Interplay of 3d and 4f Magnetism in Chiral Y 6 FeSi 2 S 14 and Tb 6 FeSi 2 S 14 Chalcogenides
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Gamage, Eranga H. [Department of Chemistry; Iowa S↗
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Probing FeSi, a d -electron topological Kondo insulator candidate, with magnetic field, pressure, and microwaves
Recently, evidence for a conducting surface state (CSS) below 19 K was reported for the correlated d -electron small gap semiconductor FeSi. In the work reported herein, the CSS and the bulk phase of FeSi were probed via electrical resistivity ρ measurements as a function of temperature T , magnetic field B to 60 T, and pressure P to 7.6 GPa, and by means of a magnetic field-modulated microwave spectroscopy (MFMMS) technique. The properties of FeSi were also compared with those of the Kondo insulator SmB 6 to address the question of whether FeSi is a d -electron analogue of an f -electron Kondo insulator and, in addition, a “topological Kondo insulator” (TKI). The overall behavior of the magnetoresistance of FeSi at temperatures above and below the onset temperature T S = 19 K of the CSS is similar to that of SmB 6 . The two energy gaps, inferred from the ρ( T ) data in the semiconducting regime, increase with pressure up to about 7 GPa, followed by a drop which coincides with a sharp suppression of T S . Several studies of ρ( T ) under pressure on SmB 6 reveal behavior similar to that of FeSi in which the two energy gaps vanish at a critical pressure near the pressure at which T S vanishes, although the energy gaps in SmB 6 initially decrease with pressure, whereas in FeSi they increase with pressure. The MFMMS measurements showed a sharp feature at T S ≈ 19 K for FeSi, which could be due to ferromagnetic ordering of the CSS. However, no such feature was observed at T S ≈ 4.5 K for SmB 6 .
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Hydrogen solubility in FeSi alloy phases at high pressures and temperatures
Abstract Light elements alloying with metallic Fe can change the properties and therefore play a key role in the structure and dynamics of planetary cores. Hydrogen and silicon are possible light elements in the rocky planets’ cores. However, hydrogen storage in Fe-Si alloy systems remains unclear at high pressures and high temperatures because of experimental difficulties. Taking advantage of pulsed laser heating combined with high-energy synchrotron X-ray diffraction, we studied reactions between FeSi and H in laser-heated diamond-anvil cells (LHDACs) up to 61.9 GPa and 3500 K. We found that under H-saturated conditions the amount of H alloying with FeSi (0.3 and <0.1 wt% for the B20 and B2 structures, respectively) is much smaller than that in pure Fe metal (>1.8 wt%). Our experiments also suggest that H remains in the crystal structure of FeSi alloy when recovered to 1 bar. Further density functional theory (DFT) calculations indicate that the low-H solubility likely results from the highly distorted interstitial sites in the B20 and B2 structures, which are not favorable for H incorporation. The recovery of H in the B20 FeSi crystal structure at ambient conditions could open up possibilities to understand geochemical behaviors of H during core formation in future experiments. The low-H content in FeSi alloys suggests that if a planetary core is Si-rich, Si can limit the ingassing of H into the Fe-rich core.
Geochemistry & Geophysics↗
Possible surface magnetism in the topological Kondo insulator candidate FeSi
Our group has previously reported the existence of a conducting surface state (CSS) in FeSi, a candidate for a d-electron topological Kondo insulator (TKI), at low temperature. In this paper, we present the electrical transport properties of single crystals of FeSi studied in the phase space of temperature (T), magnetic field (B), and the angle (θ) between the electrical current and B. The normalized T-dependent electrical resistance (R) of a successively thinned FeSi crystal provides further confirmation of the existence of a CSS. We report that, in the CSS, the magnetoresistance (MR) exhibits a hysteresis loop bounded within ±0.5 T, suggesting two-dimensional magnetic ordering. The hysteretic MR is asymmetric in B and anisotropic with respect to θ. Further exploration of R(θ) at a fixed field of 9 T reveals an initial progressive rotation of the axis for twofold rotational symmetry from 2 to 10 K, then a stabilized axis for twofold symmetry until, at T > 40 K, the anisotropy vanishes, coincident with the disappearance of the CSS. Furthermore, these observations point to a possible magnetically ordered surface state that has been reported in similar systems such as FeSi nanofilms and bulk SmB 6 .
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
FeSi 4 P 4 and CoSi 3 P 3 : Hidden Gems of Ternary Tetrel Pnictides with Outstanding Nonlinear Optical Properties
Metal silicon phosphides have shown promise as nonlinear optical materials. To be practically useful and cheap, earth-abundant 3d transition metals are preferred over their scarcer and more expensive 4d and 5d counterparts. Here, we developed a synthetic method to produce polycrystalline bulk powders and millimeter-sized single crystals of ternary compounds FeSi 4 P 4 and CoSi 3 P 3 . Both studied compounds have noncentrosymmetric and chiral crystal structures with ordered Si/P arrangements as was confirmed by single-crystal X-ray diffraction and solid-state NMR. Despite the presence of the transition metal, FeSi 4 P 4 and CoSi 3 P 3 are semiconductors with direct band gaps of 1.3 and 1.6 eV, respectively, indicating low-spin d 6 electronic configuration for octahedral Fe 2+ and Co 3+ . Relative to reported sulfide materials, FeSi 4 P 4 and CoSi 3 P 3 small band gap semiconductors demonstrate an outstanding combination of second-harmonic generation (SHG) activity and laser damage threshold (LDT). Both studied materials are phase-matchable with a 2.09 μm laser and not only exhibit 2.5–3.0 times stronger SHG signal than that of the state-of-the-art AgGaS 2 standard but also demonstrate an LDT response of 2.3–2.5 times higher than that of AgGaS 2 (at 1.09 μm laser with a pulse width of 10 ns)-which is unprecedented for small band gap semiconductors.
36 MATERIALS SCIENCE↗
Understanding the Formation of Complex Phases: The Case of FeSi 2
One of the fundamental goals of materials science is to understand and predict the formation of complex phases. In this study, FeSi 2 is considered as an illustration of complex phase formation. Although Fe and Si both crystallize with a simple structure, namely, body-centered cubic (bcc A2) and diamond (A4) structures, respectively, it is rather intriguing to note the existence of two complex structures in the Si-rich part of the phase diagram around FeSi 2 : α-FeSi 2 at high temperatures (HT) with a slight iron-deficient structure and β-FeSi2 (also referred to as Fe 3 Si 7 ) at low temperatures (LT). We re-analyze the geometry of these two phases and rely on approximant phases that make the relationship between these two phases simple. To complete the analysis, we also introduce a surrogate of the C16 phase that is observed in FeGe 2 . We clearly identify the relationship that exists between these three approximant phases, corroborated by a ground-state analysis of the Ising model for describing ordering that takes place between the transition metal element and the “vacancies”. This work is further supported by ab initio electronic structure calculations based on density functional theory in order to investigate properties and transformation paths. Finally, extension to other alloys, including an entire class of alloys, is discussed.
36 MATERIALS SCIENCE↗
Effect of Pressure on Crystal Structure and Phonon Density of States of FeSi
The strongly correlated material FeSi displays several unusual thermal, magnetic, and structural properties under varying pressure-temperature (P-T) conditions. It is a potential thermoelectric alloy and a material with several geochemical implications as a possible constituent at the Earth's core-mantle boundary (CMB). Previous theoretical studies predicted a pressure-induced B20-B2 transition at ambient temperature below 40 GPa; however, experimentally, the structural transition is observed only at high P-T conditions. In this study, we have performed high-pressure powder X-ray diffraction (XRD) up to 90 GPa and Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements up to 120 GPa to understand the phase stability and lattice dynamics. Our study provides evidence for a non-hydrostatic stress-induced B20-B2 transition in FeSi at around 36 GPa. We deduced the Fe partial phonon density of states (PDOS) and thermal parameters from NRIXS measurements up to 120 GPa and compared them with density functional theory (DFT) calculations. Furthermore, the computations show pressure-induced metallization and the band gap closing around 12 GPa.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Resonant inelastic x-ray scattering in the topological semimetal FeSi
The energy spectrum of topological semimetals contains protected degeneracies in reciprocal space that correspond to Weyl, Dirac, or multifold fermionic states. To exploit the unconventional properties of these states, one has to access the electronic structure of the three-dimensional bulk. In this paper, we present a joint theory-experiment study of the electronic structure of a candidate topological semimetal with resonant inelastic x-ray scattering (RIXS). We resolve the bulk electronic states of FeSi using momentum-dependent RIXS at the Fe L 3 edge. We observe a broad excitation continuum devoid of sharp features, consistent with particle-hole scattering in an underlying electronic band structure. Using density functional theory (DFT), we calculate the electronic structure of FeSi and derive a band theory formulation of RIXS in the fast collision approximation to model the scattering process with zero adjustable parameters. While band theory predicts an excitation continuum with broad spectral features similar to the observed ones, discrepancies between theory and experiment suggest the presence of low-energy processes that DFT alone does not account for. Further, this study of RIXS in a topological semimetal shows that RIXS is a useful tool for revealing unanticipated behavior of bulk electronic states in this class of materials.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Pressure-induced metallization and loss of surface magnetism in FeSi
Single-crystalline FeSi samples with a conducting surface state were studied under high pressure and magnetic field by means of electrical resistance measurements to explore how the bulk semiconducting state and the surface state are tuned by the application of pressure. Here, we found that the energy gap associated with the semiconducting bulk phase begins to close abruptly at a critical pressure of ~10 GPa and the bulk material becomes metallic with no obvious sign of any emergent phases or non-Fermi liquid behavior in temperature dependent electrical resistance in the neighborhood of the critical pressure above 3 K. Moreover, the metallic phase appears to remain at near-ambient pressure upon release of the pressure. Interestingly, the hysteresis in the electrical resistance vs magnetic field curve associated with the magnetically ordered conducting surface state decreases with pressure and vanishes at the critical pressure, while the slope of the electrical resistance vs magnetic field curve, which has a negative value for pressure below the critical pressure, decreases in magnitude with pressure and changes sign at the critical pressure. Thus the conducting surface state and the corresponding two-dimensional magnetic order collapse at the critical pressure where the energy gap of the bulk material starts to close abruptly, revealing the connection between the conducting surface state and the semiconducting bulk state in FeSi.
36 MATERIALS SCIENCE↗
Materials Data on Yb(FeSi)2 by Materials Project
Yb(FeSi)2 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Yb2+ is bonded in a distorted body-centered cubic geometry to eight equivalent Si4- atoms. All Yb–Si bond lengths are 3.10 Å. Fe3+ is bonded to four equivalent Si4- atoms to form a mixture of edge and corner-sharing FeSi4 tetrahedra. All Fe–Si bond lengths are 2.27 Å. Si4- is bonded in a 9-coordinate geometry to four equivalent Yb2+, four equivalent Fe3+, and one Si4- atom. The Si–Si bond length is 2.59 Å.
36 MATERIALS SCIENCE↗
Materials Data on Hf(FeSi)2 by Materials Project
Hf(FeSi)2 crystallizes in the orthorhombic Pbcm space group. The structure is three-dimensional. Hf4+ is bonded in a 6-coordinate geometry to eight Si4- atoms. There are a spread of Hf–Si bond distances ranging from 2.70–3.07 Å. There are two inequivalent Fe2+ sites. In the first Fe2+ site, Fe2+ is bonded in a 5-coordinate geometry to five Si4- atoms. There are a spread of Fe–Si bond distances ranging from 2.28–2.40 Å. In the second Fe2+ site, Fe2+ is bonded in a 7-coordinate geometry to two equivalent Fe2+ and five Si4- atoms. Both Fe–Fe bond lengths are 2.51 Å. There are a spread of Fe–Si bond distances ranging from 2.38–2.48 Å. There are two inequivalent Si4- sites. In the first Si4- site, Si4- is bonded in a 9-coordinate geometry to four equivalent Hf4+ and five Fe2+ atoms. In the second Si4- site, Si4- is bonded in a 11-coordinate geometry to four equivalent Hf4+, five Fe2+, and two equivalent Si4- atoms. Both Si–Si bond lengths are 2.51 Å.
36 MATERIALS SCIENCE↗
Materials Data on Sc(FeSi)2 by Materials Project
Sc(FeSi)2 crystallizes in the orthorhombic Pbcm space group. The structure is three-dimensional. Sc3+ is bonded in a 6-coordinate geometry to eight Si4- atoms. There are a spread of Sc–Si bond distances ranging from 2.69–3.11 Å. There are two inequivalent Fe+2.50+ sites. In the first Fe+2.50+ site, Fe+2.50+ is bonded in a 5-coordinate geometry to five Si4- atoms. There are a spread of Fe–Si bond distances ranging from 2.26–2.43 Å. In the second Fe+2.50+ site, Fe+2.50+ is bonded to five Si4- atoms to form a mixture of distorted corner and edge-sharing FeSi5 trigonal bipyramids. There are a spread of Fe–Si bond distances ranging from 2.40–2.48 Å. There are two inequivalent Si4- sites. In the first Si4- site, Si4- is bonded in a 9-coordinate geometry to four equivalent Sc3+ and five Fe+2.50+ atoms. In the second Si4- site, Si4- is bonded in a 11-coordinate geometry to four equivalent Sc3+, five Fe+2.50+, and two equivalent Si4- atoms. Both Si–Si bond lengths are 2.49 Å.
36 MATERIALS SCIENCE↗
Materials Data on Zr(FeSi)2 by Materials Project
Zr(FeSi)2 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Zr2+ is bonded in a distorted body-centered cubic geometry to eight equivalent Si4- atoms. All Zr–Si bond lengths are 2.98 Å. Fe3+ is bonded to four equivalent Si4- atoms to form a mixture of corner and edge-sharing FeSi4 tetrahedra. All Fe–Si bond lengths are 2.24 Å. Si4- is bonded in a 9-coordinate geometry to four equivalent Zr2+, four equivalent Fe3+, and one Si4- atom. The Si–Si bond length is 2.40 Å.
36 MATERIALS SCIENCE↗
Materials Data on Np(FeSi)2 by Materials Project
Np(FeSi)2 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Np3+ is bonded in a distorted body-centered cubic geometry to eight equivalent Si4- atoms. All Np–Si bond lengths are 3.05 Å. Fe+2.50+ is bonded to four equivalent Si4- atoms to form a mixture of corner and edge-sharing FeSi4 tetrahedra. All Fe–Si bond lengths are 2.28 Å. Si4- is bonded in a 9-coordinate geometry to four equivalent Np3+, four equivalent Fe+2.50+, and one Si4- atom. The Si–Si bond length is 2.47 Å.
36 MATERIALS SCIENCE↗
Materials Data on Th(FeSi)2 by Materials Project
Th(FeSi)2 crystallizes in the tetragonal I4/mmm space group. The structure is three-dimensional. Th4+ is bonded in a distorted body-centered cubic geometry to eight equivalent Si4- atoms. All Th–Si bond lengths are 3.15 Å. Fe2+ is bonded to four equivalent Si4- atoms to form a mixture of edge and corner-sharing FeSi4 tetrahedra. All Fe–Si bond lengths are 2.29 Å. Si4- is bonded in a 9-coordinate geometry to four equivalent Th4+, four equivalent Fe2+, and one Si4- atom. The Si–Si bond length is 2.66 Å.
36 MATERIALS SCIENCE↗
Materials Data on FeSi by Materials Project
FeSi crystallizes in the cubic P2_13 space group. The structure is three-dimensional. Fe is bonded in a 7-coordinate geometry to seven equivalent Si atoms. There are a spread of Fe–Si bond distances ranging from 2.28–2.50 Å. Si is bonded in a 7-coordinate geometry to seven equivalent Fe atoms.
36 MATERIALS SCIENCE↗
Understanding temperature-dependent SU(3) spin dynamics in the S = 1 antiferromagnet Ba 2 FeSi 2 O 7
Quantum magnets admit more than one classical limit and N-level systems with strong single-ion anisotropy are expected to be described by a classical approximation based on SU(N) coherent states. Here we test this hypothesis by modeling finite temperature inelastic neutron scattering (INS) data of the effective spin-one antiferromagnet Ba 2 FeSi 2 O 7 . The measured dynamic structure factor is calculated with a generalized Landau-Lifshitz dynamics for SU(3) spins. Unlike the traditional classical limit based on SU(2) coherent states, the results obtained with classical SU(3) spins are in good agreement with the measured temperature dependent spectrum. The SU(3) approach developed here provides a general framework to understand the broad class of materials comprising weakly coupled antiferromagnetic dimers, trimers, or tetramers, and magnets with strong single-ion anisotropy.
36 MATERIALS SCIENCE↗
Physical properties of the quasi-two-dimensional square lattice antiferromagnet Ba 2 FeSi 2 O 7
In this work, we report magnetization (χ, M ), magnetic specific heat ( C M ), and neutron powder diffraction results on a quasi-two-dimensional (2D) S = 2 square lattice antiferromagnet Ba 2 FeSi 2 O 7 consisting of FeO 4 tetrahedrons with highly compressive tetragonal distortion (27%). Despite of the quasi-2D lattice structure, both χ and C M present three-dimensional magnetic long-range ordering below the Néel temperature T N = 5.2 K . Neutron diffraction data show a collinear Q m = (1,0,1/2) antiferromagnetic (AFM) structure below T N but the ordered moment aligned in the a b plane is suppressed by 26% from the ionic spin S = 2 value ( 4 μ B ). Both the AFM structure and the suppressed moments are well explained by using Monte Carlo simulations with a large single-ion in-plane anisotropy D = 1 .4 meV and a rather small Heisenberg exchange J intra = 0 .15 meV in the plane. The characteristic 2D spin fluctuations are recognized in the magnetic entropy release and diffuse scattering above T N . This new quasi-2D magnetic system also displays unusual nonmonotonic dependence of T N as a function of magnetic field H .
36 MATERIALS SCIENCE↗