Here, we report on the anomalous behavior of Cr-doped single-crystal Bi2Se3 that indicates energy filtering. The solubility of chromium in Bi2Se3 was studied, which is the key parameter in the formation process of inclusions. We present recent results on the effect of Cr-doping on the transport coefficients on a wide set of single crystalline samples. Magnetic measurements were used to corroborate the conclusions drawn from the transport and X-ray measurements.

Tetradymite-type Bi2Se3 is synthesised via a solid-state method, and its phase evolution in Li, Na and K half-cells is experimentally investigated. Ex-situ X-ray diffraction data is analysed with the Rietveld method, indicating intercalation, conversion and alloying reactions for all systems. Direct evidence of alloying is observed in the cases of the Li and Na systems while alloying is inferred for the K system. In addition, a preliminary study of the performance of Bi2Se3 as an electrode material for rechargeable Li, Na and K-half cells is undertaken. High initial capacities of 560, 680 and 1000 mAh/g for Li, Na and K respectively are observed. However, capacity fade is severe in all cases, with 10th cycle capacity decreases of 68%, 79% and 90% respectively. This poor cyclability is likely attributable to the large volume changes that accompany the conversion and alloying reactions. Finally, the magnetic properties of the resultant intercalated materials are investigated. Li intercalation induces a divergence between zero-field cooled and field cooled curves below 150 ​K, whereas Na and K intercalation do not have significant effects on the observed magnetic properties.

We report on the electrical and thermal transport properties of the Sn 1− x Pb x Bi 2 Te 4 series and we discuss the potential of these materials for thermoelectric conversion applications. From the evolution of the XRD patterns, we can confidently conclude that a complete solid solution exists between SnBi 2 Te 4 and PbBi 2 Te 4 , with no miscibility gap. A crossover from p-type conduction in Sn-rich samples to n-type conduction in Pb-rich ones has been observed, with a transition between x =0.3 and 0.4. A concomitant increase of the electrical resistivity and of the Seebeck coefficient has been observed in the solid solution, which leads to almost constant values of the thermoelectric power factor. Moreover, the thermal conductivity is slightly reduced in the solid solution. The best figure of merit ZT values at room temperature have been observed for p-type Sn 0.8 Pb 0.2 Bi 2 Te 4 with ZT =0.25 and for n-type Sn 0.3 Pb 0.7 Bi 2 Te 4 with ZT =0.15.

Crystals of Ti2PTe2 have been synthesised by chemical vapour transport. Ti2PTe2 crystallises, isostructural to the mineral tetradymite (Bi2STe2), in the space group R3¯m with unit-cell parameters a=3.6387(2) A and c=28.486(2) A for the hexagonal setting. In the structure, layers of isolated phosphide and telluride anions form an ordered close sphere-packing with titanium cations filling two-thirds of the octahedral voids. From XANES fluorescence, the presence of Ti4+ is clearly established. In accordance with the ionic formula (Ti4+)2(P3−)(Te2−)2(e−) metallic conductivity (ρ=40 μΩ cm at 300 K) and nearly temperature-independent paramagnetism are found. The electronic band structure shows bands of titanium states crossing the Fermi level in directions corresponding to the ab-plane and a band gap along the c-axis.

Incorporation of the transition metal elements in the tetradymite structure of Sb{sub 2}Te{sub 3} has a strong influence on electronic properties. Recent studies have indicated that Mn substitutes on the Sb sublattice increases the carrier concentration of holes. However, the doping efficiency of Mn appears rather low in comparison to what it should be based on the measurements of magnetization, structural analysis, and transport properties. In this paper we address this issue by making detailed studies of the Hall effect and electrical resistivity and we explain the results with the aid of a model that takes into account interactions of the Mn impurity with the native defects in antimony telluride. Specifically, we find that Mn atoms interact with antisite defects (antimony atoms located on the tellurium sublattice), a process that decreases the density of antisite centers and generates free electrons. These, in turn, recombine with holes and thus decrease their concentration and the apparent Mn doping efficiency. - Graphical abstract: Temperature dependence of the electrical resistivity {rho}{sub perpendicular} {sub c} of Sb{sub 2-} {sub x} Mn {sub x} Te{sub 3} single crystals (samples are labeled according to)

The electrical transport properties of the GeBi[sub 2]Te[sub 4]-, Ge[sub 3]Bi[sub 2]Te[sub 6]-, and GeBi[sub 4]Te[sub 7]-layered compounds were characterized as a function of temperature and for samples with various stoichiometric deviations. The Hall Coefficient and electrical resistivity were measured from 77 to 800 K, and the Seebeck coefficient was determined in the 90--450 K temperature range. The onset of intrinsic conductivity behavior was observed in these ternary compounds at fairly low temperatures. On the basis of the variation of the electrical conductivity at elevated temperatures, energy gap values of 0.24, 0.20, and 0.22 eV were calculated at 0 K for the Ge[sub 3]Bi[sub 2]Te[sub 6], GeBi[sub 2]Te[sub 4], and GeBi[sub 4]Te[sub 7] compounds, respectively. To explain the carrier mobility temperature dependence, a mixed carrier scattering mechanism on acoustic phonons and point defects is proposed at low temperatures. It is found that the contribution of scattering by acoustic phonons increased with temperature. The analysis of experimental data for the GeBi[sub 4]Te[sub 7] compound in the 150--200 K temperature range demonstrated the need for an additional carrier scattering mechanism, probably connected to the order-disorder phenomena in the cation layers. An anomalous temperature dependence of the Hall coefficient was discovered for themore » stoichiometric GeBi[sub 2]Te[sub 4] compound. It is explained by mixed carrier conduction effects due to fluctuations of the composition and the existence of p- and n-type conductivity domains in the GeBi[sub 2]Te[sub 4] crystal.« less

The concentration of holes in InxBi2−xTe3 crystals for x = 0.0–01 has been determined using simplified assumptions on the basis of experimental values of the transport coefficients. These data and the calculated changes of bonding parameters were used to formulate more precisely the idea of the variation of the concentration of antisite defects in mixed crystals with increasing indium concentration. The obtained results have been employed in proposing a model that makes possible a generalization on the relations among structure, bonding, and concentration of antisite defects in AIIIxBV2−xCVI3 crystals. In agreement with the model it has been shown that the increasing content of In or Tl atoms results in an increase in the ratio c a of the lattice parameters, in a decrease of the unit cell volume, in an increase of the electron density in the outer atomic planes of the layer, and in a decrease of the concentration of both the antisite defects and the free carriers.