Tetragonal FeSe Research Articles

Al2O3-induced phase conversion regulation from hexagonal FeS to orthorhombic FeS for enhancing the Li-ion accommodation ability

Cycling lifespan/stability is one of the most important limitations for multivalent transition metal sulfides (TMSs) used in secondary batteries. However, the chaos reaction of TMSs is an important obstacle that restricts their electrochemical stability in all-enclosed batteries. Herein, taking FeS anode and lithium-ion batteries as the example, the phase-lithiation correlation is studied through TEM and DFT analysis. It's found that the pristine hexagonal FeS converts to tetragonal FeS phase accompanied by severe and rapid capacity decay. However, it converts to orthorhombic FeS with enhanced capacity and nice lifespan when introducing trace amount amorphous Al2O3 upon FeS power's surface. DFT calculations further reveal that Al2O3 induces the phase conversion from low-acitve tetragonal FeS to high-active orthorhombic FeS, which promotes the lithiation-delithiation reversibility and stability. Based on the above result, it is expected to provide useful guidelines for the in-situ structure engineering of TMSs-based anodes in all-enclosed secondary batteries.

Lifting of Gap Nodes by Disorder in Tetragonal FeSe_{1-x}S_{x} Superconductors.

The observation of time-reversal symmetry breaking and large residual density of states in tetragonal FeSe_{1-x}S_{x} suggests a novel type of ultranodal superconducting state with Bogoliubov Fermi surfaces (BFSs). Although such BFSs in centrosymmetric superconductors are expected to be topologically protected, the impurity effect of this exotic superconducting state remains elusive experimentally. Here, we investigate the impact of controlled defects introduced by electron irradiation on the superconducting state of tetragonal FeSe_{1-x}S_{x} (0.18≤x≤0.25). The temperature dependence of magnetic penetration depth is initially consistent with a model with BFSs in the pristine sample. After irradiation, we observe a nonmonotonic evolution of low-energy excitations with impurity concentrations. This nonmonotonic change indicates a transition from nodal to nodeless, culminating in gapless with Andreev bound states, reminiscent of the nodal s_{±} case. This points to the accidental nature of the possible BFSs in tetragonal FeSe_{1-x}S_{x}, which are susceptible to disruption by the disorder.

Hydrothermal synthesis of tetragonal FeS: Dome-shaped superconductivity vs Fe:S actual molar ratios

We successfully synthesized tetragonal FeS with various Fe:S molar ratios using a hydrothermal method, and the EDS and XRF results confirmed that FeS exhibits superconductivity within the Fe and S element ratio range of 1.155–1.274, rather than a strict stoichiometric ratio. The reaction temperature can not only play a crucial role in determining the purity of the obtained FeS phase but also significantly alter its micromorphology. Significantly, by analyzing the relationship between the Fe:S actual molar ratio and Tc as well as the Fe:S actual molar ratio and unit cell volume, FeS exhibits dome-shaped superconductivity and shows a negative correlation with Tc and chemical pressure. Furthermore, considering the correlation between anion height (hanion) and Tc, Tc reaches the maximum value of 4.53 K for hanion ≈ 1.28 Å, a behavior distinct from that of observed in Fe-pnictide superconductors. In short, our experimental results provide a unique perspective to deepen our understanding of FeS superconductors.

NOVEL INSIGHTS INTO PHASE FORMATION AND ELECTRICAL RESISTIVITY OF FeSe ALLOY PREPARED BY POWDER METALLURGY METHOD

This study prepared the FeSe alloy using a powder metallurgy route. The Fe and Se powders were weighed at an atomic ratio of Fe:Se = 1.025:1 and milled for 5 hours. A simultaneous thermal analysis (STA) was performed to observe the behavior of the milled powder during thermal changes. After packing the milled powder in a stainless-steel tube, it was compacted. Investigation was performed to observe how the tetragonal FeSe phase forms at sintering temperatures of 718 K, 818 K, and 918 K. At a sintering temperature of 718 K, the tetragonal FeSe phase was formed, as determined by our quantitative analysis of XRD. The highest tetragonal FeSe phase fraction of 68.46 wt.% was obtained at a temperature of 918 K. The lattice constants of the tetragonal FeSe for the best sample were a = 0.3773 nm and c = 0.5520 nm. The resistivity test demonstrated that all samples have a conductor phenomenon exceeding 16 K, with a maximum Tc-onset value of 15.11 K.

Superconducting Two-Dimensional FeSe Grown on the Fe-Enriched Interface.

Two-dimensional (2D) tetragonal FeSe has sparked extensive research interest owing to its tunable superconductivity, providing valuable insights into the design of high-temperature superconductors. Currently, the intricate Fe-Se phase diagram poses a challenge to the controlled synthesis of superconducting 2D FeSe in a pure tetragonal phase. Here, we exploit the ion-exchange property of fluorophlogopite mica to devise a straightforward approach for the phase-controlled synthesis of tetragonal FeSe on an Fe-enriched mica surface within a molten salt environment. This method successfully produces highly crystalline FeSe in a pure tetragonal phase with adjustable thickness. We investigated the surface composition of the postgrowth mica substrate using various microscopic and spectroscopic characterizations to highlight the importance of the Fe-enriched growth interface in the phase-selective synthesis of 2D tetragonal FeSe. The obtained 2D FeSe exhibited 2D superconductivity, comparable to that of FeSe mechanically exfoliated from bulk crystals, confirming the high quality of our samples. Beyond tetragonal FeSe, 2D antiferromagnetic FeTe and superconducting FeSxSeyTe1-x-y have been phase-selectively synthesized via this approach. Our study elucidates the significance of the growth interface on the phase-selective synthesis of 2D materials and presents potential opportunities for the phase-controlled synthesis of 2D multiphase materials via the rational design of the growth interface.

Investigation of pressure impact on physical characteristics of FeSe chalcogenide superconductor: Insights from first principles calculation

Understanding the high-pressure physical characteristics of iron selenium (FeSe) superconductors is crucial to comprehending their superconducting abilities. First-principles Density Functional Theory has been computed in the present calculations to analyze the pressure impact on structural, mechanical, electronic and superconducting characteristics along with the Debye and melting temperatures of tetragonal FeSe. The analysis of mechanical characteristics suggests that this compound remains mechanically stable up to a pressure of 20 GPa. The applied hydrostatic pressure is observed to have a significant impact on the elastic moduli and the anisotropic behaviour of FeSe at all pressure conditions. Similarly, the first indication of ductility is observed approximately at 8 GPa and then it increases as pressure rises. At 0 GPa, the optimized structural values are seen to line up with the available data. The variation of structural parameters and electronic factors of FeSe with applied pressure is also mentioned here. Moreover, the assessment of the compound’s electronic characteristics provides strong evidence for its metallic nature and the rise in a number of states at the Fermi energy level with applied pressure may be the cause for the increase in the superconducting critical temperature of FeSe with pressure.

Two-Dimensional Tetragonal Transition Metal Chalcogenides for High Performance Oxygen Evolution and Reduction: A DFT Study.

The pursuit of high-performance bifunctional catalysts for oxygen evolution/reduction (OER/ORR) has gained significant attention in the field of electrochemical water splitting and fuel cells. In this study, we employed density functional theory (DFT) calculations to investigate a series of 2D tetragonal TMX (TM=transition metal, X=S, Se, Te) monolayers as potential bifunctional electrocatalysts for OER/ORR. To evaluate the overall performance of OER electrocatalysts, we introduced a descriptor, Gmax. The Gmax values obtained for tetragonal CdS, CdSe, FeSe, NiSe, and NiTe monolayers were all below 1.0 V, indicative of their superior catalytic activity and selectivity. Moreover, NiSe displayed remarkable ORR capability with an overpotential (ηORR ) of 0.53 V. Based on the bifunctional index (BI), the catalytic activity ranking for the bifunctional catalysts is as follows: NiSe>NiTe>FeSe>CdS>CdSe>NiS>TiSe>ZnTe. These findings provide an insightful understanding of the electrocatalytic properties of 2D tetragonal TMX monolayers for OER/ORR, opening avenues for the future development of efficient bifunctional electrocatalysts based on 2D tetragonal transition metal chalcogenides.

The Controllable Synthesis of High-Quality Two-Dimensional Iron Sulfide with Specific Phases.

2D Fe-chalcogenides have drawn significant attention due to their unique structural phases and distinct properties in exploring magnetism and superconductivity. However, it remains a significant challenge to synthesize 2D Fe-chalcogenides with specific phases in a controllable manner since Fe-chalcogenides have multiple phases. Herein, a molecular sieve-assisted strategy is reported for synthesizing ultrathin 2D iron sulfide on substrates via the chemical vapor deposition method. Using a molecular sieve and tuning growth temperatures to control the partial pressures of precursor concentrations, hexagonal FeS, tetragonal FeS, and non-stoichiometric Fe7 S8 nanoflakes can be precisely synthesized. The 2D h-FeS, t-FeS, and Fe7 S8 have high conductivities of 5.4 × 105 S m-1 , 5.8 × 105 S m-1 , and 1.9 × 106 S m-1 . 2D tetragonal FeS shows a superconducting transition at 4 K. The spin reorientation at ≈30 K on the non-stoichiometric Fe7 S8 nanoflakes with ferrimagnetism up to room temperature has also been observed. The controllable synthesis of various phases of 2D iron sulfide may provide a route for synthesizing other 2D compounds with various phases.

Phase transformation from FeSe to Fe3Se4

We report on thermal atomic layer deposition of iron selenide (FeSe and Fe3Se4) thin films with four-inch wafer-scale uniformity using two precursors of iron bis(N, N′-diisopropylacetamidinato) [Fe(i-Pr-Me-AMD)2] and bis(triethysilyl) selenide [(Et3Si)2Se]. The temperature window is determined to be 350–400 °C. The transformation from tetragonal FeSe to monoclinic Fe3Se4 has been found. The tetragonal FeSe thin films exhibit a strong (00 l) texture deposited on amorphous silica glass, suggesting that the surface is partially parallel to the layered ab plane. By adjusting the ratio of iron and selenium, the thin films exhibit from conductors to semiconductors, and finally to insulators. All as-grown conductive samples appear metallic-like temperature dependence by low-temperature electrical transport measurements. This study opens the way to prepare various metal selenides by atomic layer deposition.

Analysis of the influence of aggressive factors and conditions on the composition of corrosive products

Data on the use of the X-ray diffraction method in the analysis of the composition of corrosion products are presented. Such knowledge makes it possible to obtain information on the mechanisms of corrosion development and the protective properties of corrosion products, being either dense (with certain protective properties against corrosion) or loose (with a low level of protection against corrosion), which doesn't prevent the penetration of corrosive media to steel surfaces. Under H2S conditions, a layer of mackinawite (tetragonal FeS) is formed on the surface of steels, and in acidic environments of formation water imitations, it was found that, in addition to it, cubic FeS is formed. Iron sulfide with a cubic crystal structure, being metastable, reduces the protective properties of the sulfide film in aggressive acidic H2S media. During carbon dioxide corrosion of steel, the main product is siderite (FeCO3), characterized by the phenomenon of isomorphism (i.e. changes in the chemical composition of the phase while maintaining its crystal structure). It is established that in the formation water model, sediments of non-stoichiometric composition CaxFeyCO3 and (XFe)CO3 are formed, where X = (Са2+, Mg2+, Mn2+). Both of them are poorly crystallized and have defects in the crystal structure, which reduce their protective properties relative to the stoichiometric FeCO3 formed in a 3%NaCl solution. A corrosion inhibitor in aqueous media promotes the adsorption of the inhibitor film, preventing the formation of corrosion products.

In situ atomic‐scale observation of size‐dependent (de)potassiation and reversible phase transformation in tetragonal FeSe anodes

AbstractPotassium‐ion batteries (PIBs) are considered promising alternatives to lithium‐ion batteries owing to cost‐effective potassium resources and a suitable redox potential of −2.93 V (vs. −3.04 V for Li+/Li). However, the exploration of appropriate electrode materials with the correct size for reversibly accommodating large K+ ions presents a significant challenge. In addition, the reaction mechanisms and origins of enhanced performance remain elusive. Here, tetragonal FeSe nanoflakes of different sizes are designed to serve as an anode for PIBs, and their live and atomic‐scale potassiation/depotassiation mechanisms are revealed for the first time through in situ high‐resolution transmission electron microscopy. We found that FeSe undergoes two distinct structural evolutions, sequentially characterized by intercalation and conversion reactions, and the initial intercalation behavior is size‐dependent. Apparent expansion induced by the intercalation of K+ ions is observed in small‐sized FeSe nanoflakes, whereas unexpected cracks are formed along the direction of ionic diffusion in large‐sized nanoflakes. The significant stress generation and crack extension originating from the combined effect of mechanical and electrochemical interactions are elucidated by geometric phase analysis and finite‐element analysis. Despite the different intercalation behaviors, the formed products of Fe and K2Se after full potassiation can be converted back into the original FeSe phase upon depotassiation. In particular, small‐sized nanoflakes exhibit better cycling performance with well‐maintained structural integrity. This article presents the first successful demonstration of atomic‐scale visualization that can reveal size‐dependent potassiation dynamics. Moreover, it provides valuable guidelines for optimizing the dimensions of electrode materials for advanced PIBs.image

Role of nematicity in controlling spin fluctuations and superconducting Tc in bulk FeSe

FeSe undergoes a transition from a tetragonal to a slightly orthorhombic phase at 90\,K, and becomes a superconductor below 8\,K. The orthorhombic phase is sometimes called a nematic phase because quantum oscillation, neutron, and other measurements detect a significant asymmetry in $x$ and $y$. How nematicity affects superconductivity has recently become a matter of intense speculation. Here we employ an advanced \emph{ab-initio} Green's function description of superconductivity and show that bulk tetragonal FeSe would, in principle, superconduct with almost the same T$_{c}$ as the nematic phase. The mechanism driving the observed nematicity is not yet understood. Since the present theory underestimates nematicity, we simulate the full nematic asymmetry by artificially enhancing the orthorhombic distortion. For benchmarking, we compare theoretical spin susceptibilities against experimentally observed data over all energies and relevant momenta. When the orthorhombic distortion is adjusted to correlate with observed nematicity in spin susceptibility, the enhanced nematicity causes spectral weight redistribution in the Fe-3d$_{xz}$ and d$_{yz}$ orbitals, but it leads to at most 10-15$\%$ increment in T$_{c}$. This is because the d$_{xy}$ orbital always remains the most strongly correlated and provides most of the source of the superconducting glue. Nematicity suppresses the density of states at Fermi level; nevertheless T$_{c}$ increases, in contradiction to both BCS and BEC theories. We show how the increase is connected to the structure of the particle-particle vertex. Our results suggest while nematicity may be intrinsic property of bulk FeSe, is not the primary force driving the superconducting pairing.

Nematic state of the FeSe superconductor

We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real and reciprocal space. We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales. It also survives up to temperatures above 90 K where reciprocal-space analysis suggests tetragonal symmetry. Additionally, the real-space pair distribution function analysis of the synchrotron x-ray diffraction data reveals a tiny broadening of the peaks corresponding to the nearest Fe-Fe, nearest Fe-Se, and the next-nearest Fe-Se bond distances as well as the tetrahedral torsion angles at a short length scale of 20 angstr\"om. This broadening appears below 20 K and is attributed to a pseudogap. However, we did not observe any further reduction in local symmetry below orthorhombic down to 3 K. Our results suggest that the superconducting gap anisotropy in FeSe is not associated with any symmetry-lowering short-range structural correlations.

Low temperature synthesis of iron selenide by thermolysis of organometallic precursor for photodegradation under direct sunlight

In this article, 1-(4-fluorobenzoyl)-3-(4-ferrocenyl-3-methylphenyl)selenourea (P-1) has been synthesized and characterized (FTIR, multinuclear NMR, CHNS and AAS) and used as a single source precursor for the fabrication of iron selenide (FeSe) by simple thermolysis at 200 °C in a vacuum furnace. By using UV–visible spectroscopy, the synthesized tetragonal FeSe has been used to degrade methyl orange (MO) and methyl violet (MV) under direct sunlight.

Iron(II) monosulfide (FeS) minerals reductively transform the insensitive munitions compounds 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO)

As military applications of the insensitive munitions compounds (IMCs) 2,4-dinitroanisole (DNAN) and 3-nitro-1,2,4-triazol-5-one (NTO) increase, there is a growing need to understand their environmental fate and to develop remediation strategies to mitigate their impacts. Iron (II) monosulfide (FeS) minerals are abundant in freshwater and marine sediments, marshes, and hydrothermal environments. This study shows that FeS solids can reduce DNAN and NTO to their corresponding amines under anoxic ambient conditions. The reactions between IMCs and the FeS minerals were surface-mediated since they did not occur when only dissolved Fe2+(aq) and S2−(aq) were present. Mackinawite, a tetragonal FeS with a layered structure, reduced DNAN mainly to 2-methoxy-5-nitroaniline (MENA), which in turn was partially reduced to 2-4-diaminoanisole (DAAN). The layered structure of mackinawite provided intercalation sites likely responsible for partial adsorption of MENA and DAAN. Mackinawite entirely reduced NTO to 3-amino-1,2,4-triazol-5-one (ATO). The reduction of IMCs showed concurrent oxidation of mackinawite to goethite and elemental sulfur. A commercial FeS product, composed mainly of pyrrhotite and troilite, reduced DNAN to DAAN and NTO to ATO. At pH 6.5, DNAN and NTO transformation rates were 667 and 912 μmol h−1 m−2, respectively, on the mackinawite surface and 417 and 1344 μmol h−1 m−2, respectively, on the commercial FeS surface. This is the first report of the reduction of a nitro-heterocyclic compound (NTO) by FeS minerals. The evidence indicates that DNAN and NTO can be rapidly transformed to their succeeding amines in anoxic subsurface environments and aquatic sediments rich in FeS minerals.

Polymorph Engineering for Boosted Volumetric Na-Ion and Li-Ion Storage.

To meet the ever-growing demand for advanced rechargeable batteries with light weight and compact size, much effort has been devoted to improving the volumetric capacity of electrodes. Herein, an effective strategy of polymorph engineering is proposed to boost the volumetric capacity of FeSe. Owing to the inherent metallic electronic conductivity of tetragonal-FeSe, a conductive additive-free electrode (hereafter denoted as CA-free) can be assembled with an enhanced sodium storage volumetric capacity of 1011 mAh cm-3 , significantly higher than semiconducting hexagonal-FeSe. Impressively, the CA-free electrode can achieve an extremely high active material utilization of 96.7wt% and high initial Coulombic efficiency of 96%, superior to most of the anodes for Na-ion storage. Moreover, the design methodology is branched out using tetragonal FeSe as the cathode for Li-ion batteries. The CA-free tetragonal-FeSe electrode can achieve a high volumetric energy density of 1373Wh L-1 and power density of 7200 W L-1 , outperforming most metal chalcogenides. Reversible conversion reactions are revealed by in situ XRD for both sodium and lithium systems. The proposed design strategy provides new insight and inspiration to aid in the ongoing quest for better electrode materials.

Influence of trace level As or Ni on pyrite formation kinetics at low temperature

Pyrite formation at low temperature during early diagenesis in (sub-)surface sediments is an essential step of Fe and S biogeochemical cycles and the presence of this ubiquitous mineral of surface environments is often used as an indicator of paleo-redox conditions. Pathways of pyrite formation are usually discussed in environmental settings by involving a variety of nanosized Fe-S mineralogical precursors as a function of the local geochemical conditions. However, the influence of trace element impurities such as Ni and As in the solution at the time of pyrite formation has been poorly studied, whereas specific chemical signatures of trace elements are commonly observed in sedimentary pyrites. A better understanding of the impact of Ni and As incorporation at trace levels on pyrite formation is essential to help refining the use of these elements as paleo-redox indicators and to evaluate the role of pyrite as a sink regulating the biogeochemical cycle of potentially toxic trace elements. In this study, we have performed syntheses of pyrite at low temperature via the polysulfide pathway using aqueous Fe(III) and H2S in the presence of trace amounts of Ni(II) (0.001 mol%Fe) and As(III) (0.001 mol%Fe). Analysis of the solids collected at different time steps over the course of the experiments using X-Ray absorption spectroscopy at both the Fe and S K-edges shows that pyrite starts to precipitate within 5 days in presence of Ni(II) and within 32 days in presence of As(III), while the control experiment showed an intermediate precipitation rate of 14 days. Shell-by-shell analysis of Fe K-edge EXAFS data shows that the initial mineralogical precursors are the same in all the experiments and correspond to poorly-crystalline FeS (3.0 ± 0.1 Fe-S@2.25 Å; 1.7 ± 0.2 Fe-Fe@2.67 Å). In addition, XANES qualitative analysis suggests the incorporation of small amounts of Fe(III) within these FeS precursors. Synchrotron-based XRD and WAXS-PDF analysis of the starting solids show that in addition to S(0), the FeS precursors correspond to a continuum of FeS particles that ranges from tetragonal nanocrystalline FeS (a = 3.70(2) Å, c = 5.24(7) Å, MCDab = 41 ± 4 Å MCDc = 21 ± 2 Å) to cluster-type FeS (MCDabc < 8.4 ± 4.3 Å). We propose that Ni(II) and As(III) have a different type of interaction with these FeS precursors, resulting respectively in an increase and a decrease in the rate of pyrite nucleation. While Ni(II) would incorporate within the structure of the FeS precursors, As would interact with (poly)sulfides in solution to form thio-As, possibly binding or precipitating onto FeS surfaces and thus slowing FeS transformation to FeS2. Given that both Ni and As were introduced at trace levels in our experiments, these results suggest that the occurrence of trace amounts of impurities could have a strong influence on pyrite precipitation kinetics in natural settings such as pore-scale microenvironments. In addition to emphasizing the importance of trace elements such as Ni or As on the persistence of mobile colloidal FeS species in anoxic conditions, the results of the present study also point to the importance of considering the actual nature of the impurities when using pyrite composition for ancient environments and past climates reconstruction.

Superconductivity at 40 K in Lithiation-Processed [(Fe,Al)(OH)2][FeSe]1.2 with a Layered Structure.

Exploration of new superconductors has always been one of the research directions in condensed matter physics. We report here a new layered heterostructure of [(Fe,Al)(OH)2][FeSe]1.2, which is synthesized by the hydrothermal ion-exchange technique. The structure is suggested by a combination of X-ray powder diffraction and the electron diffraction (ED). [(Fe,Al)(OH)2][FeSe]1.2 is composed of the alternating stacking of a tetragonal FeSe layer and a hexagonal (Fe,Al)(OH)2 layer. In [(Fe,Al)(OH)2][FeSe]1.2, there exists a mismatch between the FeSe sublayer and the (Fe,Al)(OH)2 sublayer, and the lattice of the layered heterostructure is quasi-commensurate. The as-synthesized [(Fe,Al)(OH)2][FeSe]1.2 is nonsuperconducting due to the Fe vacancies in the FeSe layer. The superconductivity with a Tc of 40 K can be achieved after a lithiation process, which is due to the elimination of the Fe vacancies in the FeSe layer. The Tc is nearly the same as that of (Li,Fe)OHFeSe although the structure of [(Fe,Al)(OH)2][FeSe]1.2 is quite different from that of (Li,Fe)OHFeSe. The new layered heterostructure of [(Fe,Al)(OH)2][FeSe]1.2 contains an iron selenium tetragonal lattice interleaved with a hexagonal metal hydroxide lattice. These results indicate that the superconductivity is very robust for FeSe-based superconductors. It opens a path for exploring superconductivity in iron-base superconductors.

Non-innocent Intercalation of Diamines into Tetragonal FeS Superconductor

We report two solvothermal pathways toward intercalated iron sulfide, [Fe8S10]Fe(en)3·en0.5 (en = ethylenediamine), featuring [Fe8S10]2– layers stacked by [Fe(en)3]2+ cations and free ethylenediamine molecules. [Fe8S10]Fe(en)3·en0.5 is synthesized in a simple single-step method from Fe and S in ethylenediamine with addition of NH4Cl mineralizer as well as from solvothermal treatment of mackinawite, tetragonal FeS. In situ synchrotron powder X-ray diffraction experiments reveal a clear transformation of tetragonal FeS into [Fe8S10]Fe(en)3·en0.5 upon reaction with ethylenediamine. In-house control synthetic experiments confirmed the chemical process, whereby ethylenediamine leaches iron solely from the tetragonal Fe–S layers to form [Fe(en)3]2+ complexes and thereby oxidize the intralayer iron to Fe2.25+. Our report emphasizes that, in layered iron chalcogenides, diamines can intercalate as charged coordination complexes in tandem with neutral diamine molecules.

Understanding electronic peculiarities in tetragonal FeSe as local structural symmetry breaking

Traditional band theory of perfect crystalline solids often uses as input the structure deduced from diffraction experiments; when modeled by the minimal unit cell this often produces a spatially averaged model. The present study illustrates that this is not always a safe practice unless one examines if the intrinsic bonding mechanism is capable of benefiting from the formation of a distribution of lower symmetry local environments that differ from the macroscopically averaged structure. This can happen either due to positional, or due to magnetic symmetry breaking. By removing the constraint of a small crystallographic cell, the energy minimization in the density functional theory finds atomic and spin symmetry breaking, not evident in conventional diffraction experiments but being found by local probes such as pair distribution function analysis. Here we report that large atomic and electronic anomalies in bulk tetragonal FeSe emerge from the existence of distributions of local positional and magnetic moment motifs. The found symmetry-broken motifs obtained by minimization of the internal energy represent what chemical bonding in tetragonal phase prefers as an intrinsic energy lowering static distortions. This explains observations of band renormalization, predicts orbital order and enhanced nematicity, and provides unprecedented close agreement with spectral function measured by photoemission and local atomic environment revealed by pair distribution function. While the symmetry-restricted strong correlation approach has been argued previously to be the exclusive theory needed for describing the main peculiarities of FeSe, we show here that the symmetry-broken mean-field approach addresses numerous aspects of the problem, provides intuitive insight into the electronic structure, and opens the door for large-scale mean-field calculations for similar d-electron quantum materials.