Chemical Manipulation of Magnetic Ordering in Mn_1–<i>x</i>Sn_<i>x</i>Bi₂Se₄ Solid–Solutions

Several compositions of manganese–tin–bismuth selenide solid–solution series, Mn_1–<i>x</i>Sn_<i>x</i>Bi₂Se₄ (<i>x</i> = 0, 0.3, 0.75), were synthesized by combining high purity elements in the desired ratio at moderate temperatures. X-ray single crystal studies of a Mn-rich composition (<i>x</i> = 0) and a Mn-poor phase (<i>x</i> = 0.75) at 100 and 300 K revealed that the compounds crystallize isostructurally in the monoclinic space group <i>C</i>2/<i>m</i> (no.12) and adopt the MnSb₂Se₄ structure type. Direct current (DC) magnetic susceptibility measurements in the temperature range from 2 to 300 K indicated that the dominant magnetic ordering within the Mn_1–<i>x</i>Sn_<i>x</i>Bi₂Se₄ solid–solutions below 50 K switches from antiferromagnetic (AFM) for MnBi₂Se₄ (<i>x</i> = 0), to ferromagnetic (FM) for Mn_0.7Sn_0.3Bi₂Se₄ (<i>x</i> = 0.3), and finally to paramagnetic (PM) for Mn_0.25Sn_0.75Bi₂Se₄ (<i>x</i> = 0.75). We show that this striking variation in the nature of magnetic ordering within the Mn_1–<i>x</i>Sn_<i>x</i>Bi₂Se₄ solid–solution series can be rationalized by taking into account: (1) changes in the distribution of magnetic centers within the structure arising from the Mn to Sn substitutions, (2) the contributions of spin-polarized free charge carriers resulting from the intermixing of Mn and Sn within the same crystallographic site, and (3) a possible long-range ordering of Mn and Sn atoms within individual {M}_<i>n</i>Se_4<i>n</i>+2 single chain leading to quasi isolated {MnSe₆} octahedra spaced by nonmagnetic {SnSe₆} octahedra

High‑<i>T</i>_c Ferromagnetism and Electron Transport in p‑Type Fe_1–<i>x</i>Sn_<i>x</i>Sb₂Se₄ Semiconductors

Single-phase polycrystalline powders of Fe_1–<i>x</i>Sn_<i>x</i>Sb₂Se₄ (<i>x</i> = 0 and 0.13) were synthesized by a solid-state reaction of the elements at 773 K. X-ray diffraction on Fe_0.87Sn_0.13Sb₂Se₄ single-crystal and powder samples indicates that the compound is isostructural to FeSb₂Se₄ in the temperature range from 80 to 500 K, crystallizing in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12). Electron-transport data reveal a marginal alteration in the resistivity, whereas the thermopower drops by ∼60%. This suggests a decrease in the activation energy upon isoelectronic substitution of 13% Fe by Sn. Magnetic susceptibility and magnetization measurements from 2 to 500 K reveal that the Fe_1–<i>x</i>Sb₂Sn_<i>x</i>Se₄ phases exhibit ferromagnetic behavior up to ∼450 K (<i>x</i> = 0) and 325 K (<i>x</i> = 0.13). Magnetotransport data for FeSb₂Se₄ reveal large negative magnetoresistance, suggesting spin polarization of free carriers in the sample. The high-<i>T</i>_c ferromagnetism in Fe_1–<i>x</i>Sn_<i>x</i>Sb₂Se₄ phases and the decrease in <i>T</i>_c of the Fe_0.87Sn_0.13Sb₂Se₄ sample are rationalized by taking into account (1) the separation between neighboring magnetic centers in the crystal structures and (2) the formation of bound magnetic polarons, which overlap to induce long-range ferromagnetic ordering

Geometrical Spin Frustration and Ferromagnetic Ordering in (Mn_<i>x</i>Pb_2–<i>x</i>)Pb₂Sb₄Se₁₀

Engineering the atomic structure of an inorganic semiconductor to create isolated one-dimensional (1D) magnetic subunits that are embedded within the semiconducting crystal lattice can enable chemical and electronic manipulation of magnetic ordering within the magnetic domains, paving the way for (1) the investigation of new physical phenomena such as the interactions between electron transport and localized magnetic moments at the atomic scale and (2) the design and fabrication of geometrically frustrated magnetic materials featuring cooperative long-range ordering with large magnetic moments. We report the design, synthesis, crystal structure and magnetic behavior of (Mn_<i>x</i>Pb_2–<i>x</i>)­Pb₂Sb₄Se₁₀, a family of three-dimensional manganese-bearing main-group metal selenides featuring quasi-isolated [(Mn_<i>x</i>Pb_2–<i>x</i>)₃Se₃₀]_∞ hexanuclear magnetic ladders coherently embedded and uniformly distributed within a purely inorganic semiconducting framework, [Pb₂Sb₄Se₁₀]. Careful structural analysis of the magnetic subunit, [(Mn_<i>x</i>Pb_2–<i>x</i>)₃Se₃₀]_∞ and the temperature dependent magnetic susceptibility of (Mn_<i>x</i>Pb_2–<i>x</i>)­Pb₂Sb₄Se₁₀, indicate that the compounds are geometrically frustrated 1D ferromagnets. Interestingly, the degree of geometrical spin frustration (f) within the magnetic ladders and the strength of the intrachain antiferromagnetic (AFM) interactions strongly depend on the concentration (<i>x</i> value) and the distribution of the Mn atom within the magnetic substructure. The combination of strong intrachain AFM interactions and geometrical spin frustration in the [(Mn_<i>x</i>Pb_2–<i>x</i>)₃Se₃₀]_∞ ladders results in a cooperative ferromagnetic order with exceptionally high magnetic moment at around 125 K. Magnetotransport study of the Mn₂Pb₂Sb₄Se₁₀ composition over the temperature range from 100 to 200 K revealed negative magnetoresistance (NMR) values and also suggested a strong contribution of magnetic polarons to the observed large effective magnetic moments

Pb₇Bi₄Se₁₃: A Lillianite Homologue with Promising Thermoelectric Properties

Pb₇Bi₄Se₁₃ crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3) Å, <i>b</i> = 4.262(2) Å, <i>c</i> = 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 <i>N</i>₁ = 5 and <i>N</i>₂ = 4 [<i>N</i> = number of edge-sharing (Pb/Bi)­Se₆ octahedra along the central diagonal] are arranged along the <i>c</i> 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 Pb₇Bi₄Se₁₃ 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 Pb₇Bi₄Se₁₃ 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 Pb₇Bi₄Se₁₃ and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples

Pb₇Bi₄Se₁₃: A Lillianite Homologue with Promising Thermoelectric Properties

Pb₇Bi₄Se₁₃ crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (No. 12) with <i>a</i> = 13.991(3) Å, <i>b</i> = 4.262(2) Å, <i>c</i> = 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 <i>N</i>₁ = 5 and <i>N</i>₂ = 4 [<i>N</i> = number of edge-sharing (Pb/Bi)­Se₆ octahedra along the central diagonal] are arranged along the <i>c</i> 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 Pb₇Bi₄Se₁₃ 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 Pb₇Bi₄Se₁₃ 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 Pb₇Bi₄Se₁₃ and its tunable electronic properties point to a high thermoelectric figure of merit for properly optimized samples

Coexistence of High‑<i>T</i>_c Ferromagnetism and <i>n</i>‑Type Electrical Conductivity in FeBi₂Se₄

The discovery of <i>n</i>-type ferromagnetic semiconductors (<i>n</i>-FMSs) exhibiting high electrical conductivity and Curie temperature (<i>T</i>_c) above 300 K would dramatically improve semiconductor spintronics and pave the way for the fabrication of spin-based semiconducting devices. However, the realization of high-<i>T</i>_c <i>n</i>-FMSs and <i>p</i>-FMSs in conventional high-symmetry semiconductors has proven extremely difficult due to the strongly coupled and interacting magnetic and semiconducting sublattices. Here we show that decoupling the two functional sublattices in the low-symmetry semiconductor FeBi₂Se₄ enables unprecedented coexistence of high <i>n</i>-type electrical conduction and ferromagnetism with <i>T</i>_c ≈ 450 K. The structure of FeBi₂Se₄ consists of well-ordered magnetic sublattices built of [Fe_<i>n</i>Se_4<i>n</i>+2]_∞ single-chain edge-sharing octahedra, coherently embedded within the three-dimensional Bi-rich semiconducting framework. Magnetotransport data reveal a negative magnetoresistance, indicating spin-polarization of itinerant conducting electrons. These findings demonstrate that decoupling magnetic and semiconducting sublattices allows access to high-<i>T</i>_c <i>n</i>- and <i>p</i>-FMSs as well as helps unveil the mechanism of carrier-mediated ferromagnetism in spintronic materials

Indium Preferential Distribution Enables Electronic Engineering of Magnetism in FeSb_2–<i>x</i>In_<i>x</i>Se₄ p‑Type High-Tc Ferromagnetic Semiconductors

Single-phase samples of the solid-solution series FeSb_2–<i>x</i>In_<i>x</i>Se₄ (0 ≤ <i>x</i> ≤ 0.25) were synthesized using solid-state reaction of the elements to probe the effect of electronic structure engineering on the magnetic behavior of the p-type semiconductor, FeSb₂Se₄. Powder X-ray diffraction data suggest that all samples are isostructural with FeSb₂Se₄. Rietveld refinements of the distribution of In atoms at various metal positions indicate a preferential substitution of Sb at the M1­(<i>4i</i>) position within the magnetic layer A for In concentration up to <i>x</i> = 0.1. FeSb_2–<i>x</i>In_<i>x</i>Se₄ compositions with higher In content show the distribution of In atoms at all metal positions, except for the M3­(<i>2d</i>), which is fully occupied by Fe atoms. Interestingly, the ordering of Fe atoms within the crystal structure of FeSb_2–<i>x</i>In_<i>x</i>Se₄ remains essentially unaffected by the degree of substitution (<i>x</i> values) and is comparable to the distribution of Fe atoms reported in FeSb₂Se₄. X-ray photoelectron spectroscopy confirms the oxidation states of various metal atoms In­(+3), Sb­(+3), Fe­(+2) in the structure. Electronic transport properties indicate p-type semiconducting behavior for all samples. The electrical conductivity above 300 K first increases with In content, reaches the maximum value for <i>x</i> = 0.1, then decreases with further increase in In content. A reverse trend is observed for the thermopower. All samples show drastically low thermal conductivity with room temperature values ranging from 0.45 Wm^–1 K^–1 for <i>x</i> = 0 to 0.27 Wm^–1 K^–1 for the sample with <i>x</i> = 0.25. Magnetic susceptibility data suggest ferromagnetic-like behavior from 2–300 K for all samples. The magnitude of the magnetic susceptibility rapidly increases with In content, reaches a maximum for <i>x</i> = 0.1, and marginally decreases with further increase in In concentration. The observed surprising change in the magnetic and electronic behavior of samples with high In content (<i>x</i> > 0.1) is rationalized using the concept of antiferromagnetic scattering of charge carriers at the interfaces between overlapping bound magnetic polarons from adjacent layers A and B