arXiv subjects

Samira Sheykhi

Publications and source records attributed to Samira Sheykhi.

6 recordsLinked to original sources

arXiv2025

"Adiabatic" Elastic Constants in Hubbard-Corrected Density-Functional Theory DFT+U: case UO$_2$

Since in DFT+U there are multiple self-consistent electronic solutions, the so called metastable states, the elastic constants computed from stress-vs-strain will be incorrect if some of the strained configurations fall into a different local electronic minimum than the equilibrium non-strained state. So, it is crucial to carefully take steps to keep the same electronic Hubbard occupation branch when computing the stresses for small strained geometries. In this work, we have explained this "adiabatic" method of calculation for elastic constants and applied for UO$_2$ crystal described within two different unit cells of cubic 12-atom and tetragonal 6-atom basis sets. The calculation results for the two different unit cells are the same within 0.1 GPa, and agreement with experiment is excellent.

cond-mat.mtrl-sci↗

arXiv2025

Calculation of Elastic Constants of UO$_2$ using the Hubbard-Corrected Density-Functional Theory DFT+U

Uranium dioxide which is used as a fuel in light water nuclear reactors, is continually exposed to radiation damage originated from the collision of high-energy particles. Accumulation of the resulting defects gives rise to the evolution in the micro-structure of the fuel which in turn brings about local tensions and strains in the fuel. One of the after effects due to evolution of micro-structure is the swelling of fuel which can damage the fuel cladding and cause environmental contamination by leakage of radioactive particles. Hence, it is vital to continually monitor the evolution of micro-structure and to analyze the changes in mechanical properties of the fuel. The study of elastic constants and analysis of their behavior is very helpful in understanding the mechanical properties of the fuel. In this research, using the Hubbard-corrected first-principles density-functional theory method, we have calculated the elastic constants of the uranium dioxide single crystal and compared the results with existing experimental data. In addition, using the Voigt, Reuss, and Hill models, we have estimated the mechanical properties for the poly-crystalline fresh fuel. The results show a very good agreement between the theory and experiment. Accordingly, we can reliably extend our method of calculations to the complicated system of irradiated fuel pellet, which is in the form of a poly-crystal and hosts various defects.

cond-mat.mtrl-sci↗

arXiv2023

Self-consistent on-site and inter-site Hubbard parameters within DFT+U+V for UO$_2$ using density-functional perturbation theory

To apply the Hubbard-corrected density-functional theory for predicting some known materials' properties, the Hubbard parameters are usually so tuned that the calculations give results in agreement with some experimental data and then one uses the tuned model to predict unknown properties. However, in designing new unknown novel materials there is no data to fit the parameters and therefore self-consistent determination of these parameters is crucial. In this work, using the new method formulated by others, which is based on density-functional perturbation theory, we have calculated self-consistently the Hubbard parameters for UO$_2$ crystal within different popular exchange-correlation approximations. The calculated ground-state lattice constants and electronic band-gaps are compared with experiment and shown that PBE-sol lead to results in best agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2023

Relativistic effects in the study of structure and electronic properties of UO$_2$ within DFT+U method

To study crystals that contain heavy atoms, it is important to consider the relativistic effects, as electrons in orbitals close to the atom's nucleus can reach speeds comparable to that of light in a vacuum. In this study, we utilized the first-principles DFT+U method to analyze the electronic structure and geometric properties of uranium dioxide (UO2) using three formulations: full-relativistic, scalar-relativistic, and non-relativistic. Our findings demonstrate that the non-relativistic scheme produces results that deviate significantly from experimental values for both lattice constant and band gap. In contrast, the scalar-relativistic regime yields highly accurate results for the geometric properties of UO2, and is therefore sufficient for most studies. However, for a more precise analysis, the full-relativistic calculations with spin-orbit effects should be employed, which result in a $6.2\%$ increase in the Kohn-Sham band-gap and a $0.05\%$ decrease in the lattice constant compared to the scalar-relativistic approach.

cond-mat.mtrl-sci↗

arXiv2020

First-principles study of UO$_2$ lattice thermal-conductivity: A simple description

Modeling the high-$T$ paramagnetic state of bulk UO$_2$ by a non-spin-polarized calculation and neglecting the Hubbard-U correction for the $f$ electrons in U atoms, the lattice thermal conductivity of bulk UO$_2$ is investigated by the exact solution of the Boltzmann transport equation for the steady-state phonon distribution function. The results show that TA branches corresponding to U-atoms vibrations have the largest lifetimes and therefore have dominant role in thermal conductivity, while the optical branches corresponding mainly to O-atoms vibrations have the shortest lifetimes. Using this simple model, our results for the thermal conductivity show a very good agreement with the experiments. The calculations are repeated for bulk UO$_2$ with different U-235 concentrations of 3\%, 5\%, 7\%, and 20\%, and the results show a small decrease of thermal conductivity which arise from scattering of phonons by impurities.

cond-mat.mtrl-sci↗

arXiv2019

Electronic Structure Properties of UO2 as a Mott Insulator

In this work using the density functional theory (DFT), we have studied the structural, electronic and magnetic properties of uranium dioxide with antiferromagnetic 1k-, 2k-, and 3k-order structures. Ordinary approximations in DFT, such as the local density approximation (LDA) or generalized gradient approximation (GGA), usually predict incorrect metallic behaviors for this strongly correlated electron system. Using Hubbard term correction for f-electrons, LDA+U method, as well as using the screened Heyd-Scuseria-Ernzerhof (HSE) hybrid functional for the exchange-correlation (XC), we have obtained the correct ground-state behavior as an insulator, with band gaps in good agreement with experiment.

cond-mat.mtrl-sci↗