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,
Pb(7)Bi(4)Se(13) crystallizes in the monoclinic space group C2/m (No. 12) with a = 13.991(3) Å, b = 4.262(2) Å, c = 23.432(5) Å, and β = 98.3(3)° at 300 K. In its three-dimensional structure, two NaCl-type layers A and B with respective thicknesses N(1) = 5 and N(2) = 4 [N = number of edge-sharing (Pb/Bi)Se6 octahedra along the central diagonal] are arranged along the c axis in such a way that the bridging monocapped trigonal prisms, PbSe7, are located on a pseudomirror plane parallel to (001). This complex atomic-scale structure results in a remarkably low thermal conductivity (∼0.33 W m(-1) K(-1) at 300 K). Electronic structure calculations and diffuse-reflectance measurements indicate that Pb(7)Bi(4)Se(13) is a narrow-gap semiconductor with an indirect band gap of 0.23 eV. Multiple peaks and valleys were observed near the band edges, suggesting that Pb(7)Bi(4)Se(13) is a promising compound for both n- and p-type doping. Electronic-transport data on the as-grown material reveal an n-type degenerate semiconducting behavior with a large thermopower (∼-160 μV K(-1) at 300 K) and a relatively low electrical resistivity. The inherently low thermal conductivity of Pb(7)Bi(4)Se(13) and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples.
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
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