Large reductions in the thermal conductivity of thermoelectrics using nanostructures have been widely demonstrated. Some enhancements in the thermopower through nanostructuring have also been reported. However, these improvements are generally offset by large drops in the electrical conductivity due to a drastic reduction in the mobility. Here, we show that large enhancements in the thermopower and electrical conductivity of half-Heusler (HH) phases can be achieved simultaneously at high temperatures through coherent insertion of nanometer scale full-Heusler (FH) inclusions within the matrix. The enhancements in the thermopower of the HH/FH nanocomposites arise from drastic reductions in the "effective" carrier concentration around 300 K. Surprisingly, the mobility increases drastically, which compensates for the decrease in the carrier concentration and minimizes the drop in the electrical conductivity. Interestingly, the carrier concentration in HH/FH nanocomposites increases rapidly with temperature, matching that of the HH matrix at high temperatures, whereas the temperature dependence of the mobility significantly deviates from the typical T(-α) law and slowly decreases (linearly) with rising temperature. This remarkable interplay between the temperature dependence of the carrier concentration and mobility in the nanocomposites results in large increases in the power factor at 775 K. In addition, the embedded FH nanostructures also induce moderate reductions in the thermal conductivity leading to drastic increases in the ZT of HH(1 - x)/FH(x) nanocomposites at 775 K. By combining transmission electron microscopy and charge transport data, we propose a possible charge carrier scattering mechanism at the HH/FH interfaces leading to the observed anomalous electronic transport in the synthesized HH(1 - x)/FH(x) nanocomposites.

The thermopower (S) and electrical conductivity (σ) in conventional semiconductors are coupled adversely through the carriers' density (n) making it difficult to achieve meaningful simultaneous improvements in both electronic properties through doping and/or substitutional chemistry. Here, we demonstrate the effectiveness of coherently embedded full-Heusler (FH) quantum dots (QDs) in tailoring the density, mobility, and effective mass of charge carriers in the n-type Ti(0.1)Zr(0.9)NiSn half-Heusler matrix. We propose that the embedded FH QD forms a potential barrier at the interface with the matrix due to the offset of their conduction band minima. This potential barrier discriminates existing charge carriers from the conduction band of the matrix with respect to their relative energy leading to simultaneous large enhancements of the thermopower (up to 200%) and carrier mobility (up to 43%) of the resulting Ti(0.1)Zr(0.9)Ni(1+x)Sn nanocomposites. The improvement in S with increasing mole fraction of the FH-QDs arises from a drastic reduction (up to 250%) in the effective carrier density coupled with an increase in the carrier's effective mass (m*), whereas the surprising enhancement in the mobility (μ) is attributed to an increase in the carrier's relaxation time (τ). This strategy to manipulate the transport behavior of existing ensembles of charge carriers within a bulk semiconductor using QDs is very promising and could pave the way to a new generation of high figure of merit thermoelectric materials.

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,

No abstract