scholarly journals The Impact of Vibrational Entropy on the Segregation of Cu to Antiphase Boundaries in Fe3Al

2021 ◽  
Vol 7 (8) ◽  
pp. 108
Author(s):  
Martin Friák ◽  
Miroslav Černý ◽  
Mojmír Šob
Keyword(s):  
Magnetic Moments ◽  
Energy Difference ◽  
Extended Defects ◽  
Vibrational Entropy ◽  
Phonon Modes ◽  
Antiphase Boundaries ◽  
Related Energy ◽  
Quantum Mechanical Study ◽  
Nonlinear Trends ◽  
The Impact

We performed a quantum mechanical study of segregation of Cu atoms toward antiphase boundaries (APBs) in Fe3Al. The computed concentration of Cu atoms was 3.125 at %. The APBs have been characterized by a shift of the lattice along the ⟨001⟩ crystallographic direction. The APB energy turns out to be lower for Cu atoms located directly at the APB interfaces and we found that it is equal to 84 mJ/m2. Both Cu atoms (as point defects) and APBs (as extended defects) have their specific impact on local magnetic moments of Fe atoms (mostly reduction of the magnitude). Their combined impact was found to be not just a simple sum of the effects of each of the defect types. The Cu atoms are predicted to segregate toward the studied APBs, but the related energy gain is very small and amounts to only 4 meV per Cu atom. We have also performed phonon calculations and found all studied states with different atomic configurations mechanically stable without any soft phonon modes. The band gap in phonon frequencies of Fe3Al is barely affected by Cu substituents but reduced by APBs. The phonon contributions to segregation-related energy changes are significant, ranging from a decrease by 16% at T = 0 K to an increase by 17% at T = 400 K (changes with respect to the segregation-related energy difference between static lattices). Importantly, we have also examined the differences in the phonon entropy and phonon energy induced by the Cu segregation and showed their strongly nonlinear trends.

scholarly journals Impact of Antiphase Boundaries on Structural, Magnetic and Vibrational Properties of Fe3Al

Materials ◽  
2020 ◽  
Vol 13 (21) ◽  
pp. 4884
Author(s):  
Martin Friák ◽  
Miroslav Černý ◽  
Monika Všianská ◽  
Mojmír Šob
Keyword(s):  
Magnetic Moments ◽  
Harmonic Approximation ◽  
Interface Energy ◽  
Vibrational Properties ◽  
Phonon Modes ◽  
Antiphase Boundaries ◽  
Mechanical Study ◽  
Quantum Mechanical Study ◽  
Lower Entropy ◽  
Induced Changes

We performed a quantum-mechanical study of the effect of antiphase boundaries (APBs) on structural, magnetic and vibrational properties of Fe3Al compound. The studied APBs have the {001} crystallographic orientation of their sharp interfaces and they are characterized by a 1/2⟨111⟩ shift of atomic planes. There are two types of APB interfaces formed by either two adjacent planes of Fe atoms or by two adjacent planes containing both Fe and Al atoms. The averaged APB interface energy is found to be 80 mJ/m2 and we estimate the APB interface energy of each of the two types of interfaces to be within the range of 40–120 mJ/m2. The studied APBs affect local magnetic moments of Fe atoms near the defects, increasing magnetic moments of FeII atoms by as much as 11.8% and reducing those of FeI atoms by up to 4%. When comparing phonons in the Fe3Al with and without APBs within the harmonic approximation, we find a very strong influence of APBs. In particular, we have found a significant reduction of gap in frequencies that separates phonon modes below 7.9 THz and above 9.2 THz in the defect-free Fe3Al. All the APBs-induced changes result in a higher free energy, lower entropy and partly also a lower harmonic phonon energy in Fe3Al with APBs when compared with those in the defect-free bulk Fe3Al.

scholarly journals A Quantum–Mechanical Study of Clean and Cr–Segregated Antiphase Boundaries in Fe3Al

Materials ◽  
2019 ◽  
Vol 12 (23) ◽  
pp. 3954 ◽  
Author(s):  
Martin Friák ◽  
Monika Všianská ◽  
Mojmír Šob
Keyword(s):  
Magnetic Properties ◽  
Magnetic Moments ◽  
Lattice Parameter ◽  
Quantum Mechanical ◽  
Strong Impact ◽  
Antiphase Boundaries ◽  
Cubic Symmetry ◽  
Mechanical Study ◽  
Quantum Mechanical Study ◽  
The Impact

We present a quantum-mechanical study of thermodynamic, structural, elastic, and magnetic properties of selected antiphase boundaries (APBs) in Fe 3 Al with the D0 3 crystal structure with and without Cr atoms. The computed APBs are sharp (not thermal), and they have {001} crystallographic orientation. They are characterized by a mutual shift of grains by 1/2⟨100⟩a where a is the lattice parameter of a cube-shaped 16-atom elementary cell of Fe 3 Al, i.e., they affect the next nearest neighbors (APB-NNN type, also called APB-D0 3 ). Regarding clean APBs in Fe 3 Al, the studied ones have only a very minor impact on the structural and magnetic properties, including local magnetic moments, and the APB energy is rather low, about 80 ± 25 mJ/m 2 . Interestingly, they have a rather strong impact on the anisotropic (tensorial) elastic properties with the APB-induced change from a cubic symmetry to a tetragonal one, which is sensitively reflected by the directional dependence of linear compressibility. The Cr atoms have a strong impact on magnetic properties and a complex influence on the energetics of APBs. In particular, the Cr atoms in Fe 3 Al exhibit clustering tendencies even in the presence of APBs and cause a transition from a ferromagnetic (Cr-free Fe 3 Al) into a ferrimagnetic state. The Fe atoms with Cr atoms in their first coordination shell have their local atomic magnetic moments reduced. This reduction is synergically enhanced (to the point when Fe atoms are turned non-magnetic) when the influence of clustering of Cr atoms is combined with APBs, which offer specific atomic environments not existing in the APB-free bulk Fe 3 Al. The impact of Cr atoms on APB energies in Fe 3 Al is found to be ambiguous, including reduction, having a negligible influence or increasing APB energies depending on the local atomic configuration of Cr atoms, as well as their concentration.

scholarly journals A Quantum-Mechanical Study of Antiphase Boundaries in Ferromagnetic B2-Phase Fe2CoAl Alloy

2021 ◽  
Vol 7 (10) ◽  
pp. 137 ◽  
Author(s):  
Martin Friák ◽  
Josef Gracias ◽  
Jana Pavlů ◽  
Mojmír Šob
Keyword(s):  
Computational Models ◽  
Poisson Ratio ◽  
Magnetic Moments ◽  
Quantum Mechanical ◽  
Random Structure ◽  
Antiphase Boundaries ◽  
Mechanical Study ◽  
Quantum Mechanical Study ◽  
Similar Formation ◽  
Type Lattice

In this study, we performed a quantum mechanical examination of thermodynamic, structural, elastic, and magnetic properties of single-phase ferromagnetic Fe2CoAl with a chemically disordered B2-type lattice with and without antiphase boundaries (APBs) with (001) crystallographic orientation. Fe2CoAl was modeled using two different 54-atom supercells with atoms on the two B2 sublattices distributed according to the special quasi-random structure (SQS) concept. Both computational models exhibited very similar formation energies (−0.243 and −0.244 eV/atom), B2 structure lattice parameters (2.849 and 2.850 Å), magnetic moments (1.266 and 1.274 μB/atom), practically identical single-crystal elastic constants (C11 = 245 GPa, C12 = 141 GPa, and similar C44 = 132 GPa) and auxetic properties (the lowest Poisson ratio close to −0.1). The averaged APB interface energies were observed to be 199 and 310 mJ/m2 for the two models. The studied APBs increased the total magnetic moment by 6 and 8% due to a volumetric increase as well as local changes in the coordination of Fe atoms (their magnetic moments are reduced for increasing number of Al neighbors but increased by the presence of Co). The APBs also enhanced the auxetic properties.

scholarly journals Quantum-Mechanical Study of Nanocomposites with Low and Ultra-Low Interface Energies

Nanomaterials ◽  
2018 ◽  
Vol 8 (12) ◽  
pp. 1057 ◽  
Author(s):  
Martin Friák ◽  
David Holec ◽  
Mojmír Šob
Keyword(s):  
Intermetallic Compound ◽  
Elastic Properties ◽  
Magnetic Moments ◽  
Perfect Match ◽  
Interface Energy ◽  
Α Phase ◽  
Error Bar ◽  
Mechanical Study ◽  
Quantum Mechanical Study ◽  
The Impact

We applied first-principles electronic structure calculations to study structural, thermodynamic and elastic properties of nanocomposites exhibiting nearly perfect match of constituting phases. In particular, two combinations of transition-metal disilicides and one pair of magnetic phases containing the Fe and Al atoms with different atomic ordering were considered. Regarding the disilicides, nanocomposites MoSi 2 /WSi 2 with constituents crystallizing in the tetragonal C11 b structure and TaSi 2 /NbSi 2 with individual phases crystallizing in the hexagonal C40 structure were simulated. Constituents within each pair of materials exhibit very similar structural and elastic properties and for their nanocomposites we obtained ultra-low (nearly zero) interface energy (within the error bar of our calculations, i.e., about 0.005 J/m 2 ). The interface energy was found to be nearly independent on the width of individual constituents within the nanocomposites and/or crystallographic orientation of the interfaces. As far as the nanocomposites containing Fe and Al were concerned, we simulated coherent superlattices formed by an ordered Fe 3 Al intermetallic compound and a disordered Fe-Al phase with 18.75 at.% Al, the α -phase. Both phases were structurally and elastically quite similar but the disordered α -phase lacked a long-range periodicity. To determine the interface energy in these nanocomposites, we simulated seven different distributions of atoms in the α -phase interfacing the Fe 3 Al intermetallic compound. The resulting interface energies ranged from ultra low to low values, i.e., from 0.005 to 0.139 J/m 2 . The impact of atomic distribution on the elastic properties was found insignificant but local magnetic moments of the iron atoms depend sensitively on the type and distribution of surrounding atoms.

scholarly journals Single Crystal Growth and Physical Properties of Pyroxene CoGeO3

Crystals ◽  
2021 ◽  
Vol 11 (4) ◽  
pp. 378
Author(s):  
Li Zhao ◽  
Zhiwei Hu ◽  
Hanjie Guo ◽  
Christoph Geibel ◽  
Hong-Ji Lin ◽  
...  
Keyword(s):  
High Pressure ◽  
Magnetic Properties ◽  
Magnetic Susceptibility ◽  
Crystal Growth ◽  
Physical Properties ◽  
Single Crystal ◽  
Single Crystals ◽  
Magnetic Moments ◽  
Single Crystal Growth ◽  
The Impact

We report on the synthesis and physical properties of cm-sized CoGeO3 single crystals grown in a high pressure mirror furnace at pressures of 80 bar. Direction dependent magnetic susceptibility measurements on our single crystals reveal highly anisotropic magnetic properties that we attribute to the impact of strong single ion anisotropy appearing in this system with TN∼33.5 K. Furthermore, we observe effective magnetic moments that are exceeding the spin only values of the Co ions, which reveals the presence of sizable orbital moments in CoGeO3.

scholarly journals Non-Parametric Generalized Additive Models as a Tool for Evaluating Policy Interventions

Mathematics ◽  
2021 ◽  
Vol 9 (4) ◽  
pp. 299
Author(s):  
Jaime Pinilla ◽  
Miguel Negrín
Keyword(s):  
Linear Regression ◽  
Regression Models ◽  
Linear Trend ◽  
Generalized Additive Models ◽  
Additive Models ◽  
Linear Regression Models ◽  
Non Linear ◽  
Nonlinear Trends ◽  
The Impact ◽  
Non Parametric

The interrupted time series analysis is a quasi-experimental design used to evaluate the effectiveness of an intervention. Segmented linear regression models have been the most used models to carry out this analysis. However, they assume a linear trend that may not be appropriate in many situations. In this paper, we show how generalized additive models (GAMs), a non-parametric regression-based method, can be useful to accommodate nonlinear trends. An analysis with simulated data is carried out to assess the performance of both models. Data were simulated from linear and non-linear (quadratic and cubic) functions. The results of this analysis show how GAMs improve on segmented linear regression models when the trend is non-linear, but they also show a good performance when the trend is linear. A real-life application where the impact of the 2012 Spanish cost-sharing reforms on pharmaceutical prescription is also analyzed. Seasonality and an indicator variable for the stockpiling effect are included as explanatory variables. The segmented linear regression model shows good fit of the data. However, the GAM concludes that the hypothesis of linear trend is rejected. The estimated level shift is similar for both models but the cumulative absolute effect on the number of prescriptions is lower in GAM.

scholarly journals First-Principles Study of the Impact of Grain Boundary Formation in the Cathode Material LiFePO4

2019 ◽  
Vol 4 (3) ◽  
pp. 80 ◽  
Author(s):  
Jan Kuriplach ◽  
Aki Pulkkinen ◽  
Bernardo Barbiellini
Keyword(s):  
Grain Boundary ◽  
Cathode Material ◽  
First Principles ◽  
Magnetic Moments ◽  
Lithium Ion ◽  
Coincident Site Lattice ◽  
Interface Energy ◽  
First Principles Study ◽  
Lifepo4 Cathode ◽  
The Impact

Motivated by the need to understand the role of internal interfaces in Li migration occurring in lithium-ion batteries, a first-principles study of a coincident site lattice grain boundary in LiFePO4 cathode material and in its delithiated counterpart FPO4 is performed. The structure of the investigated grain boundary is obtained, and the corresponding interface energy is calculated. Other properties, such as ionic charges, magnetic moments, excess free volume, and the lifetime of positrons trapped at the interfaces are determined and discussed. The results show that while the grain boundary in LiFePO4 has desired structural and bonding characteristics, the analogous boundary in FePO4 needs to be yet optimized to allow for an efficient Li diffusion study.

scholarly journals The Electronic Properties of Extended Defects in SrTiO3—A Case Study of a Real Bicrystal Boundary

Crystals ◽  
2020 ◽  
Vol 10 (8) ◽  
pp. 665 ◽  
Author(s):  
Christian Rodenbücher ◽  
Dominik Wrana ◽  
Thomas Gensch ◽  
Franciszek Krok ◽  
Carsten Korte ◽  
...  
Keyword(s):  
Electronic Properties ◽  
Boundary Region ◽  
Thermal Reduction ◽  
Fluorescence Lifetime Imaging ◽  
Extended Defects ◽  
Force Microscopy ◽  
Ab Initio Simulations ◽  
Complementary Method ◽  
Inhomogeneous Properties ◽  
The Impact

This study investigates the impact of extended defects such as dislocations on the electronic properties of SrTiO3 by using a 36.8° bicrystal as a model system. In order to evaluate the hypothesis that dislocations can serve as preferential reduction sites, which has been proposed in the literature on the basis of ab initio simulations, as well as on experiments employing local-conductivity atomic force microscopy (LC-AFM), detailed investigations of the bicrystal boundary are conducted. In addition to LC-AFM, fluorescence lifetime imaging microscopy (FLIM) is applied herein as a complementary method for mapping the local electronic properties on the microscale. Both techniques confirm that the electronic structure and electronic transport in dislocation-rich regions significantly differ from those of undistorted SrTiO3. Upon thermal reduction, a further confinement of conductivity to the bicrystal boundary region was found, indicating that extended defects can indeed be regarded as the origin of filament formation. This leads to the evolution of inhomogeneous properties of defective SrTiO3 on the nano- and microscales.

scholarly journals Impact of Nano-Scale Distribution of Atoms on Electronic and Magnetic Properties of Phases in Fe-Al Nanocomposites: An Ab Initio Study

Nanomaterials ◽  
2018 ◽  
Vol 8 (12) ◽  
pp. 1059 ◽  
Author(s):  
Ivana Miháliková ◽  
Martin Friák ◽  
Yvonna Jirásková ◽  
David Holec ◽  
Nikola Koutná ◽  
...  
Keyword(s):  
Ab Initio ◽  
Density Functional ◽  
Nearest Neighbor ◽  
Magnetic Moments ◽  
Generalized Gradient ◽  
Functional Theory ◽  
Α Phase ◽  
Ordered Intermetallic ◽  
Random Structures ◽  
The Impact

Quantum-mechanical calculations are applied to examine magnetic and electronic properties of phases appearing in binary Fe-Al-based nanocomposites. The calculations are carried out using the Vienna Ab-initio Simulation Package which implements density functional theory and generalized gradient approximation. The focus is on a disordered solid solution with 18.75 at. % Al in body-centered-cubic ferromagnetic iron, so-called α -phase, and an ordered intermetallic compound Fe 3 Al with the D0 3 structure. In order to reveal the impact of the actual atomic distribution in the disordered Fe-Al α -phase three different special quasi-random structures with or without the 1st and/or 2nd nearest-neighbor Al-Al pairs are used. According to our calculations, energy decreases when eliminating the 1st and 2nd nearest neighbor Al-Al pairs. On the other hand, the local magnetic moments of the Fe atoms decrease with Al concentration in the 1st coordination sphere and increase if the concentration of Al atoms increases in the 2nd one. Furthermore, when simulating Fe-Al/Fe 3 Al nanocomposites (superlattices), changes of local magnetic moments of the Fe atoms up to 0.5 μ B are predicted. These changes very sensitively depend on both the distribution of atoms and the crystallographic orientation of the interfaces.

Formation and Properties of Schottky Diodes on 4H-SiC after High Temperature Annealing with Graphite Encapsulation

2006 ◽  
Vol 527-529 ◽  
pp. 915-918 ◽  
Author(s):  
Y. Wang ◽  
M.K. Mikhov ◽  
B.J. Skromme
Keyword(s):  
High Temperature ◽  
Ideality Factor ◽  
Schottky Diodes ◽  
Extended Defects ◽  
High Temperature Annealing ◽  
Force Microscopy ◽  
Atomic Force ◽  
Current Voltage ◽  
Encapsulation Method ◽  
The Impact

The impact of high temperature annealing using graphite encapsulation (formed by baking photoresist) on the electrical properties of Ni Schottky diodes formed on the annealed surfaces is studied. The surface morphology is also characterized by atomic force microscopy (AFM). Annealing for 10 minutes at temperatures up to 1800 °C with graphite encapsulation actually reduces the high-current ideality factor of the diodes while raising the current-voltage barrier height (linearly extrapolated to unity ideality factor) from 1.453 V to 1.67-1.73 V. Excess leakage current occurs only in a subset of diodes, which are believed to be affected by extended defects. The AFM images show no significant surface roughening, and the graphite can be removed after processing. This encapsulation method is found to be highly effective in preserving the electronic properties of the surface during high temperature annealing.

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