Kirill A. Nekrasov

Molecular dynamics modeling of uranium, plutonium, and thorium cation diffusion in the volume of isolated (U _0.25 Pu _0.5 Th _0.25 )O ₂ nanocrystals consisting of 5460 particles was performed in the temperature range from 2700 K to 3200 K that corresponds to the superionic state. It was shown that cations of all types moved together during the passage of a cation vacancy through the crystal bulk. The temperature dependences of the diffusion coefficients of cations of all types coincided within the calculation error. The diffusion coefficient values lie in the region to which low-temperature experimental data on UO ₂ and ThO ₂ crystals can be extrapolated. The calculated value of the diffusion activation energy ( E _D = 7.3 ± 0.8 eV) is somewhat overestimated relative to low-temperature measurements.

Oxygen diffusion in (Pu_xTh_1−x)O₂ mixed oxide crystals was investigated using molecular dynamics simulation. The model systems were isolated nanocrystals of 5460 and 15,960 particles, featuring a free surface. The oxygen diffusion coefficient D increased with decreasing thorium content, in accordance with the decrease in the melting temperature of (Pu_xTh_1−x)O₂ as x varied from 0 to 1. The temperature dependences D(T) exhibited non-linearity in the Arrhenius coordinates lnD = f(1/kT). The three linear segments of the plots corresponded to the superionic state, a transitional region, and the low-temperature crystalline phase. The transitional region was characterized by maximum values of the effective diffusion activation energy E_D(PuO₂) = 3.47 eV, E_D(ThO₂) = 5.24 eV and a complex collective mechanism of oxygen migration, which involved the displacement of anions into interstitial sites. At lower temperatures, an interstitialcy mechanism of oxygen diffusion was observed. The temperature dependence of D(PuO₂) showed quantitative agreement with low-temperature experimental data.

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Oxygen diffusion in (Pu_xTh_1−x)O₂ mixed oxide crystals was investigated using molecular dynamics simulation. The model systems were isolated nanocrystals of 5460 and 15960 particles, featuring a free surface. The oxygen diffusion coefficient D increased with decreasing thorium content, in accordance with the decrease in the melting temperature of (Pu_xTh_1−x)O₂ as x varied from 0 to 1. The temperature dependences D(T) exhibited non-linearity in the Arrhenius coordinates lnD = f(1/kT). The three linear segments of the plots corresponded to the superionic state, a transitional region, and the low-temperature crystalline phase. The transitional region was characterized by maximum values of the effective diffusion activation energy E_D(PuO₂) = 3.47 eV, E_D(ThO₂) = 5.24 eV and a complex collectiv e mechanism of oxygen migration, which involved the displacement of anions into interstitial sites . At lower temperatures, an interstitialcy mechanism of oxygen diffusion was observed. T he temperature dependence of D(PuO₂) showed quantitative agreement with low-temperature experimental data.

A classical molecular dynamics simulation of cation diffusion in isolated crystals (U_xPu_yTh_1−x−y)O₂ bounded by a free surface was performed. It was shown that in the bulk of the same model crystallite, the diffusion coefficients of cations of all types were practically identical. At the same time, the cation diffusion coefficients changed with the melting temperature of nanocrystals, which increased with increasing thorium content. At a given temperature, the diffusion coefficients were the higher, the lower were the melting points of the (U_xPu_yTh_1−x−y)O₂ crystallites. The temperature dependences of the diffusion coefficients in crystallites of different compositions converged when using coordinates normalized to the melting points.

Classical molecular dynamics simulation of cation diffusion in isolated crystals (UxPuyTh1xy)O2 bounded by a free surface was performed. It was shown that in the bulk of the same model crystallite the diffusion coefficients of cations of all types were practically identical. At the same time, the cation diffusion coefficients changed with the melting temperature of nanocrystals, which increased with increasing thorium content. At a given temperature, the diffusion coefficients were the higher, the lower were the melting points of the (UxPuyTh1xy)O2 crystallites. The temperature dependences of the diffusion coefficients in crystallites of different compositions converged when using coordinates normalized to the melting points.

Defects like vacancies are a viable method to modify the properties of 2D materials. Thus, for more insights into the defect mechanism, herein we have explored the impact of nitrogen vacancies on the electronic and magnetic properties of 2D SiN nanosheet using density functional theory (DFT) calculations. The pristine SiN monolayer is a nonmagnetic semiconductor having a band gap of 1.73[ ]eV. However, the variation in the band gap and total magnetic moment is observed when the pristine SiN nanosheet is subjected to different concentrations of nitrogen vacancies (3.125%, 6.25% and 9.375%). A transition of SiN nanosheet from a nonmagnetic semiconductor to a magnetic half-metal is observed due to the introduction of nitrogen vacancy. We believe that this study will provide a good insight into the defect mechanism for engineering the material’s properties for applications like spintronics and optoelectronics.

A set of empirical pair interaction potentials for molecular dynamics (MD) modeling of the YPO₄ crystal is proposed. The parameters of the potentials, as well as the effective charges of the ions, are recovered based on experimental data on the lattice constants of YPO₄, as well as on the structure of the PO₄ complex within this crystal. Using these potentials, an MD simulation of YPO₄ crystallites isolated in vacuum is performed in the temperature range from 300 K to 2200 K. A quantitative agreement between the thermal expansion coefficient of the crystal and experimental data is obtained. Simulation of the formation of Frenkel defects in the yttrium, oxygen, and phosphorus sublattices is carried out. The formation energy of the Frenkel defects in the oxygen and yttrium sublattices is in quantitative agreement with ab initio calculations.

Molecular dynamics modeling of the self-diffusion of thorium cations in the volume of isolated ThO2 crystallites with a free surface was carried out. The model systems consisted of 15960 particles, the duration of the computational experiments reached 2 μs. In the superionic state of crystals, a quantitative coincidence of the diffusion coefficients of thorium and oxygen with experimental data extrapolated to high temperatures was obtained. In the temperature range from 2725 K to 3600 K, which included the superionic and crystalline phases, the diffusion of thorium cations in the presence of specially created cation vacancies was studied, and an estimate of the migration energy of cations was obtained equal to (5.9±0.3) eV. The effective activation energies for the diffusion of thorium and oxygen are compared with experimental data.