Tuning Fe-Se Tetrahedral Frameworks by a Combination of [Fe(en)[subscript 3]][superscript 2+] Cations and Cl[superscript -] Anions
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Gamage, Eranga H.↗
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Understanding the Reactions Between Fe and Se Binary Diffusion Couples
Spurred by recent discoveries of high-temperature superconductivity in Fe-Se based materials, the magnetic, electronic, and catalytic properties of iron-chalcogenides have drawn significant attention. Furthermore, much remains to be understood about the sequence of phase formation in these systems. In this work, we shed light on this issue by preparing a series of binary Fe-Se ultrathin diffusion couples via designed thin film precursors and investigating their structural evolution as a function of composition and annealing temperature. Two previously unreported Fe-Se phases crystallized during the deposition process on a nominally room-temperature Si substrate in the 27-33% and 37-47% Fe (atomic percent) composition regimes. Both phases completely decompose after annealing to 200°C in a nitrogen glovebox. At higher temperatures, the sequence of phase formation is governed by Se loss in the annealing process, consistent with what would be expected from the phase diagram. Films rich in Fe (53-59% Fe) crystalized during deposition as β-FeSe (P4/nmm) with preferred c-axis orientation to the amorphous SiO 2 substrate surface, providing a means to non-epitaxial self-assembly of crystallographically aligned, iron-rich β-FeSe for future research. Our findings suggest the crystallization of binary Fe-Se compounds at room temperature via near diffusionless transformations should be a significant consideration in future attempts to prepare metastable ternary and higher order compounds containing Fe and Se.
36 MATERIALS SCIENCE↗
Suppression of Superconductivity and Nematic Order in Fe 1–y Se 1–x S x (0 ≤ x ≤ 1; y ≤ 0.1) Crystals by Anion Height Disorder
Connections between crystal chemistry and critical temperature T c have been in the focus of superconductivity, one of the most widely studied phenomena in physics, chemistry and materials science alike. In most Fe-based superconductors materials chemistry and physics conspire so that T c correlates with the average anion height above the Fe plane, i.e. with the geometry of the FeAs 4 or FeCh 4 (Ch = Te, Se, S) tetrahedron. Here, by synthesizing Fe 1-y Se 1-x S x (0 ≤ x ≤ 1, y ≤ 0.1) we find that in alloyed crystals T c is not correlated with the anion height as most other Fe superconductors. Instead, changes in T c (x) and tetragonal-to-orthorombic (nematic) transition T s (x) on cooling are correlated with disorder in Fe vibrations in direction orthogonal to Fe planes, along the crystallographic c-axis. The disorder stems from the random nature of S substitution, causing deformed Fe(Se,S) 4 tetrahedra with different Fe-Se and Fe-S bond distances. Our results provide evidence of T c and T s suppression by disorder in anion height. The connection to local crystal chemistry may be exploited in computational prediction of new superconducting materials with FeSe/S building blocks.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Electronic Origin of T c in Bulk and Monolayer FeSe
FeSe is classed as a Hund’s metal, with a multiplicity of d bands near the Fermi level. Correlations in Hund’s metals mostly originate from the exchange parameter J, which can drive a strong orbital selectivity in the correlations. The Fe-chalcogens are the most strongly correlated of the Fe-based superconductors, with d xy the most correlated orbital. Yet little is understood whether and how such correlations directly affect the superconducting instability in Hund’s systems. By applying a recently developed ab initio theory, we show explicitly the connections between correlations in d xy and the superconducting critical temperature T c . Starting from the ab initio results as a reference, we consider various kinds of excursions in parameter space around the reference to determine what controls T c . We show small excursions in J can cause colossal changes in T c . Additionally we consider changes in hopping by varying the Fe-Se bond length in bulk, in the free standing monolayer M-FeSe, and M-FeSe on a SrTiO 3 substrate (M-FeSe/STO). The twin conditions of proximity of the dxy state to the Fermi energy, and the strength of J emerge as the primary criteria for incoherent spectral response and enhanced single- and two-particle scattering that in turn controls T c . Using c-RPA, we show further that FeSe in monolayer form (M-FeSe) provides a natural mechanism to enhance J. We explain why M-FeSe/STO has a high T c , whereas M-FeSe in isolation should not. Our study opens a paradigm for a unified understanding what controls Tc in bulk, layers, and interfaces of Hund’s metals by hole pocket and electron screening cloud engineering.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Materials Data on Fe3Se4 by Materials Project
Fe3Se4 is MAX Phase-derived structured and crystallizes in the monoclinic C2/m space group. The structure is two-dimensional and consists of one Fe3Se4 sheet oriented in the (0, 0, 1) direction. there are two inequivalent Fe+2.67+ sites. In the first Fe+2.67+ site, Fe+2.67+ is bonded to six Se2- atoms to form a mixture of corner, edge, and face-sharing FeSe6 octahedra. The corner-sharing octahedra tilt angles range from 47–49°. There are three shorter (2.40 Å) and three longer (2.48 Å) Fe–Se bond lengths. In the second Fe+2.67+ site, Fe+2.67+ is bonded to six equivalent Se2- atoms to form a mixture of corner, edge, and face-sharing FeSe6 octahedra. The corner-sharing octahedra tilt angles range from 47–49°. There are four shorter (2.53 Å) and two longer (2.55 Å) Fe–Se bond lengths. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to six Fe+2.67+ atoms to form a mixture of distorted corner and edge-sharing SeFe6 pentagonal pyramids. In the second Se2- site, Se2- is bonded in a distorted T-shaped geometry to three equivalent Fe+2.67+ atoms.
36 MATERIALS SCIENCE↗
Materials Data on Fe3Se4 by Materials Project
Fe3Se4 crystallizes in the triclinic P-1 space group. The structure is three-dimensional. there are two inequivalent Fe+2.67+ sites. In the first Fe+2.67+ site, Fe+2.67+ is bonded in a 4-coordinate geometry to four Se2- atoms. There are a spread of Fe–Se bond distances ranging from 2.33–2.59 Å. In the second Fe+2.67+ site, Fe+2.67+ is bonded in a square co-planar geometry to four Se2- atoms. There are two shorter (2.32 Å) and two longer (2.41 Å) Fe–Se bond lengths. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a 4-coordinate geometry to four Fe+2.67+ atoms. In the second Se2- site, Se2- is bonded in a distorted linear geometry to two Fe+2.67+ atoms.
36 MATERIALS SCIENCE↗
Materials Data on Fe7Se3 by Materials Project
Fe7Se3 crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. there are five inequivalent Fe sites. In the first Fe site, Fe is bonded in a 2-coordinate geometry to three Se atoms. There are a spread of Fe–Se bond distances ranging from 2.34–2.65 Å. In the second Fe site, Fe is bonded in a 3-coordinate geometry to four Se atoms. There are a spread of Fe–Se bond distances ranging from 2.45–2.93 Å. In the third Fe site, Fe is bonded in a 2-coordinate geometry to two equivalent Se atoms. Both Fe–Se bond lengths are 2.57 Å. In the fourth Fe site, Fe is bonded in a 4-coordinate geometry to four Se atoms. There are a spread of Fe–Se bond distances ranging from 2.55–2.71 Å. In the fifth Fe site, Fe is bonded in a 3-coordinate geometry to three Se atoms. There are two shorter (2.47 Å) and one longer (2.49 Å) Fe–Se bond lengths. There are two inequivalent Se sites. In the first Se site, Se is bonded in a 9-coordinate geometry to nine Fe atoms. In the second Se site, Se is bonded in a 7-coordinate geometry to seven Fe atoms.
36 MATERIALS SCIENCE↗
Materials Data on Fe3Se by Materials Project
Fe3Se is Uranium Silicide structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Fe is bonded to eight equivalent Fe and four equivalent Se atoms to form distorted FeFe8Se4 cuboctahedra that share corners with twelve equivalent FeFe8Se4 cuboctahedra, edges with eight equivalent SeFe12 cuboctahedra, edges with sixteen equivalent FeFe8Se4 cuboctahedra, faces with four equivalent SeFe12 cuboctahedra, and faces with fourteen equivalent FeFe8Se4 cuboctahedra. All Fe–Fe bond lengths are 2.54 Å. All Fe–Se bond lengths are 2.54 Å. Se is bonded to twelve equivalent Fe atoms to form SeFe12 cuboctahedra that share corners with twelve equivalent SeFe12 cuboctahedra, edges with twenty-four equivalent FeFe8Se4 cuboctahedra, faces with six equivalent SeFe12 cuboctahedra, and faces with twelve equivalent FeFe8Se4 cuboctahedra.
36 MATERIALS SCIENCE↗