The effect of Bi (semimetal) nanoinclusions in nanostructured Bi2Te3 matrices is investigated. Bismuth nanoparticles synthesized by a low temperature solvothermal method are incorporated into Bi2Te3 matrix phases, synthesized by planetary ball milling. High density pellets of the Bi nanoparticle/Bi2Te3 nanocomposites are created by hot pressing the powders at 200 °C and 100 MPa. The effect of different volume fractions (0–7%) of Bi semimetal nanoparticles on the Seebeck coefficient, electrical conductivity, thermal conductivity and carrier concentration is reported. Our results show that the incorporation of semimetal nanoparticles results in a reduction in the lattice thermal conductivity in all the samples. A significant enhancement in power factor is observed for Bi nanoparticle volume fraction of 5% and 7%. We show that it is possible to reduce the lattice thermal conductivity and increase the power factor resulting in an increase in figure of merit by a factor of 2 (from ZT = 0.2 to 0.4). Seebeck coefficient and electrical conductivity as a function of carrier concentration data are consistent with the electron filtering effect, where low‐energy electrons are preferentially scattered by the barrier potentials set up at the semimetal nanoparticle/semiconductor interfaces.

Transition metal compounds exhibiting spontaneous drops in magnetization are being investigated for use as molecular switches, sensors, and data storage devices. This phenomenon of magnetization change is generally associated with spin transition or spin crossover (high spin to low spin) induced by temperature, pressure, or irradiation, and is generally found in insulating antiferromagnetic oxides [1][2][3][4][5] and in transition metal complexes containing 3d n (4 n 7) ions, such as iron(II), iron(III), or cobalt(III), [3,[6][7][8][9][10][11][12][13] in octahedral or squareplanar coordination. [2,4,11,14] Spontaneous loss of magnetization can also be induced by other mechanisms, such as the spin dimerization observed in CuIr 2 S 4 , [15] the so-called spin-Peierls transition [16][17][18][19] observed in CuGeO 3 , and the Verwey transition, [20][21][22] which is commonly observed in mixed-valence transition metal oxides with the AB 2 X 4 spinel or inverse spinel structures such as magnetite (Fe 3 O 4 ).[23] The loss of magnetization in Verwey compounds is accompanied by a metal-to-insulator transition, which is interpreted as resulting from long-range ordering of the mixed-valence ions within the B sites of the spinel structure.[24]Herein we present the observation of room-temperature ferromagnetism, semiconductivity, and reversible, cooperative magnetic and semiconductor-to-insulator (SI) transitions in FeSb 2 Se 4 . To the best of our knowledge, the coexistence of these phenomena in a single transition metal chalcogenide compound has not been reported to date. Despite ) 4 (magnetite), the crystal structures of these compounds are profoundly different and none of the mechanisms mentioned above is suitable for the interpretation of the phase transitions observed in the three-dimensional monoclinic structure of FeSb 2 Se 4 . Therefore, alternative mechanisms to explain the observed transitions must be explored. Because the nature of the phase transitions in FeSb 2 Se 4 can be rather complex, we have tackled the problem by performing systematic investigations of 1) the crystal structure above and below the transition temperature, 2) the thermal evolution of unit cell parameters using X-ray diffraction on powder and on single-crystal samples, 3) the electrical resistivity, and 4) the magnetic properties across the transition temperature. FeSb 2 Se 4 (see Supporting Information [25] for experimental details) crystallizes in the monoclinic space group C2m (No. 12) with lattice parameters a = 13.069(3) , b = 3.9671(8) , c = 15.192(4) , and b = 114.99(3)8, and it is isostructural with MnSb 2 S 4 .[26] The structure contains four crystallographically independent metal positions and four Se positions. All metal sites located at special positions (Fe(3) at (0,

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Keywords: Manganese / Antimony / Selenium / Semiconductors / Magnetic properties / Thermoelectricity A single phase of MnSb 2 Se 4 was synthesized by combining high-purity elements at 773 K. Single-crystal X-ray diffraction revealed that MnSb 2 Se 4 is isostructural with FeSb 2 Se 4 crystallizing in the monoclinic space group C2/m with a = 13.076(3) Å, b = 3.965(2) Å, c = 15.236(4) Å, β = 115.1(2)°, Z = 4. MnSb 2 Se 4 melts congruently at 790 K and is thermally stable up to 1000 K. Electronic band structure calculations, infrared diffuse reflectance spectroscopy, and low-temperature electronic transport data indicate that MnSb 2 Se 4 is a narrow-bandgap p-type semiconductor with an energy

Thermoelectric properties, X-ray photoelectron spectroscopy, Raman spectroscopy, and electronic structures have been studied for Mn-substituted CuInSe 2 chalcopyrites. Raman spectroscopy verifies the lattice disorder due to the introduction of Mn into the CuInSe 2 matrix, leading to a slight suppression of thermal conductivity. On the other hand, the Mn substitution significantly increases the electrical conductivity and Seebeck coefficient. Therefore the thermoelectric figure of merit, ZT, has been enhanced by over two orders of magnitude by the introduction of Mn into CuInSe 2 . These materials are p-type degenerate semiconductors, containing divalent Mn species as confirmed by X-ray photoelectron spectroscopy. The crystal structure of Mn-substituted CuInSe 2 , as well as related ternary and quaternary diamond-like semiconductors, can be viewed as a combination of an electrically conducting unit, the

A new family of quasi-one-dimensional ferromagnetic selenides with general formula Fe(x)Pb(4-x)Sb(4)Se(10) (0 < or = x < or = 2) was generated by isoelectronic substitution in octahedral positions of Pb atoms by Fe within the structure of Pb(4)Sb(4)Se(10). Two members of this family with x = 0.75 and x = 1 were synthesized as a single phase through direct combination of the elements at 823 K. Single crystal X-ray diffraction revealed that Fe(0.75)Pb(3.25)Sb(4)Se(10) crystallizes with the orthorhombic space group Pnma, whereas Fe(0.96)Pb(3.04)Sb(4)Se(10) adopts the lower symmetry monoclinic subgroup P2(1)/m (#11). Both compounds are isomorphous with Pb(4)Sb(4)Se(10), and their crystal structures consist of corrugated layers of edge-sharing bicapped trigonal prisms and octahedra around Pb atoms. Adjacent layers are interconnected by NaCl-type {SbSe} ribbons. The voids left by this arrangement are filled by the novel one-dimensional {Fe(2)Se(10)} double chains (ladder) of edge-sharing octahedra running along [010]. Temperature dependent magnetic susceptibility as well as field dependent magnetization isotherms showed that both Fe(0.75)Pb(3.25)Sb(4)Se(10) and FePb(3)Sb(4)Se(10) are ferromagnetic below 300 K and exhibit superparamagnetism at higher temperatures. A dramatic reduction in the magnetic moment per Fe(2+), approximately 0.40 micro(B), was observed in Fe(0.75)Pb(3.25)Sb(4)Se(10) and FePb(3)Sb(4)Se(10) suggesting that the Fe(x)Pb(4-x)Sb(4)Se(10) (0 < or = x < or = 2) phases are not ordinary ferromagnets where all the magnetic spins are parallel at low temperatures. Analysis of the magnetic coupling of spins located on adjacent Fe atoms (within a localized Fe(2+) moment picture) using Goodenough-Kanamori rules suggested that the magnetism within the ladder and ladder-single chain systems in Fe(x)Pb(4-x)Sb(4)Se(10) phases is controlled by competing interactions.

The concept of band structure engineering near the Fermi level through atomic-scale alteration of a bulk semiconductor crystal structure using coherently embedded intrinsic semiconducting quantum dots provides a unique opportunity to manipulate the transport behavior of the existing ensembles of carriers within the semiconducting matrix. Here we show that in situ growth of coherent nanometer-scale full-Heusler quantum dots (fH-QDs) within the p-type Ti(0.5)Hf(0.5)CoSb(0.9)Sn(0.1) half-Heusler (hH) matrix induces a drastic decrease of the effective hole density within the hH/fH-QD nanocomposites at 300 K followed by a sharp increase with rising temperature. This behavior is associated with the formation of staggered heterojunctions with a valence band (VB) offset energy, ΔE at the hH/fH phase boundaries. The energy barrier (ΔE) discriminates existing holes with respect to their energy by trapping low energy (LE) holes, while promoting the transport of high energy (HE) holes through the VB of the fH-QDs. This "hole culling" results in surprisingly large increases in the mobility and the effective mass of HE holes contributing to electronic conduction. The simultaneous reduction in the density and the increase in the effective mass of holes resulted in large enhancements of the thermopower whereas; the increase in the mobility minimizes the drop in the electrical conductivity.

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