Физика. Рубрика в журнале - Вестник Южно-Уральского государственного университета. Серия: Математика. Механика. Физика
Ab initio calculation of vacancy formation energy in antiperovskite Mn3GaC
Статья научная
In this article, we presented the ab initio calculation of vacancy formation energy according to Schottky in the alloy Mn3GaC. Calculations were carried out in the frameworks of the density functional theory (DFT), implemented in VASP software package. For approximation of the exchange-correlation functional, the generalized gradient approximation in the Perdew-Burke-Ernzerhof formulation was used. It was shown that for the alloy under research, the most energetically favorable formation of a vacancy is in the place of C atom; formation of vacancies in places of Mn atoms is also beneficial, whereas the Ga vacancies are energetically unfavorable. Also, the concentration of vacancies at a finite temperature was calculated. It was shown that Mn and C vacancies have almost identical equilibrium concentration at a nonzero temperature; at that, the concentration of Ga vacancies is negligibly small. In addition, elastic moduli for various magnetic orderings (ferromagnetic, noncollinear, and antiferromagnetic) in the alloy under research were calculated. Using the quasi-harmonic Debye model, the Helmholtz free energy curves were constructed. Using these curves, it was also shown that Schottky monovacancies do not destabilize the ferromagnetic phase. Stability of the ferromagnetic phase is due to the large contribution of magnetic entropy to the Helmholtz free energy for the alloy under research.
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Ab initio modeling of interactions of P, H, C, S with grain boundaries in a-iron
Статья научная
The results of modeling from the first principles of interaction of non-metallic impurities of interstitial (H, C) and substitutional (P, S) with grain boundaries in α-iron are presented. The modeling has been conducted within the framework of the density functional theory (DFT) by the full-potential linearized augmented plane waves (FP LAPW) method with consideration to the generalized gradient approximation (GGA'96) in the WIEN2k software package. Three grain boundaries of the slope Σ3 (111), Σ5 (210) and Σ5 (310) are studied. The supercells of the tilt grain boundaries using the coincidence site lattice model is constructed. The values of the energy characteristics of various grain boundaries with impurities are influenced by a number of factors, namely, the volume of the Voronoi polyhedron per impurity, magnetic moments, and the symmetry of the surrounding matrix. The results show that symmetric grain boundaries Σ3 (111) and Σ5 (310) are embrittled by phosphorus, hydrogen, and sulfur, while carbon strengthens interatomic bonds at the grain boundary, which coincides with the data available in the work. In the case of an asymmetric grain boundary Σ5 (210), phosphorus and hydrogen weaken bonds at the grain boundary, while sulfur strengthens them. This is primarily explained by the geometry of the surrounding matrix. The magnetic moments at the impurity atoms are very small and, in most cases, are antiparallel to the magnetic moments at the neighboring Fe atoms.
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Ab initio modelling of a bilayer graphene
Статья научная
Using the electron density functional theory, numerical modelling of bilayer graphene has been performed. The structure and binding energy of the layers depending on the representation of the wave function of the system have been studied: plane waves (VASP package) and atomic-like orbitals (SIESTA package); and choice of approximation for the exchange-correlation (XC) functional. It has been shown that being in the free form the system creates a stable AB structure of bilayer graphene. The calculation of the layer binding energy has shown that the results of modelling performed with different basis for the wave function are consistent when using XC functionals corresponding to each other and considering the correction to the basis set superposition error in the tight binding method. As expected, the generalized gradient approximation (GGA) has shown underestimated values of the interaction energy of graphene layers. Comparison with experimental data has shown that the energy and geometric characteristics of bilayer graphene are best described by the local electron density approximation (LDA). The 2nd and 3rd generation semi-empirical Grimme corrections for GGA have given estimates of the binding energy higher than LDA, but also close to the experimental results.
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Ab initio studies of hydrogen physisorption on clear and Li-doped carbon nanotubes
Статья научная
The hydrogen adsorption on internal and external surfaces of clear and Li-doped carbon nanotubes of different radii are investigated to assess the effects of concavity and doping on hydrogen uptake and binding energy. We make density functional calculations with the exchange-correlation functionals PBE and CA. Modeling of H2 adsorption on clear carbon tubes shows that only in case of internal sorption on narrow (5,5) nanotube energy of adsorption fall within the desirable range of 300-400 meV per H2 molecule. But in this case hydrogen uptake is too low and constitutes about 1,6 wt %. Doping with Li atom increases the adsorption energy of hydrogen molecule by 30-100 meV and in case of external sorption this energy enlarges several times. Nevertheless, the optimal range of binding energy can be achieved only in case of hydrogen adsorption inside quite narrow (5,5) and (7,7) Li-doped nanotubes.
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Ab initio моделирование взаимодействия водорода с точечными дефектами в ОЦК-железе
Статья научная
Проведено ab initio моделирование взаимодействия водорода с точечными дефектами в ОЦК-железе. Определено равновесное положение и энергия захвата водорода в ячейке с вакансией. Показано, что связь водорода с комплексом дефектов вакансия + атом замещения в основном определяется взаимодействием водорода с вакансией.
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Ab initio моделирование влияния кремния на образование карбида Fe3C в ОЦК-железе
Статья научная
Представлены результаты первопринципного моделирования влияния кремния на энергию формирования цементита и парциальную энтальпию. Моделирование проводилось в рамках теории функционала плотности (DFT) полнопотенциальным методом линеаризованных присоединенных плоских волн (FP LAPW) с учетом обобщенного градиентного приближения (GGA’96) в программном пакете WIEN2k. Были изучены различные концентрации примеси кремния в цементите, а именно, 1,6, 3,2 и 6 ат. % как в позиции замещения атома железа (S и G-позиции), так и атома углерода (С-позиция). Была выполнена объемная оптимизация структур. Найдены равновесные параметры решетки как цементита без примеси, так и в присутствии кремния, которые отлично согласуются с экспериментальными и теоретическими данными. Энергия формирования для концентрации 3,2 ат. % в С-позиции оказалась -0,03 эВ, что может говорить о стабилизации цементита. Но при этом парциальная энтальпия для всех положений кремния положительна, и значит, кремний остается в твердом растворе ОЦК-Fe, что находится в хорошем согласии с результатами других теоретических и экспериментальных работ. Было получено, что чем больше концентрация кремния, тем ниже средний магнитный момент на атомах железа.
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Ab initio моделирование энергии формирования границы зерна в ОЦК-железе
Статья научная
С помощью программного пакета WIEN2k проведено первопринципное моделирование границы зерна Σ5 ОЦК-железа. Получены оптимальные параметры и рассчитаны энергии формирования границы зерна для двух углов разориентировки.
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Статья научная
В программном пакете WIEN2k проведено первопринципное моделирование равновесной структуры и свойств ГЦК-железа. Получены оптимальные параметры, позволяющие построить наиболее точную модель. Произведено вычисление энергий растворения атомов углерода, их взаимного расположения и вклада упругих эффектов в энергию системы для немагнитного (НМ) и антиферромагнитного двухслойного (АФМД) состояний ГЦК железа.
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Статья научная
Using the DFT method, we simulated the adsorption of a single hydrogen molecule on pristine low-dimensional carbon nanomaterials: carbon nanotubes (CNT), en-yne (CEY), and graphdiyne (GDY). For wave function decomposition, we employed two approaches: localized pseudoatomic orbitals (SIESTA package) and plane waves (VASP package). For CNT, CEY, GDY, and bulk carbon (graphite), we optimized atomic basis sets. Delta test of used DFT packages showed a good agreement for carbon: ∆С = 0,36 meV/atom. We demonstrated that after atomic basis set optimization the value of counterpoise (CP) correction of basis set superposition error (BSSE) in calculations of hydrogen adsorption energies reduces. Moreover, this CP correction could be by several times bigger than the corrected hydrogen adsorption energy. Therefore, to obtain reasonable results in weakly interacting systems, CP-corrected adsorption energies in the optimized PAOs are needed. In considered systems, hydrogen adsorption energies, which were calculated in this way, agree with the energies obtained using the BSSE-free plane-wave basis set.
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DFT моделирование взаимодействия водорода с вакансией в ОЦК-железе
Статья научная
Впервые представлены результаты первопринципного моделирования взаимодействия атома водорода с вакансией ОЦК-железа в парамагнитном состоянии, которое сопоставляется с аналогичной величиной в магнитоупорядоченном ферромагнитном состоянии. Показано, что расстояния между атомом водорода и соответствующей октапорой для ферромагнитного и парамагнитного состояния оказываются одинаковыми и составляют 0,23 Å. Энергия связи комплекса водород-вакансия составила 0,60 и 0,27 для ферромагнитного и парамагнитного ОЦК-железа, соответственно.
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Effects of basis set superposition error on DFT model of C2N/graphene bilayer
Статья научная
We investigated the structural and energetic properties of the C2N/graphene bilayer using the electron density functional theory. We compared two approaches for wave function decomposition: plane waves (PW) and localized pseudoatomic orbitals (PAOs). We showed that for the weakly bonded bilayer, it is essential to consider correction to the basis set superposition error in binding energy calculations and geometry optimization. Otherwise, the interlayer binding energy and layer separation could be overestimated by 45-90 % and underestimated by 4-12 %, respectively. Also, to have the quantitative agreement between PAOs and PW results, the atomic-like basis set should be optimized. Overall, calculated with dispersion corrections, the interlayer binding energy (0,17-0,22 J/m2) is of the van der Waals nature.
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Elaboration of atomic model for ab initio calculation of the ferrite / cementite interface
Статья научная
The work describes constructing a supercell for ab initio modelling of ferrite / cementite interface in pearlite microsctructure of iron-carbon alloys. Bagaryatskii type orientation relationship between the two phases was accepted, with habit plane (101) c||(112) α. Optimization of ab initio modelling parameters was also performed using WIEN2k package.
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Статья научная
Intense irradiation and high-speed collision of metals results in the formation and dissemination of shock compression pulses in them. The recent development of experimental technology using high-power subpicosecond laser pulses makes it possible to obtain shock pulses of the picosecond range. A molecular dynamics simulation of high-speed collisions for aluminium samples is conducted. The presence of a nanorelief or precipitated nanoparticles on the back superficies of the sample may essentially enhance the rear splitting threshold. The cooperation of a shock wave with a nanorelief or precipitated nanoparticles results in strong plastic deformation. Consequently, part of the compression pulse energy is spent on plastic deformation, which prevents spall destruction. The effect of increasing the threshold can reach hundreds of meters per second in terms of collision speed and tens of gigapascals in amplitude of the incident shock wave. The distribution of shear strain and temperature in the sample is considered. It is shown that the maximum degree of deformation and maximum heating are observed in those parts of the nanorelief, for which the greatest change in shape is observed. The maximum temperature reaches the melting point, but no obvious traces of melting are found, which may be related to the speed of the processes.
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First-principles investigations of reference states of Co2CrIn Heusler alloys
Статья научная
In order to identify the reasons for the differences between the available theoretical and experimental values of the total magnetic moment of Co2CrIn Heusler alloy, in this work we studied the effects of various magnetic reference states on the magnetic and electronic properties of the alloys by means of ab initio and Monte Carlo methods. It is shown that the calculated ground state in the L21 phase is ferromagnetic. However, the values for both lattice parameter and magnetic moment calculated for the ferrimagnetic state, where the Cr atoms are ordered antiferromagnetically, are found to be in good agreement with the available experimental data. It is shown that the half-metallic behavior is realized only in the case of the ferromagnetic order. By using the calculated exchange coupling parameters in the Heisenberg Hamiltonian, the temperature dependences of magnetization were simulated.
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First-principles studies of the phase transitions in Fe-Si alloys
Статья научная
In this paper, the structural and magnetic properties of Fe100-xSix alloys (10 ≤ x ≤ 25,0 at. %) were calculated. The structural phase transition temperatures for the crystal structures A2, B2, and D03 were estimated from the geometry optimization. The Curie temperatures were calculated in a molecular-field approximation using the constants of magnetic exchange interaction calculated ab initio. For all the considered concentrations, with the temperature increase, we observed the structural transitions from the ordered cubic phase to a disordered structure, with the intermediate stage of a partially disordered state. The ferromagnet-paramagnet transition was observed for all the compositions, though in various crystal phases.
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Статья научная
Using the solid-phase reaction method we obtained bismuth ferrite based compounds generated by partial substitution of bismuth ions with rare earth metal ions (lanthanum and praseodymium). We used thermogravimetric analysis and X-ray analysis at the temperature range from 297 to 1123 K to study the peculiarities of the processes of phase formation of reaction-products synthesis in the systems which contain oxides of iron, bismuth, lanthanum, and praseodymium; their compositions were also determined. For the final synthesis temperature of 1123 K we determined a homogeneous concentration area, where bismuth ferrite solid solutions with the structure of distorted perovskite exist. We studied the influence of lanthanum and praseodymium oxides on phase composition of bismuth ferrite compounds in the process of isovalent alloying of a part of bismuth ions by lanthanum and praseodymium ions. In the frequency range of 20 Hz - 1 MHz we determined the key parameters, which characterize electrical and magnetic properties of perovskite-like phases. We found that for the considered samples the frequency dependencies of complex magnetic permeability and electrical parameters (real and alleged part of permittivity) are of almost the same type. We showed that for the given frequency range both electrical (complex permittivity) and magnetic (complex magnetic permeability) parameters decrease with the frequency increase.
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Importance of atomic-like basis set optimization for DFT modelling of nanomaterials
Статья научная
Atomic-like orbital basis sets allow efficient calculation of nanomaterial’s surface properties within the density functional theory. However, unlike plane wave basis sets, they require thorough optimization on a reference system before modelling systems of interest. We considered the basis set optimization procedure for several structures: bulk tantalum carbide, oxygen molecule, bulk lithium, and α-carbyne. We showed that during the optimization procedure not only the total energy of a reference system should be monitored but other physical characteristics (bond length and atomic charges) too. Moreover, optimal basis parameters could not correspond to the minimum total energy of a reference system to get the correct physical properties. We obtained optimal orbital parameters, which can be used for modelling of the following systems: oxygen adsorption on tantalum carbide surface, and Li-functionalized carbyne. Considering oxygen adsorption on TaC surface and Li-functionalization of carbyne, we also demonstrated that the basis set optimization influences binding energies and atomic charges of an adsorbent and a surface.
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Investigation of the diffraction of a focused Gaussian beam by a half-plane near the beam waist
Статья научная
Diffraction of a focused Gaussian beam in the vicinity of the waist region at the edge of the screen is considered on the basis of numerical simulation and experimental investigation. The Gaussian beam propagation has been simulated on the basis of the Helmholtz equation solution by the spectral method. The diffraction of coherent laser radiation (wavelength of 0,63 μm) at the edge of a rectangular screen overlapping half the beam in the transverse direction has been analyzed experimentally. The laser beam is focused by the lens of focal length of 4 cm. A dependence of the diffraction pattern on the screen position relative to the focal waist position is observed. It is found that if the screen is located at distances less than the focal length, then the diffraction pattern is observed in the dark region and represents a semicircle with diffraction fringes. If the screen is placed in the waist region, then the diffraction pattern becomes symmetrical with respect to the screen edge, and if the screen is located at a distance greater than the focal length, then the diffraction pattern is observed in bright area with the diffraction fringes appearing on the other side. The results can be used for the accurate determination of the focused Gaussian beams waist position.
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Magnetic and structural properties of all-D metal Mn-Ni-Ti Heusler alloys
Статья научная
The paper presents a theoretical study of the effect of different atomic and magnetic orderings on the structural and magnetic properties of Mn2Ni1+xTi1-x alloys, which are composed entirely of transition metals. Using the density functional theory, we predict the structural ground states and magnetic reference states of compounds with x = 0, 0,25, 0,5, and 0,75 in both cubic austenite and tetragonal martensite phases. Partial substitution of Ti atoms with Ni leads to an increase in the energy barrier between structural phases, to a change from a layered atomic ordering to an alternating staggered order, and to a change from antiferromagnetic to ferromagnetic spin alignment in the cubic phase. All compounds with tetragonally distorted structures reveal the out-of-plane spin configuration and easy axis magnetocrystalline anisotropy except tetragonal L10 phase of Mn2NiTi. For the latter structure, easy-plane magnetic anisotropy is observed. The calculated values of anisotropy are comparable with those of tetragonal L10-FeNi.
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Population kinetics of localized states in nanocomposite materials under exposure to radiation
Статья научная
We simulated the population of localized states in nanocomposite materials using Rouse-Fowler model. The following radiation effects were considered: prolonged irradiation (over 3 s) with the low absorbed dose rate (0,002 W/kg) and pulsed irradiation (100 ns) with the high absorbed dose rate (over 105 W/kg) of ionizing radiation. We investigated the role of localized states in electrical conductive properties of nanocomposite materials on the example of nanocomposite materials with hole conductivity (polymathimethacrilate (PMMA) + CdS) and electron conductivity (α-Al2O3+SrO), as well as in pure PMMA and α-Al2O3. Our results indicate that the small traps influence the speed of relaxation to the equilibrium radiation induced electrical conductivity, while the deep traps, the depth of which is much greater than kT, have an impact on the sensitivity to an absorbed dose of ionizing radiation. Moreover, pure PMMA and nanocomposite materials based on it are unsuitable for dosimetry due to a large share of the small traps in the spectrum of intrinsic localized states. On the contrary, aluminum oxide is an almost perfect material for the accumulation of the information about the ionizing radiation, since its spectrum of localized states includes only deep traps. On the whole, the most interesting materials from the dosimetry viewpoint are nanocomposites based on aluminum oxide, where the concentration of impurity centers does not exceed the concentration of intrinsic states, and the nanoparticle radius is no more than 2 nm in case of small share of the small traps in impurity spectrum.
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