Structural‐Distortion‐Driven Cooperative Magnetic and Semiconductor‐to‐Insulator Transitions in Ferromagnetic FeSb₂Se₄

Abstract

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,

Crystal Structure, Charge Transport, and Magnetic Properties of MnSb2Se4

“…The rapid increase in the resistivity is similar to the one observed in FeSb 2 Se 4 . [14] From the Arrhenius plot ( Figure 5, b, inset), we calculate the bandgap value to be 0.52 eV. This value is consistent with the observed optical bandgap, thereby confirming again the semiconducting character of MnSb 2 Se 4 at 300 K. …”

“…The bandgap estimated from the absorption coefficient/scattering coefficient (α/S) ratio versus energy plot is about 0.32 eV, which indicates that the compound is a narrow-bandgap semiconductor at 300 K ( Figure 5, a). The observed value of the energy gap is comparable to the optical bandgap of FeSb 2 Se 4 (E g = 0.33 eV) [14] and is consistent with the black color of the MnSb 2 Se 4 crystals. The temperature dependence of the electrical resistivity (ρ) of MnSb 2 Se 4 was measured from 100 to 300 K ( Figure 5, b).…”

“…The compound crystallizes isostructurally with FeSb 2 Se 4 [14] and exhibits Mn/Sb mixed occupancy at the Mn-rich M(4)(2a) site, as well as in all Sb-rich M(1) and M(2) positions. Charge-transport data, diffuse reflectance infrared spectroscopy, and electronic structure calculations revealed that MnSb 2 Se 4 is a narrow-gap p-type semiconductor with E g = 0.32 eV and exhibits a sharp increase in the resistivity below 180 K. The compound displays a low lattice thermal conductivity (ca.…”

“…Intensity data were recorded at 300 K with a STOE Imaging Plate Diffraction System (IPDS-2T) using graphite-monochromated Mo-K α radiation (λ = 0.71073 Å) and were indexed in the monoclinic crystal system with cell parameters a = 13.076(6) Å, b = 3.9651(8) Å, c = 15.236(4) Å and β = 115.1(1)°, which are similar to the unit-cell parameters of FeSb 2 Se 4 . [14] Therefore, atomic positions of FeSb 2 Se 4 were used as starting model for the structure refinement in the space group C2/m (no. 12) using the SHELTXL package.…”

“…[9][10][11] Among known complex transition-metal chalcogenides is the fascinating family of ternary MPn 2 Q 4 (M = Fe, Mn; Pn = Sb, Bi; and Q = S, Se) compounds with crystal structures that show 1D to 3D connectivity (depending on composition and synthetic method) as well as interesting physical properties. [12][13][14][15][16][17][18][19][20] For instance, MnSb 2 S 4 synthesized by solid-state reaction crystallizes in the monoclinic lattice (mC28) [18] and is isotypic with HgBi 2 S 4 , [21] whereas an orthorhombic modification (oP28), [12,15] isostructural with FeSb 2 S 4 , [13] bandgap of approximately 0.31 eV and exhibits a sharp increase in the resistivity near 230 K. A large Seebeck coefficient (S = +945 μV K -1 ), high electrical resistivity (ρ ≈ 9 Ω m), and low thermal conductivity (κ = 1.4 W m -1 K -1 ) were observed at 300 K. Direct current (DC) magnetic-susceptibility measurements indicated that MnSb 2 Se 4 is paramagnetic at 300 K and undergoes an order/disorder antiferromagnetic transition with a Néel temperature of approximately 20 K. Alternating current (AC) susceptibility at various frequencies suggested a spin-glass-like behavior.…”