Editor’s column

D.N. Sokolov, O.V. Polev, V.S. Myasnichenko, K.G. Savina, N.Yu. Sdobnyakov
Tver State University

Abstract: This work is devoted to the problem of thermal stability of mono- and binary metal nanocages consisting of gold and silver atoms. The number of atoms in the studied nanocages was 1744, 2150, 2470 and 3370 atoms. The characteristic size (outer diameter) of nanocages is from 4,4 to 5,1 nm. Interatomic interaction was described by the tight-binding potential. Analysis of the caloric dependences of the specific potential part of the internal energy made it possible to identify the temperature regions of «healing» of cavities (pores) on the faces and in the internal regions (cores) of nanocages. An example of the structural collapse of a nanocage is described in detail, as a result of which crystalline and quasicrystalline phases are identified in the nanoparticle core for a temperature corresponding to melting for the chosen size. Segregation in a binary Au-Ag nanocage before and after its collapse was also studied.
Keywords: atomistic simulation, tight binding potential, metallic nanocages, porеs, thermal effects, stability/instability, structure formation

N.Yu. Sdobnyakov, A.Yu. Kolosov, D.N. Sokolov, K.G. Savina, A.N. Bazulev, S.A. Veresov, S.V. Serov
Tver State University

Abstract: The melting and crystallization phase transitions in the five-component metallic Au-Ag-Cu-Pd-Pt equiatomic nanosystem were investigated. The complex approach to atomistic modeling is due to the use of alternative methods of computer simulation – the molecular dynamics and Monte Carlo methods. The interatomic interactions were described by the tight-binding potential. According to the results of a series of computer experiments, it was established that five-component nanoparticles of equiatomic composition can form crystalline phases during cooling. Melting and crystallization temperatures for the investigated five-component nanoparticles were determined. The values obtained by alternative methods are in good agreement. For five-component nanoparticles, the concept of fixing the temperatures corresponding to the beginning and end of the phase transition process is confirmed. The metals that make up five-component nanoparticles, the atoms of which in the process of crystallization form the central part of the nanoparticle (core) and the peripheral regions, including the surface of the nanoparticle, are determined.
Keywords: molecular dynamics method, Monte Carlo method, tight binding potential, five-component nanoparticles, structure formation, melting point, crystallization temperature

V.V. Samsonov, S.A. Vasilyev, I.V. Talyzin, V.V. Puitov
Tver State University

Abstract: Taking into account results of our molecular dynamics experiments, we have concluded thatof the three  commonly considered alternative models of nanoparticle melting (homogeneous melting, liquid shell, nucleation of liquid and growth), the latter is the most adequate. At the same time, a more adequate model corresponds to a combination of continuous melting at the initial stage of the process with its subsequent abrupt completion. In other words, nucleation and growth of a liquid-like surface layer occur until a certain critical radius of the crystalline core of the particle is reached, and then melting is completed very quickly, almost abruptly (in fractions of a nanosecond) at a temperature interpreted as the nanoparticle melting temperature T_m. Then, the role of surface melting in nanoparticle sintering is discussed. According to our results, the sintering of metal nanoparticles at high temperatures cannot be reduced to a single mechanism: a certain role play surface melting, surface and bulk diffusion, deformation in the contact zone, and collective effects associated with the displacements of groups (clusters) of atoms rather than of individual atoms. We also have put forward and substantiated the hypothesis that the previously introduced redetermined Tamman temperature T_T=0,5T_m corresponds to the switching of the scenario of sintering of metal nanoparticles from formation of a dumbbell-shaped nanocrystal at low temperatures to the scenario corresponding to coalescence of solid nanoparticles resulting in the formation of a defective nanocrystal of a shape close to spherical.
Keywords: surface melting, metal nanoparticles, melting mechanisms, coalescence and sintering, molecular dynamics, thermodynamics

V.M. Samsonov, I.V. Talyzin, S.A. Vasilyev, V.V. Puitov
Tver State University

Abstract: Being the first part of a two-part series, published in this issue of the journal, this paper combines a brief overview of theoretical and experimental studies, as well as the results of atomistic simulations of surface melting in bulk bodies and nanoparticles with presentation of our own molecular dynamics results. We have studied the patterns and mechanisms of surface melting in metal nanoparticles (gold, silver, copper, lead and nickel). The patterns and mechanisms of this phenomenon were studied in most detail on gold and silver nanoparticles. It has been established that the effect of surface premelting is characteristic for nanoparticles of all the above metals, although with decreasing particle size this effect manifests itself to a lesser extent. In addition, our molecular dynamics results do not confirm theoretical predictions of some authors about the existence of a quite definite characteristic (critical) radius of nanoparticles, below which the effect of surface melting is completely absent.
Keywords: surface melting (premelting), metal nanoparticles, molecular dynamics, embedded atom method, LAMMPS

K.G. Savina, R.E. Grigoryev, A.D. Veselov, S.S. Bogdanov, P.M. Ershov, S.A. Veresov, D.R. Zorin, V.S. Myasnichenko, N.Yu. Sdobnyakov
Tver State University

Abstract: The processes of structure formation in Co-Au and Ti-V metal nanoparticles as well as factors affecting the crystallization process are considered. The objects of the study were Co-Au and Ti-V binary nanoparticles containing N = 400, 800, 1520 and 5000 atoms with the equiatomic composition. The computer experiment was carried out using method of molecular dynamics. The interatomic interaction was described by the tight-binding potential. According to the results of a series of computer experiments, it was found that the main factors influencing the possibility of obtaining crystalline phases are: the cooling rate of binary nanoparticles, their size and the size mismatch of atoms included in the composition, as well as the nature of the interaction of metal atoms. The manifestation of stability/instability in binary nanoparticles may be due to patterns of formation of crystalline phases. Moreover, the tendency to segregate one of the components in a binary system may not be the main factor determining the stability/instability of such a system.
Keywords: molecular dynamics method, tight binding potential, binary nanoparticles, cobalt, gold, titanium, vanadium, dimensional mismatch, crystallization

D.A. Ryzhkova
Katanov Khakass State University

Abstract: Phase-change random access memory is an excellent candidate for next-generation nonvolatile memory technology. In order to meet the needs of the industry, its capacity must be improved, for which it is necessary to reduce the volume of a unit cell. Proceeding from this, in this work, the possibility of using nanoparticles of the Ag-Au binary alloy as individual phase-change random access memory cells was evaluated by computer simulation. The method of molecular dynamics with a modified tight binding potential was used. For this, an analysis was made of the crystallization processes of these nanoparticles with a diameter of 2,0 to 8,0 nm with different rates of thermal energy removal. It was shown that the addition of gold to the composition makes it possible to solve the problem of the complex reproduction of the amorphous structure, which is characteristic of pure Ag nanoparticles. Due to this, stable switching between the amorphous and crystalline phases can be achieved at a nanocluster diameter of ≥4 nm and ≥6 nm with an Au content in the composition of ≥40% and ≥20%, respectively, which is significantly lower than the cut-off value of 10 nm characteristic of silver nanoparticles.
Keywords: nanoclusters, silver, gold, crystallization, structure, computer simulation, tight-binding, PCM cell

V.A. Polukhin, S. Kh. Estemirova, E.D. Kurbanova, R.M. Belyakova
Ural Branch of the Russian Academy of Science

Abstract: The principles of designing high-entropy alloys related to the selection of elements areanalyzed. When selecting elements, a parametric approach is used, including chemical and topological parameters. The main chemical parameter is the enthalpy of mixing of elements, the main topological parameter is the atomic radius. It is emphasized that the use of modified atomic radii (which take into account the local electronic environment) better predicts the formation of either amorphous or crystalline high-entropy alloys. Four main effects that determine the properties of high-entropy alloys are considered: the high entropy effect, the lattice distortion effect, the delayed diffusion effect, and the «cocktail» effect. Obtaining nanosized high-entropy materials based on high-entropy alloys is a new promising direction that allows one to significantly expand their areas of application related to energy (catalysis, energy storage, etc.), nanoelectronics, etc. The article analyzes some methods for the synthesis of nanosized high-entropy alloys and materials based on them. basis, developed as catalysts. The improved performance over conventional catalysts is explained in terms of the effects and features specific to multicomponent systems.
Keywords: multicomponent, amorphous and nanocrystalline alloys, high-entropy alloys and nanosized high-entropy alloys, morphology, catalysts, fcc and hcp structures, bcc, strength, thermal stability

N.I. Nepsha, D.N. Sokolov, E.S. Mitinev, A.A. Taktarov, N.Yu. Sdobnyakov
Tver State University

Abstract: In this work, scenarios of structure formation in ternary nanoparticles based on platinum and palladium of four stoichiometric compositions of different sizes were studied, with nickel acting as a dopant. Two alternative methods were used: the molecular dynamics method (implemented in the open source software LAMMPS) and the Monte Carlo method (implemented in the Metropolis scheme). In addition, to describe the interatomic interaction, two versions of force fields were used: the modified tightbinding potential (when implementing the molecular dynamics and Monte Carlo methods) and the embedded atom potential (when implementing the molecular dynamics method). Based on the results of a series of computer experiments, it was found that palladium atoms have increased segregation to the surface. At a cooling rate of 0,1 K/ps, an ordered crystalline FCC structure with inclusions of the HCP phase is formed. With an increase in the nickel dopant content to 20% in the ternary Pd-Pt-Ni nanoparticle, the identifiable local structure becomes more complex, both in terms of the number of phases and in terms of structural segregation.
Keywords: molecular dynamics method, Monte Carlo method, embedded atom potential, modified tightbinding potential, polyhedral template matching method, bimetallic and ternary nanoparticles, nickel, palladium, platinum, structure formation, melting and crystallization temperatures

V.S. Myasnichenko, P.M. Ershov, S.A. Veresov, A.N. Bazulev, N.Yu. Sdobnyakov
Tver State University

Abstract: The final configurations obtained during crystallization in ternary metal nanoalloys Ti_x-Al_96-x-V₄ of various compositions were studied. The molecular dynamics method was used as an atomistic simulation method. Interatomic interaction was described by the tight-binding potential. The size dependence of melting temperatures, as well as changes in melting and crystallization temperatures with changes in the composition of ternary nanoparticles, have been determined. Based on the results of a series of computer experiments, differences in the crystallization scenarios of Ti_x-Al_96-x-V₄ ternary nanoparticles were established. A classification based on internal structure and degree of crystallinity was proposed and tested. For Ti_x-Al_96-x-V₄ ternary nanoparticles, five main classes are identified based on the number of (semi) axes of 5th order symmetry. Despite the fact that studying the segregation of components of Ti_x-Al_96-x-V₄ ternary nanoparticles was not the goal of the work, atomic configurations corresponding to different temperatures during the cooling process were constructed and described.
Keywords: molecular dynamics method, tight-binding potential, ternary nanoparticles, structure formation, melting and crystallization temperatures

N.E. Malysheva¹, E.V. Dyakova², O.V. Malyshkina^2
1 Military Academy of Air and Space Defence named after Marshal of the Soviet Union G.K. Zhukov
² Tver State University

Abstract: We have analyzed the complex permittivity of a porous (10 volume % of pores) sodiumlithium niobate ceramic with the results of computer simulation. The calculation was based on the Cole–Cole formula, which took into account the presence of various mechanisms of relaxation processes in the low-frequency (linear dispersion) and mid-frequency regions, an additional term from the Debye formula was added to take into account the mixed polarization, and a term taking into account the damping factor was added for resonant-type polarization. The simulation was carried out with and without the conductivity taken into account. To take into account the contribution of conductivity to the dynamic dielectric response, the expression σ* = (ε″ + iε′)ε0ω was used. It is shown that the linear part of the frequency dependence in the range from 50 Hz to 1 MHz is equally well described both with and without conductivity. At the same time, the behavior of the dielectric response of piezoelectric ceramics in the high-frequency region, where resonance effects are observed, and the low-frequency region, where volume-charge polarization predominates, is much better described taking into account the contribution of conductivity to the dielectric response of the system.
Keywords: piezoelectric ceramics, complex permittivity, complex conductivity, dielectric spectroscopy