Fe-As Bond Research Articles

Effect of aliovalent substitution on the local structure of CaKFe4As4 superconductor

We have investigated the local structure of the iron-based CaKFe4As4superconductor featuring distinct aliovalent substitutions at the Ca and K sites, that is CaKFe4As4, CaK0.9Sr0.1Fe4As4, CaK0.9Ba0.1Fe4As4and Ca0.9Na0.1K0.9Ba0.1Fe4As4. Temperature-dependent Fe K-edge extended x-ray absorption fine structure (EXAFS) measurements are used to determine the near-neighbors bondlengths and their stiffness. The EXAFS analysis reveals that the Fe-As bondlength undergoes negligible changes by substitution, however, the Fe-Fe bondlength and the As height are affected by the Sr substitution. The superconducting transition temperatures of CaK0.9Sr0.1Fe4As4and CaK0.9Ba0.1Fe4As4are very similar even if the mean As heights are significantly different suggesting that the anion height may not be a unique parameter to describe the superconductivity in CaKFe4As4. The mean As heights show a peculiar temperature dependence characteristic of CaKFe4As4system. Furthermore, the temperature-dependent mean square relative displacements reveal similar Fe-Fe bond stiffness in all samples, instead the Fe-As bond is substantially stiffer in case of CaK0.9Sr0.1Fe4As4. The local structure results are discussed in relation to the differing transport properties of aliovalent substituted 1144 superconductor.

Insight into the molecular interaction of trace As (III) and As(V) onto the hybrid anion exchanger impregnated with Fe/Mn nanoparticles (HA502P-Fe/Mn)

In this study, a hybrid anion exchanger impregnated with Fe/Mn nanoparticles (HA502P-Fe/Mn) was synthesized via two-step mixed metal oxide precipitation using 50:50 % DI: ethanol as solvent. Trace As(III) and As(V) removal efficiency was comprehensively validated in both equilibrium adsorption isotherms and kinetics. The maximum adsorption capacities of HA502P-Fe/Mn toward As(III) and As(V) were 21,590 and 18,930 µg/g, respectively. The kinetics of As(III) removal were best predicted (R2 = 0.986) using a pseudo-second order model (k2 = 0.001 g/µg As(III)·min). Adsorption mechanisms were further characterized using X-ray absorption spectroscopy (XANES and EXAFS), including a comparison with the density functional theory (DFT) data. The XANES results showed complex redox transformation of the adsorbed As(V) on HA502P-Fe/Mn. The EXAFS spectra indicate predominant inner-sphere complex formation of bidentate mononuclear edge-sharing (2E) complexes for Mn-As (bond distance 2.94 Å), and bidentate binuclear corner-sharing (2C) complexes for Fe-As (bond distance 3.34 Å) on HA502P-Fe/Mn surfaces. Based on the DFT simulation, the obtained 3D structures were optimized, and the bond lengths between Fe-As and Mn-As were comparable with the EXAFS. For practical purposes, the fixed-bed column experiments were fully investigated for both As(III) and As(V) removal using NSF standard 53 test water (As(III) 300 ppb, pH 6.5), including the regeneration. The HA502P-Fe/Mn demonstrated 2X treatable bed volumes (4,300 vs 2,000 BVs) more than the commercially available As selective adsorbent. The experiments demonstrated that HA502P-Fe/Mn is a promising novel adsorbent for As(III) removal owing to its relatively high As adsorption capacity and regenerating ability.

Theoretical investigation the growth of Fe3Si on GaAs: Stability and electronic properties of Fe3Si/GaAs(0 0 1), (1 1 0) via DFT

In this paper, we study the difference in stability between two low refractive index interfaces (0 0 1) and (1 1 0) of Fe3Si/GaAs heterostructures, the optimal geometry, adhesion work, interfacial energy, interfacial distance and interface electronic structure were calculated by using density functional theory (DFT). We constructed four different Fe3Si/GaAs heterostructures models by combining different stacking methods with two interfaces. Changes in interfaces and stacking methods can affect adhesion work, interfacial distance, and interfacial energy. The research shows that the hollow-site has a larger adhesion work and smaller interfacial energy than the top-site. Based on the data of the adhesion work and the interfacial energy, the mechanical properties and ductility of the (0 0 1) interface with hollow-site is stronger than others. In order to enhance the stability and fracture toughness of the system, more (0 0 1) interface with hollow-site need to be formed. At the same time, it has the smallest interfacial energy and is easier to form. The electronic structure of the (0 0 1) interface with hollow-site and the contribution of electron orbital hybridization to the formation of covalent bonds at the interface are also discussed. A new covalent bond is formed at the interface, and Fe-As bond is easier to form than Si-As bond.

Isovalent Substitution Effects of Arsenic on Structural and Electrical Properties of Iron-Based Superconductor NdFeAsO0.8F0.2

In this paper, nominal compositions of NdFeAsO0.8F0.2, NdFeAs0.95Sb0.05O0.8F0.2 and NdFeAs0.95P0.05O0.8F0.2 were prepared by one step solid state reaction method. The structural, electrical and morphological properties of samples were characterized through the XRD pattern, the 4 probe method and SEM, respectively. The crystal structure of our samples was tetragonal with P4/nmm:2 symmetry group. Also, the (x, y, z) and occupancy of ions and lattice parameters were changed by isovalent substitution of Phosphorus P and Antimony Sb in the NdFeAsO0.8F0.2 sample. The alfa and beta bond angles and Fe-As bond length are changed from the corresponding value of alfa and beta regular FeAs4 tetrahedron by isovalent doping p/As and Sb/As, that they are effective on the superconductivity transition temperature. The microstrain and crystalline size of samples were studied by the Williamson Hall method. The superconducting critical temperatures were attained at 56K and 46K for NdFeAsO0.8F0.2, NdFeAs0.95Sb0.05O0.8F0.2, respectively. The NdFeAs0.95P0.05O0.8F0.2 showed the structural transition temperature at 140K. It seems that there is a relation between the superconductivity and shrinkage of the crystal lattice. The flake type of grains was observed by SEM pictures of samples.

Collapsed tetragonal phase as a strongly covalent and fully nonmagnetic state: Persistent magnetism with interlayer As–As bond formation in Rh-doped Ca0.8Sr0.2Fe2As2

A well-known feature of CaFe$_{2}$As$_{2}$-based superconductors is the pressure-induced collapsed tetragonal phase that is commonly ascribed to the formation of an interlayer As-As bond. Using detailed X-ray scattering and spectroscopy, we find that Rh-doped Ca$_{0.8}$Sr$_{0.2}$Fe$_{2}$As$_{2}$ does not undergo a first-order phase transition and that local Fe moments persist despite the formation of interlayer As-As bonds. Our density functional theory calculations reveal that the Fe-As bond geometry is critical for stabilizing magnetism and that the pressure-induced drop in the $c$ lattice parameter observed in pure CaFe$_{2}$As$_{2}$ is mostly due to a constriction within the FeAs planes. These phenomena are best understood using an often overlooked explanation for the equilibrium Fe-As bond geometry, which is set by a competition between covalent bonding and exchange splitting between strongly hybridized Fe $3d$ and As $4p$ states. In this framework, the collapsed tetragonal phase emerges when covalent bonding completely wins out over exchange splitting. Thus the collapsed tetragonal phase is properly understood as a strong, covalent phase that is fully nonmagnetic with the As-As bond forming as a byproduct.

Electronic origin of structural transition in 122 Fe based superconductors

Direct quantitative correlations between the orbital order and orthorhombicity is achieved in a number of Fe-based superconductors of 122 family. The former (orbital order) is calculated from first principles simulations using experimentally determined doping and temperature dependent structural parameters while the latter (the orthorhombicity) is taken from already established experimental studies; when normalized, both the above quantities quantitatively corresponds to each other in terms of their doping as well as temperature variations. This proves that the structural transition in Fe-based materials is electronic in nature due to orbital ordering. An universal correlations among various structural parameters and electronic structure are also obtained. Most remarkable among them is the mapping of two Fe–Fe distances in the low temperature orthorhombic phase, with the band energies Edxz, Edyz of Fe at the high symmetry points of the Brillouin zone. The fractional co-ordinate zAs of As which essentially determines anion height is inversely (directly) proportional to Fe-As bond distances (with exceptions of K doped BaFe2As2) for hole (electron) doped materials as a function of doping. On the other hand, Fe-As bond-distance is found to be inversely (directly) proportional to the density of states at the Fermi level for hole (electron) doped systems. Implications of these results to current issues of Fe based superconductivity are discussed.

Temperature-Dependent As K-Edge EXAFS Studies of LaFe 1−x Co x AsO (x = 0.0 and 0.11) Single Crystals

We report the experimental results of temperature dependent polarized As K-edge extended x-ray absorption fine structure (EXAFS) of LaFe1-xCoxAsO (x=0.0 and 0.11) single-crystals. By aligning the Fe-As bond direction in the direction of the x-ray beam polarization we have been able to identify an anomaly in the Fe-As bond correlations at the tetragonal to orthorhombic transition at 150K, while previous investigations with standard unpolarized EXAFS of undoped LaFeAsO powder samples were not able to detect any such anomaly. Using our approach we have been able to identify in the superconducting doped sample, LaFe0.89Co0.11AsO, a broad anomaly around 60 K. The low temperature anomaly has good correlations with the temperature dependence of several properties like resistivity, magnetic susceptibility, linear thermal expansion, etc indicating the emergence of the dynamical oscillations of the Fe - As pairs

Local structural distortions and their role in superconductivity in SmFeAsO1−xFx superconductors

EXAFS studies at the As K edge as a function of temperature were carried out in SmFeAsO1−xFx (x = 0 and 0.2) compounds to understand the role of local structural distortions in superconductivity observed in F-doped compounds. A significant correlation between the thermal variation of local structural parameters such as anion height and superconducting onset is found in the fluorinated compounds. Such a variation in anion height is absent in the non-superconducting compound. An increase in the Fe-As bond distance just below the superconducting onset temperature indicates a similarity between the distortions observed in the high-T cuprates and these Fe-based superconductors.

Charge transfer and local structure distortion in Zn-doped LaFe1−xCoxAsO

To understand the significant suppression of the superconducting transition temperature Tc by non-magnetic Zn impurity, Zn doping effect on the electronic structure and local structure distortion for the superconductor LaFe1−x−yCoxZnyAsO have been investigated by X-ray absorption spectroscopy (XAS). Fe and As K-edge X-ray absorption near-edge spectroscopy (XANES) point out the charge distribution between Fe and As atoms in the presence of Zn impurities. Moreover, doping by Zn impurities the local lattice structure is finely modulated probed by Fe and As extend X-ray absorption fine structure (EXAFS). With Zn doping the FeAs bond is rigid, while the distance of FeFe pairs increases. That leads to a more distorted FeAs4 tetrahedron in the presence of Zn impurities. Therefore, in LaFe1−xCoxZnyAsO system the severe suppression of Tc by Zn impurity is mostly associated with the more distorted FeAs4 tetrahedron.

Effect of pressure, temperature, fluorine doping, and rare earth elements on the phonon density of states ofLFeAsO studied by nuclear inelastic scattering

We have performed systematic studies of the lattice dynamics in $L$FeAsO ($L=\text{La}$, Ce, Pr, Nd, Sm) in the parent and in the $\ensuremath{\sim}$10% F-doped compounds as a function of pressure and temperature. We have found that the modifications in the partial Fe density of phonon states are mainly governed by the Fe-As bond length. The change of this bond length explains the change of the Fe density of phonon states above 25 meV. We further observe anomalies in the behavior of the phonon mode near 16 meV. In the parent phase, this mode softens anomalously upon cooling through the structural phase transition. Upon F doping, this mode hardens indicating a strong electron-phonon coupling. This suggests that the corresponding phonons play an important role in the competition between superconductivity and magnetism in these materials.

Effective five band analysis on Tc vs. lattice structure correlation in iron pnictides

In the present study, we investigate the effect of the lattice structure in the iron pnictides within the spin fluctuation pairing theory based on the effective five band models of several hypothetical lattice structures of LaFeAsI We show that the presence and absence of Fermi surface pockets is sensitive to the Fe-As-Fe bond angle due to the multiorbital nature of the system, which is reflected to the nodeless/nodal form of the superconducting gap and Tc. Superconductivity is maximized within the maximum hole Fermi surface multiplicity regime where the arsenic atoms form a regular tetrahedron. Superconductivity has an overall tendency of increasing upon increasing Fe-As bond length. We provide a guideline of higher Tc from the lattice structure point of view.

Superconductivity in fluorine and yttrium co-doped SmFeAsO

Polycrystalline fluorine and yttrium co-doped SmFeAsO samples are synthesized by solid state sintering and their physical properties are studied. The lattice parameters of the Sm1−yYyFeAsO0.8F0.2 samples decrease with the increasing y due to the smaller Y ions and the stiffness of the Y-O bond. The maximum critical temperature Tc of the samples is at y = 0.05. This may be due to the fact that the strong interaction between Sm and Fe of the Fe-As bond is being re-disturbed by the doped Y ions.

Lattice expansion from isotope substitution in iron-based superconductors

We estimate the changes in lattice parameters due to iron isotope substitution in the iron-based high temperature superconductors using a variational calculation based on the anharmonicity of the Fe-As(Se) bond. For BaFe2As2 and 57Fe to 54$Fe substitution we find a c-axis expansion of 1 10^{-3}{\AA} and in-plane a-axis contraction of 1 10^{-5}{\AA} at 250K in good agreement with experimental values on (Ba,K)Fe2As2 for which a negative isotope exponent \alpha=-\frac{d\ln T_c}{d\ln m}<0 was found [P.M. Shirage et al., Phys. Rev. Lett. 103, 257003 (2009)]. For FeSe in contrast we find an expansion for both a and c lattice parameters. We discuss the relevance of these isotope induced changes of lattice parameters to the isotope effect in light of the well established sensitivity to the Fe-As(Se) coordination.

Maximizing the Fermi-surface multiplicity optimizes the superconducting state of iron pnictide compounds

We study the condition for optimizing superconductivity in the iron pnictides from the lattice structure point of view. Studying the band structure of the hypothetical lattice structure of LaFeAsO, the hole Fermi surface multiplicity is found to be maximized around the Fe-As-Fe bond angle regime where the arsenic atoms form a regular tetrahedron. Superconductivity is optimized within this three hole Fermi surface regime, while the stoner factor of the antiferromagnetism has an overall tendency of increasing upon decreasing the bond angle. Combining also the effect of the varying the Fe-As bond length, we provide a guiding principle for obtaining high $T_c$.

Different response of the crystal structure to isoelectronic doping inBaFe2(As1−xPx)2and(Ba1−xSrx)Fe2As2

Superconductivity up to 30 K in charge neutrally doped ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ has been ascribed to chemical pressure caused by the shrinking unit cell. But the latter induces no superconductivity in $({\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}){\text{Fe}}_{2}{\text{As}}_{2}$ in spite of the same volume range. We show that the spin-density-wave (SDW) state of ${\text{BaFe}}_{2}{\text{As}}_{2}$ becomes suppressed in ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ by a subtle reorganization of the crystal structure, where arsenic and phosphorus are located at different coordinates ${z}_{\text{As}}$ and ${z}_{\text{P}}$. High-resolution x-ray diffraction experiments with ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ single crystals reveal almost unchanged Fe-P bonds, but a contraction of the Fe-As bonds, which remain nearly unchanged in $({\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}){\text{Fe}}_{2}{\text{As}}_{2}$. Since the Fe-As bond length is a gauge for the magnetic moment, our results show why the SDW is suppressed by P doping, but not by Sr doping. Only the Fe-P interaction increases the width of the iron $3d$ bands, which destabilizes the magnetic SDW ground state. The simultaneous contraction of the Fe-As bonds is rather a consequence of the vanishing magnetism. Ordered structure models of ${\text{BaFe}}_{2}{({\text{As}}_{1\ensuremath{-}x}{\text{P}}_{x})}_{2}$ obtained by density-functional theory calculations agree perfectly with the single-crystal x-ray structure determinations. The contraction of the Fe-As bonds saturates at doping levels above $x\ensuremath{\approx}0.3$, which corrects the unreasonable linear decrease in the so-called pnictide height.

Electronic and lattice structures inSmFeAsO1−xFxprobed by x-ray absorption spectroscopy

Local lattice and electronic structures in the Fe-As layer of ${\text{SmFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ superconductors were studied by x-ray absorption spectroscopy, the $\text{Fe}\text{ }K$-edge and the $\text{As}\text{ }K$-edge extended x-ray absorption fine-structure, and x-ray absorption near-edge-structure experiments, respectively. Temperature-dependent local lattice distortions were observed in the Fe-As bond mean-square relative displacement of the superconducting samples. A strong coupling of the carrier-induced local lattice distortion (polaron) to the superconducting transition temperature in the oxypnictide superconductors is indicated. The near-edge spectra showed systematic temperature-dependent energy shifts, which indicate an intralayer electron redistribution from $\text{Fe}\text{ }d$ states to $\text{As}\text{ }p$ states due to orbital-selective band filling at low temperatures.

Gutzwiller Density Functional Studies of FeAs-Based Superconductors: Structure Optimization and Evidence for a Three-Dimensional Fermi Surface

The electronic structures of FeAs compounds are sensitive to FeAs bonding, which is described unsuccessfully by the local density approximation (LDA). Treating the multiorbital fluctuations from ab inito LDA+Gutzwiller method, we can now predict the correct FeAs bond length and bonding strength, which will explain the observed "soft phonon." The bands are narrowed by a factor of 2 from their LDA widths. The d{3z{2}-r{2}} orbital is pushed up to cross the Fermi level, forming a three-dimensional Fermi surface, which reduces the anisotropy. The interorbital Hund's coupling J rather than U plays a crucial role in obtaining these results.

First-principles investigation of the effect of pressure onBaFe2As2

On the experimental side, ${\text{BaFe}}_{2}{\text{As}}_{2}$ without doping has been made superconducting by applying appropriate pressure (2--6 GPa). Here, we use a full-potential linearized augmented plane-wave method within the density-functional theory to investigate the effect of pressure on its crystal structure, magnetic order, and electronic structure. Our calculations show that the striped antiferromagnetic order observed in experiment is stable against pressure up to 13 GPa. Calculated antiferromagnetic lattice parameters are in good agreements with experimental data, while calculations with nonmagnetic state underestimate Fe-As bond length and $c$-axis lattice constant. The effects of pressure on crystal structure and electronic structure are investigated for both the antiferromagnetic state and the nonmagnetic one. We find that the compressibility of the antiferromagnetic state is quite isotropic up to about 6.4 GPa. With increasing pressure, the ${\text{FeAs}}_{4}$ tetrahedra are hardly distorted. We observe a transition of Fermi-surface topology in the striped antiferromagnetic state when the compression of volume is beyond 8% (or pressure 6 GPa), which corresponds to a large change in $c/a$ ratio. These first-principles results should be useful to understand the antiferromagnetism and electronic states in the FeAs-based materials and may have some useful implications to the superconductivity.

Removal of inorganic As5+ from a small drinking water system

The drinking water from a small drinking water system contained arsenic in a concentration of about 50 ?g/L. Chemical analyses showed that the pentavalent form of arsenic was present. Since the MCL value is 10 ?g/L, it was necessary to implement a technological treatment to make the water suitable for drinking. In order to do so, two technologies were suggested: activated alumina and ?-FeOOH (TehnoArz, TA) adsorption media. Experiments using both adsorption media were performed on a laboratory scale. It was possible to remove arsenic to below 1 ?g/L. The maximal adsorption capacity was found to be 12.7 mg of As5+ per gram of ?-FeOOH. Moreover, all the important physico-chemical parameters of the water remained practically unchanged after the treatment. Only a slight release of iron from the media was observed. The Fe-As bond was studied by means of chemical analysis and X-ray powder diffraction. Finally, in addition to showing the capability of arsenic removal by ?-FeOOH, a comprehensive optimization of the technological parameters of the selected technology is provided.

Low-temperature lattice structure anomaly in theLaFeAsO0.93F0.07superconductor by x-ray absorption spectroscopy: Evidence for a strong electron-phonon interaction

The local lattice structure in ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ superconductors is studied by extended x-ray absorption fine-structure measurements. An anomalous upturn of the mean-square relative displacement of the Fe-As bond is detected below $\ensuremath{\sim}70\text{ }\text{K}$ as electron carriers are introduced, reflecting the occurrence of Fe-As bond local lattice fluctuation. Similar to that in cuprates, this lattice fluctuation exhibits an abrupt depression at the onset superconducting transition temperature. The results indicate that strong electron-lattice interaction is involved in the superconducting transition in oxypcnictide superconductors, putting a strict limitation on possible theoretical models.