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The predicted Mössbauer spectroscopy of γ′-FeC, η-Fe 2 C, ζ-Fe 2 C, h-Fe 7 C 3 , o-Fe 7 C 3 , χ-Fe 5 C 2 , θ-Fe 3 C, γ′-Fe 4 C , γ″-Fe 4 C, and α′-Fe 16 C 2 .
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The Mössbauer spectroscopy of iron carbides (α-Fe, γ'-FeC, η-Fe2C, ζ-Fe2C, χ-Fe5C2, h-Fe7C3, θ-Fe3C, o-Fe7C3, γ'-Fe4C, γ''-Fe4C, and α'-Fe16C2) is predicted utilizing the all electron full-potential linearized augmented plane wave (FLAPW) approach across various functionals from LDA to GGA (PBE, PBEsol, and GGA + U) to meta-GGA to hybrid functional...
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... All the calculations were performed within the spin-polarized approximation [32]. The optimized lattice parameters, for example of the orthorhombic - [33], which confirms the reliability of the present computational scheme. ...
... Crystal structures Table 1 lists the crystallographic information of potential iron carbides precipitated in the Fe-C-Mn-Si alloys, including the crystal system, space group, and Pearson symbol of the -Fe 2 C-Pnnm, -Fe 2 C-P6 3 /mmc, -Fe n C-P6 3 22 (n = 2.4, 3), -Fe 3 C-Pnma, and -Fe 5 C 2 -C2/c phases, together with the presently predicted lattice parameters and volumes in comparison with those reported in the literature. It shows that the present DFT results are in accordance with the previous experimental and theoretical values [11,13,[33][34][35][36][37]. The calculated equilibrium volume (10.13 Å 3 /atom) of -Fe 2 C-P6 3 /mmc phase agrees with that (10.28 Å 3 / atom) reported in Ref. [13], which is larger than the experimental value [11]. ...
The Fe–C–Mn–Si alloys are promising advanced high strength steels (AHSSs) with their performances greatly influenced by iron carbides precipitated in quenching and partitioning process. To date, the structural characteristics and physicochemical properties of iron carbides, particularly the non-stoichiometric carbides like η and ε phases, still remain elusive. In the present work, the atomic structures, phase stability, mechanical properties, and temperature-dependent thermodynamic properties of potential iron carbides precipitated in Fe–C–Mn–Si alloys, including η-Fe2C-Pnnm, ε-Fe2C-P63/mmc, ε-FenC-P6322, θ-Fe3C-Pnma and χ-Fe5C2-C2/c, are investigated by performing the first-principles calculations based on density functional theory. It turns out that the hexagonal Fe2.4C and Fe3C carbides have the same space group of P6322, with their local atomic structure being featured by the C-centered CFe6 principal cluster with coordination number of six. Thus, these two phases are regarded as the same ε\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varepsilon$$\end{document} iron carbide. The orthorhombic η-Fe2C and θ-Fe3C phases are more stable than the hexagonal ε-Fe2C and ε-FenC (n = 2.4, 3) carbides, resulting in a potential phase transition from ε to η and/or θ. Analysis on stabilities indicate that the C atoms in iron carbides prefer to occupy the ordered octahedral and prismatic interstices. In comparison with the ε-FenC (n = 2.4, 3) carbides, which is stiffer at low temperatures, the precipitation of the θ-Fe3C and η-Fe2C carbides can benefit to strengthen the Fe–C–Mn–Si alloys at elevated temperatures. The present work provides an important insight for understanding the atomic structures and thermodynamic properties of iron carbides along with their contribution to the mechanical performances of AHSSs.
... The difference between the hyperfine field parameters of the Mössbauer spectra of the samples with carbon additives of both modifications did not exceed statistical error. According to [47,48], the formation of cementite Fe3C phase is possible in similar systems. However, its presence was not confirmed by the Mössbauer spectra. ...
The processing route of Sm2Fe17 carbides is shorter than that of nitrides, which can potentially be used for cost-effective mid-performance magnets’ production. The magnetic properties of Sm2Fe17Cx compounds can be controlled at the annealing step, which allows them to be used for a variety of applications. In this work, X-ray diffraction (XRD) analysis, Mössbauer spectroscopy, scanning and transmission electron microscopy (SEM, TEM) and vibrating sample magnetometry (VSM) were used for characterization of the structure and magnetic properties of Sm2Fe17Cx compounds. The powder samples were prepared by high-energy ball milling of Sm2Fe17 mixtures with carbon nanotubes (CNT) or graphite with subsequent annealing. The formation of Sm2Fe17Cx compounds after annealing was followed by the formation of α-Fe and amorphous Sm2O3. The hyperfine field values of Fe atoms of all the Sm2Fe17 lattice sites increased by 12% on average after annealing that was caused by carbon diffusion. The coercivity of the samples peaked after annealing at 375 °C. The samples with CNT demonstrated an increase of up to 14% in coercivity and 5% in specific remanence in the range of 250–375 °C annealing temperatures.
... However, for complex multielement solids such as q-Fe 3 C and c-Fe 5 C 2 , the C atoms are located in the trigonal prismatic sites of the distorted hexagonally closed-packed structure of Fe atoms. 41 This structural feature leads to the coexistence of multiple inequivalent Fe sites accompanied by a variety of FeÀC bond lengths and bond angles, so that there is no longer a simple relationship between the surface stability and surface undercoordination. For complex multielement solids with a small training set, GPR is expected to be a more reliable model than the linear and NN models. ...
Solid surfaces usually reach thermodynamic equilibrium through particle exchange with their environment under reactive conditions. A prerequisite for understanding their functionalities is detailed knowledge of the surface composition and atomistic geometry under working conditions. Owing to the large number of possible Miller indices and terminations involved in multielement solids, extensive sampling of the compositional and conformational space needed for reliable surface energy estimation is beyond the scope of ab initio calculations. Here, we demonstrate, using the case of iron carbides in environments with varied carbon chemical potentials, that the stable surface composition and geometry of multielement solids under reactive conditions, which involve large compositional and conformational spaces, can be predicted at ab initio accuracy using an approach that combines the bond valence model, Gaussian process regression, and ab initio thermodynamics. Determining the atomistic structure of surfaces under working conditions paves the way toward identifying the true active sites of multielement catalysts in heterogeneous catalysis.
... The calculated Mössbauer parameters are summarized in Table 5. The parameters of the three sextets correspond to the three iron positions in Hägg carbide, χ-Fe 5 C 2 [38,39]. ...
Nanocrystalline materials with the composition of (Cu0.5Ni0.5)yFe3−yO4 and a spinel structure were synthesized by the auto-combustion sol–gel method. The materials were characterized by powder X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and N2 physisorption. A decrease in the unit cell parameter and increase in the crystallite size with a decrease in the copper and nickel content in ferrites were evidenced. Mössbauer analysis determined that iron ions are in the 3+ states in all compositions. Transmission electron microscopy showed that synthesized ferrite materials consisted of nanoparticles with narrow size distributions. The catalytic properties of synthesized ferrites were studied in the reaction of ethyl acetate oxidation and methanol decomposition. The conversion of ethyl acetate and CO2 selectivity increased with temperature, and this effect was most pronounced for (Cu0.5Ni0.5)0.5Fe2.5O4, for which the main part of the particles possessed sizes below 10 nm, and the mean diameter was calculated to be 4.3 nm. The catalytic activity in the reaction of methanol decomposition was the highest for (Cu0.5Ni0.5)0.25Fe2.75O4, and it decreased with the increase in Cu and Ni content in the samples. The analysis of the samples after the catalytic test indicated significant reduction transformations within the catalysts. Under the reaction medium, the spinel phase decomposed through the formation of Hägg carbide.
... Therefore, atomic-level structural change can be detected with Mössbauer spectroscopy. With the help of Mössbauer spectroscopy, the chemical bond between the Mössbauer nucleus, such as iron or tin and surrounding elements, is reflected as a Mössbauer parameter of isomer shift (IS, δ) which reflects the 4s electron density respective to the standard material of α-Fe at the nuclear site [23]. The local distortion of polyhedral units composed of iron or tin oxides is reflected as quadrupole splitting (QS, ∆) which is associated with the asymmetrical parameter, (η xx − η yy )/η zz ), where η xx , η yy , and η zz are electric field gradients on the x-, y-, and z-axis, respectively. ...
Debye temperatures of α-SnxFe1−xOOH nanoparticles (x = 0, 0.05, 0.10, 0.15 and 0.20, abbreviated as Sn100x NPs) prepared by hydrothermal reaction were estimated with 57Fe- and 119Sn-Mössbauer spectra measured by varying the temperature from 20 to 300 K. Electrical properties were studied by solid-state impedance spectroscopy (SS-IS). Together, the charge–discharge capacity of Li- and Na-ion batteries containing Sn100x NPs as a cathode were evaluated. 57Fe-Mössbauer spectra of Sn10, Sn15, and Sn20 measured at 300 K showed only one doublet due to the superparamagnetic doublet, while the doublet decomposed into a sextet due to goethite at the temperature below 50 K for Sn 10, 200 K for Sn15, and 100 K for Sn20. These results suggest that Sn10, Sn15 and Sn20 had smaller particles than Sn0. On the other hand, 20 K 119Sn-Mössbauer spectra of Sn15 were composed of a paramagnetic doublet with an isomer shift (δ) of 0.24 mm s−1 and quadrupole splitting (∆) of 3.52 mm s−1. These values were larger than those of Sn10 (δ: 0.08 mm s−1, ∆: 0.00 mm s−1) and Sn20 (δ: 0.10 mm s−1, ∆: 0.00 mm s−1), suggesting that the SnIV-O chemical bond is shorter and the distortion of octahedral SnO6 is larger in Sn15 than in Sn10 and Sn20 due to the increase in the covalency and polarization of the SnIV-O chemical bond. Debye temperatures determined from 57Fe-Mössbauer spectra measured at the low temperature were 210 K, 228 K, and 250 K for Sn10, Sn15, and Sn20, while that of α-Fe2O3 was 324 K. Similarly, the Debye temperature of 199, 251, and 269 K for Sn10, Sn15, and Sn20 were estimated from the temperature-dependent 119Sn-Mössbauer spectra, which were significantly smaller than that of BaSnO3 (=658 K) and SnO2 (=382 K). These results suggest that Fe and Sn are a weakly bound lattice in goethite NPs with low crystallinity. Modification of NPs and addition of Sn has a positive effect, resulting in an increase in DC conductivity of almost 5 orders of magnitude, from a σDC value of 9.37 × 10–7 (Ω cm)–1 for pure goethite Sn (Sn0) up to DC plateau for samples containing 0.15 and 0.20 Sn (Sn15 and Sn20) with a DC value of ~4 × 10–7 (Ω cm)–1 @423 K. This non-linear conductivity pattern and levelling at a higher Sn content suggests that structural modifications have a notable impact on electron transport, which is primarily governed by the thermally activated via three-dimensional hopping of small polarons (SPH). Measurements of SIB performance, including the Sn100x cathode under a current density of 50 mA g−1, showed initial capacities of 81 and 85 mAh g−1 for Sn0 and Sn15, which were larger than the others. The large initial capacities were measured at a current density of 5 mA g−1 found at 170 and 182 mAh g−1 for Sn15 and Sn20, respectively. It is concluded that tin-goethite NPs are an excellent material for a secondary battery cathode and that Sn15 is the best cathode among the studied Sn100x NPs.
... The partitioning process shows expansion, which can be explained by carbon partitioning from martensite into austenite and the formation of unstable remaining austenite (stage iii). At 325 • C, martensite tempering initiates slowly and accelerates, while bainitic transformation takes place faster and then slows down [26]. The addition of Si suppresses carbide precipitation and inhibits growth during bainitic transformation at 325 • C. Thus, the applied Q&P process is likely to produce a mixture of tempered martensite and bainite laths. ...
... Martensite/austenite blocks originate from residual stable austenite, partially transformed to martensite during Q&P cooling at 5 • C/s. Pre-formed martensite, banded, and partially tempered during partitioning, also exist, along with excess carbonrejected carbide particles [26]. Fig. 4b provides a bright field TEM image, distinguishing bainitic ferrite and martensite phases. ...
In the pursuit of lightweight, durable steel, we have successfully developed a multicomponent structure in AISI 9254 spring steel using a two-stage quenching and partitioning (Q&P) process. The primary objective of this process was to engineer an optimized microstructure consisting of nanobainite, martensite, and nano-carbides. Utilizing the insights gained from the results of advanced techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT) performed on the as-received AISI 9254 spring steel, we refined the quenching and partitioning (Q&P) path, leading to the successful establishment of a bainitic transformation for superior mechanical properties. Our tensile tests revealed a high yield strength (≈ 1600 ± 25 MPa) and ultimate tensile strength (≈ 1850 ± 50 MPa), along with considerable elongation (≈ 11.15 ± 0.25%). We also identified that pre-formed martensite lath defects and high silicon content play crucial roles during the Q&P process, preventing carbide coalescence and increasing strain-hardening capacity. This study demonstrates the potential of a Q&P process to generate high-strength, ductile steel for automotive and aerospace applications.
... gr. Pnma [47]. The sextet observed in the spectrum of sample S1000 with parameters δ = 0.113 mm/s and H hf = 338.8 ...
... Iron atoms with carbon atom in the nearest environment give the Doublet1 line in the Mossbauer spectra, and iron atoms without carbon give the singlet line ( Table 2, Table 3). Small contributions corresponding to iron carbide Fe 3 C with local fields of about 57 H loc = 180-210 kOe [10,38] are present on the spectra of samples with ≤34% cobalt content (Fig.4 d, e) in the initial particles. This indirectly confirms the proposed scheme of the formation of fcc-Fe(C) regions. ...
A series of nanoparticles of the bimetallic core and carbon shell type FexCo1-x@C (x = 0.4 – 0.8) have been obtained by the gas condensation synthesis method. The microstructural peculiarities and magnetic behaviour of the nanoparticles is analysed depending on the composition and thermal treatment. Collateral phases, which are difficult to detect by traditional methods, are identified. The 59Co,57Fe NMR and 57Fe Mossbauer spectroscopies have shown that homogeneity of the composition and removal of carbon from the metallic core can be reached by annealing. The combination of methods has quantified the fraction of superparamagnetic particles, the proportion of paramagnetic inclusions. A simplified visual interpretation of the 59Co NMR spectra is proposed for binary FeCo alloys.
... 57 For the spent Co1Fe2 and Co2Fe1 catalysts, no Fe 3 O 4 phase is detected, and the three sextets can be attributed to χ-Fe 5 C 2 within a reasonable range, which is in accord with the discussion about XRD pattern (Fig. 5a, b). The detailed Mössbauer parameters shown in Table S6 goes from 110 kOe (Fe1) to 104 kOe (Co1Fe2) and further to 93 kOe (Co2Fe1), implying that the C/Fe molar ratio in measured iron carbides increases, further justifying the formation of χ-(Co x Fe 1− x ) 5 C 2 alloy carbide [36,49]. The proportion of χ-(Co x Fe 1− x ) 5 C 2 alloy carbide in spent Co1Fe2 catalyst is determined to be 97.2%, which is higher than those of spent Co2Fe1 (84.4%) and spent Fe1 catalyst (84.4%) (Fig. 5d). ...
... 11,12 Although these techniques provide the important surface structures of catalysts, it is still hard to obtain the structures of surface species on catalysts due to the limitation of experimental instruments, especially for the catalysts with metastability and strong magnetism. 13,14 Temperature-programed desorption (TPD) spectroscopy is a powerful and widely used technique 15−18 to determine the interactions of surface species with catalysts by exploring the kinetic desorption processes and thermodynamic parameters. By recording the signals of surface species as a function of time upon TPD, the interaction intensity between surface species and catalysts and the amount of surface species can be directly derived from the peak position and area of the TPD spectrum. ...
... To show its effectiveness, the TPD spectrum of CO on a χ-Fe 5 C 2 catalyst which is the key active phase 34−36 in converting syngas derived from coal, natural gas, and biomass into liquid fuels and value-added chemicals (Fischer−Tropsch synthesis, FTS) 37,38 was selected as a demonstration. The detailed interaction information between CO and Fe 5 C 2 is hard to obtain using other characterization techniques because the Fe 5 C 2 phase is metastable 35,39 with a variety of exposed surfaces 40−42 and strong magnetism, 13,14 enhancing the difficulty in deconvolution of the characterization spectrum. Based on the extensive DFT calculation of CO adsorption on nine facets of χ-Fe 5 C 2 , the TPD spectrum of the χ-Fe 5 C 2 nanoparticle was obtained by kMC simulation. ...
