arXiv subjects

Mahmoud Payami

Publications and source records attributed to Mahmoud Payami.

14 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

Prediction of Magnetic State of UO2 within Hubbard-corrected Density-Functional Theory: A self-consistent approach

The magnetic state of UO$_2$ was determined experimentally to be anti-ferromagnetic. Starting from this experimental fact, researchers have calculated other properties within the Hubbard-corrected density-functional theory, DFT+U. Up to now, the Hubbard parameters for UO$_2$ were usually so chosen that the calculations give good results for some experimental data. Also, to our knowledge there exists no valid theoretical research report on the energetically stable magnetic state of this system. In present work, employing the new method which is based on density-functional perturbation theory, we have determined self-consistently the Hubbard parameters and ground-state energies for UO$_2$ crystal in both ferromagnetic and anti-ferromagnetic configurations, and the calculated results show that UO$_2$ crystal energetically favors an anti-ferromagnetic state with a small energy difference. In all the calculations the PBE-sol approximation was used for the exchange-correlation energy functional.

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

DFT+U study of UO$_2$: Correct lattice parameter and electronic band-gap

Hubbard-corrected density functional theory, denoted by DFT+U method, was developed to enable correct prediction of insulating properties for strongly-correlated electron systems. UO$_2$ is an example having O-$2p$, U-$6d$, and U-$5f$ incomplete electronic shells. Usually, researchers apply the Hubbard correction only to the localized incomplete $5f$ electrons of U atoms and succeed to predict insulating property and good geometric properties by tweaking the Hubbard-U parameter. However, it turned out that in such a way it was impossible to obtain reasonable values for both geometry and electronic band-gap at the same time. In this work, we show that it is possible to produce good values for those properties just by applying and tuning the Hubbard corrections to all incomplete shells of O-$2p$, U-$6d$, and U-$5f$.

cond-mat.str-el↗

arXiv2023

Comparison of SMC and OMC results in determining the ground-state and meta-stable states solutions for UO$_2$ in DFT+U method

Correct prediction of the behavior of UO2 crystal, which is an antiferromagnetic system with strongly-correlated electrons, is possible by using a modified density functional theory, the DFT+U method. In the context of DFT+U, the energy of crystal turns out to be a function with several local minima, the so-called meta-stable states, and the lowest energy state amongst them is identified as the ground state. OMC was a method that were used in DFT+U to determine the ground state. Recently the SMC method was proposed which using only the oxygen electronic spin-polarization degrees of freedom also revealed the multi-minima structure of energy in the DFT+U approach and led to results in good agreement with experiment. In this work, we compare the SMC and OMC results and show that although the ground states of the two methods have similar energies and geometries, the electronic structures have significant differences. Moreover, we show that the GS obtained from SMC is by 0.0022 Ry/(formula unit) above that of OMC. The different GS results from the two methods implies that they search for the minimum-energy state over different subspaces of electron densities and each method alone is not capable to locate the global minimum of the energy. Therefore, to obtain the global-minimum state of energy one has to search over larger subspaces that involve both occupation matrices of U atoms and starting magnetization of O atoms.

cond-mat.mtrl-sci↗

arXiv2022

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↗

arXiv2021

Spin-Symmetry Broken Ground-State of UO$_2$ in DFT+U Approach: The SMC Method

It turns out that the ground states of some systems are symmetry-broken states in which some property is not symmetrically distributed. In the case of strongly correlated electron systems, that were studied by the DFT+U method, researchers had shown that the total energy of the system is a multi-minima function of input parameters and one has to single out the ground state out of the couples of minimum-energy states. However, the methods already introduced to determine these local minimum states were not able to predict all such states which may include the "true" ground state. In this work, we introduce a new simple and straight-forward method of SMC to find the GS as well as the meta-stable states of 1k-order anti-ferromagnetic configuration for UO$_2$. Using this method, it is shown that the ground state of UO$_2$ system is a spin-symmetry broken state of the electron spin magnetizations of oxygen atoms. Depending on the way we apply the SMC method, we obtain different numbers of meta-stable states, but the same ground states. The energetic properties, geometric properties, the electronic density distributions, and the electronic polarization density distributions of the ground state and the meta-stable states are shown to be different from each other. These properties also are shown to be sensitive to the magnitude of the initial opposite magnetizations of U1 and U2 atoms in the 1k-order anti-ferromagnetic configuration, but the number of meta-stable states as well as the ground-state properties are insensitive to this magnitude. Using the PBEsol-GGA approximation for the exchange-correlation we obtain the ground-state properties in excellent agreement with experiments.

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

Elastic stiffness tensors of Zr-$x$Nb alloy in presence of defects: A molecular dynamics study

In a nuclear reactor, the Zr-$x$Nb alloy, which is used as a structural material in the core region, is irradiated by energetic particles that cause the atoms to be displaced from their lattice sites and giving rise to crystal defects. The local changes in the atomic arrangements lead to local deformations of the solid and thereby changes of its local mechanical properties. Understanding the mechanisms behind this evolution in the core region of a reactor, and its monitoring or controlling is a critical task in nuclear industry. In this work, using extensive molecular dynamics simulations, we have studied the effects of radiation damage on the local mechanical properties of Zr-$x$Nb alloy. In the first step, the effect of Nb-concentration on the mechanical stability of homogeneous Zr-$x$Nb alloy is investigated. In the second step, we have studied the local changes of the elastic constants due to local changes of the microstructure. These local changes include presence and accumulation of vacancies in the form of dislocation loops or voids, accumulation of Nb atoms in the form of clusters of different morphologies. This study covers both cases of $T=0^\circ$K and finite temperatures up to $T=600^\circ$K.

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↗

arXiv2017

First-Principles Calculation of Electronic Energy Level Alignment at Electrochemical Interfaces

Energy level alignment at solid-solvent interfaces is an important step in determining the properties of electrochemical systems. The positions of conduction and valence band edges of a semiconductor are affected by its environment. In this study, using first-principles DFT calculation, we have determined the level shifts of the semiconductors TiO$_2$ and ZnO at the interfaces with MeCN and DMF solvent molecules. The level shifts of semiconductor is obtained using the potential difference between the clean and exposed surfaces of asymmetric slabs. In this work, neglecting the effects of present ions in the electrolyte solution, we have shown that the solvent molecules give rise to an up-shift for the levels, and the amount of this shift varies with coverage. It is also shown that the shapes of density of states do not change sensibly near the gap. Molecular dynamics simulations of the interface have shown that at room temperatures the semiconductor surface is not fully covered by the solvent molecules, and one must use intermediate values in an static calculations.

cond-mat.mtrl-sci↗

arXiv2015

Theoretical description of the efficiency enhancement in DSSC sensitized by newly synthesized heteroleptic Ru complexes

Recently, some new series of heteroleptic ruthenium-based dyes, the so-called RD dyes, were designed and synthesized showing better performances compared to the well-known homoleptic N719. In this work, using the density-functional theory and its time-dependent extension, we have investigated the electronic structure and absorption spectra of these newly synthesized dyes, and compared the results to those of N3 dye to describe the variations of the properties due to the molecular engineering of ancillary ligand. We have shown that the calculation results of the absorption spectra for these dyes using the PBE0 for the exchange-correlation functional are in a better agreement with the experiment than using B3LYP or range-separated CAM-B3LYP. We have also derived a formula based on the DFT and used it to visually describe the level shifts in a solvent. The higher $J_{sc}$ observed in these new dyes is explained by the fact that here, in contrast to N3, the excitation charge was effectively transferred to the anchoring ligand. Furthermore, we have shown that the difference dipole moment vectors of the ground and excited states can be used to determine the charge-transfer direction in an excitation process. Finally, the different electron lifetimes observed in these dyes is explained by investigating the adsorption geometries and the relative orientations of iodine molecules in different ``dye$\cdots$I$_2$'' complexes.

cond-mat.mtrl-sci↗

arXiv2014

Efficiency enhancement of black dye-sensitized solar cell by newly synthesized D-$π$-A coadsorbents: A theoretical study

In this work, using the DFT and TDDFT, we have theoretically studied the electronic and optical properties of the two recently synthesized coadsorbents Y1 and Y2, which were aimed to enhance the efficiency of the black dye-sensitized solar cells. To determine the solvatochromic shifts, both the implicit and mixed implicit-explicit models have been used. The connection between the solvatochromic shifts and the changes of dipole moments in the excitation process is discussed. The difference in excitation charge transfer is utilized to explain the experimentally observed difference in $J_{sc}$ for Y1 and Y2. Investigating the interactions of I$_2$ molecules in the electrolyte solution with the coadsorbents showed that with Y1 the recombination loss was weakened through decreasing the I$_2$ concentration near the TiO$_2$ surface, whereas with Y2 it was increased. As a result, the higher values of both $J_{sc}$ and $V_{oc}$ with Y1 coadsorbent explains its experimentally observed higher efficiency. The present study sheds light on how to design and engineer newer coadsorbents or organic dyes for higher efficiencies.

cond-mat.mtrl-sci↗