Ab Initio Monte Carlo Simulations Research Articles

Phase segregation and nanoconfined fluid O2 in a lithium-rich oxide cathode

Lithium-rich oxide cathodes lose energy density during cycling due to atomic disordering and nanoscale structural rearrangements, which are both challenging to characterize. Here we resolve the kinetics and thermodynamics of these processes in an exemplar layered Li-rich (Li1.2–xMn0.8O2) cathode using a combined approach of ab initio molecular dynamics and cluster expansion-based Monte Carlo simulations. We identify a kinetically accessible and thermodynamically favourable mechanism to form O2 molecules in the bulk, involving Mn migration and driven by interlayer oxygen dimerization. At the top of charge, the bulk structure locally phase segregates into MnO2-rich regions and Mn-deficient nanovoids, which contain O2 molecules as a nanoconfined fluid. These nanovoids are connected in a percolating network, potentially allowing long-range oxygen transport and linking bulk O2 formation to surface O2 loss. These insights highlight the importance of developing strategies to kinetically stabilize the bulk structure of Li-rich O-redox cathodes to maintain their high energy densities.

Magnetic and magnetocaloric effect of Ba[formula omitted]FeReO[formula omitted] double perovskite: Ab-initio and Monte Carlo studies

In this paper, the double perovskite Ba2FeReO6 was studied using ab initio calculations and Monte Carlo simulation within the Ising model framework. The band structure and density of states of the compound were investigated. Moreover, energy calculations were employed to determine the magnetic anisotropy and exchange interactions for conducting a Monte Carlo study. The calculated curie temperature Tc of the compound is found 296 K, which agrees with the experimental value (Tc = 303 K). We have studied also the magnetic and magnetocaloric properties, the maximum entropy change (ΔSmax) and the relative cooling power (RCP) at a magnetic field of h=60 kOe are 4.28 J/kg K and 255 J/kg, respectively. Furthermore, the energy product was calculated using hysteresis loops. These findings suggest that Ba2FeReO6 is a competitive candidate material for magnetic refrigeration applications.

Study of magnetic properties and magnetocrystalline anisotropy of CoPb3 alloys by Ab initio calculations and Monte Carlo simulations for three-dimensional Ising, XY and Heisenberg models

The electronic and magnetic properties of CoPb3 compound have been calculated using the frst-principles density functional calculations and Monte Carlo simulation of Ising, XY and Heisenberg models. This compound exhibit metallic character. The total magnetic moments of CoPb3 using GGA and GGA + U approximations are 1.53874 and 2.1487 μB/f.u, respectively. The magnetic anisotropy shows that the easy axis is along the axis [100]. CoPb3 has perpendicular magnetic anisotropy (PMA) at T = 0 K. The critical temperature follows the trend Ising model > XY model > Heisenberg model using the MCS method. The isothermal magnetic entropy change was calculated. The maximum magnetic entropy changes are 0.24 J K−1 kg−1, 8.69 J K−1 kg−1 and 1.16 J K−1 kg−1 for Ising, XY and Heisenberg models respectively.

Robustness of Trion State in Gated Monolayer MoSe2 under Pressure.

Quasiparticles consisting of correlated electron(s) and hole(s), such as excitons and trions, play important roles in the optical phenomena of van der Waals semiconductors and serve as unique platforms for studies of many-body physics. Herein, we report a gate-tunable exciton-to-trion transition in pressurized monolayer MoSe2, in which the electronic band structures are modulated continuously within a diamond anvil cell. The emission energies of both the exciton and trion undergo large blueshifts over 90 meV with increasing pressure. Surprisingly, the trion binding energy remains constant at 30 meV, regardless of the applied pressure. Combining ab initio density functional theory calculations and quantum Monte Carlo simulations, we find that the remarkable robustness of the trion binding energy originates from the spatially diffused nature of the trion wave function and the weak correlation between its constituent electron-hole pairs. Our findings shed light on the optical properties of correlated excitonic quasiparticles in low-dimensional materials.

Particle growth across the scales: A combination of ab initio, molecular dynamics, and kinetic Monte Carlo simulations for Fe2Al5 in liquid Zn

We study the growth of Fe2Al5 particles in liquid Zn with a multi-scale approach from ab initio molecular statics and dynamics to kinetic Monte Carlo (kMC) simulations. This includes computation of the transition rates, i.e. adsorption rates from diffusion of elements in liquid Zn and desorption rates from surface desorption energies and interaction energies of atoms at the surface. Adsorption and desorption rates together with the correct crystal structure are the main input for the kMC simulations. With systematic kMC simulations, we then map out the growth rates of Fe2Al5 at different temperatures and varying chemical compositions. Altogether, this constitutes an approach that is also applicable to other studies focusing e.g. on deposition processes or dissolution of particles.

Probing octupolar hidden order via Janus impurities

Quantum materials with non-Kramers doublets are a fascinating venue to realize multipolar hidden orders. Impurity probes which break point group symmetries, such as implanted muons or substitutional impurities, split the non-Kramers degeneracy and exhibit a Janus-faced influence in such systems: they can destroy the very order they seek to probe. Here, we explore this duality in cubic osmate double perovskites which are candidates for exotic d-orbital octupolar order competing with quadrupolar states. Using ab initio computations, Landau theory, and Monte Carlo simulations, we show that Janus impurities induce local strain fields, nucleating quadrupolar puddles and suppressing the octupolar Tc. At the same time, strains mix the non-Kramers doublet with an excited magnetic triplet, creating parasitic dipole moments which directly expose the hidden octupolar order parameter. Our work unravels this Janus duality in recent impurity nuclear magnetic resonance (NMR) experiments, with important implications for uncovering hidden order in diverse multipolar materials.

Corrosion Inhibition Properties of Lawsone Derivatives againts Mild Steel: A Theoretical Study

Theoretical studies have been carried out using DFT, ab initio MP2 and Monte Carlo (MC) simulations of corrosion inhibitors from lawsone derivatives against carbon steel. The research focuses on studying the effect of substituent groups in the lawsone structure on the efficiency of corrosion inhibition in mild steel. Quantum chemical parameters of lawstone inhibitors in neutral and protonated conditions have been calculated. Fukui’s function analysis predicts that the active side of the inhibitor will be adsorbed on the mild steel surface. MC simulation is used to understand the adsorption patterns of lawsone compounds on metal surfaces. The organic inhibitor L-NH2 has better performance as a corrosion inhibitor for mild steel in neutral or protonated conditions.

Probing Local Environments of Oxygen Vacancies Responsible for Hydration in Sc-Doped Barium Zirconates at Elevated Temperatures: In Situ X-ray Absorption Spectroscopy, Thermogravimetry, and Active Learning Ab Initio Replica Exchange Monte Carlo Simulations

Proton-conducting oxides, specifically heavily Sc-doped barium zirconate perovskite, have attracted attention as electrolytes for intermediate-temperature protonic ceramic fuel cells because of their high proton conductivity and high chemical stability against carbon dioxide in that temperature regime. Hydration is a key reaction for incorporating protons by filling oxygen vacancies, VO, with hydroxyl groups and activating proton conduction in the perovskite. However, probing the local environment of oxygen vacancies responsible for hydration is challenging because the behavior depends on the temperature and water partial pressure, which necessitates in situ observations and calculations of the local environments at elevated temperatures. To obtain such information, we combined in situ X-ray absorption spectroscopy (XAS) for both the Sc and Zr K-edges, thermogravimetry, X-ray diffractometry, and active learning ab initio replica exchange Monte Carlo (RXMC) simulations in undoped and 20–40 at % Sc-doped barium zirconates at and below 800 °C. The presence of oxygen vacancies adjacent to Sc and Zr in the dehydrated samples and the hydration of these oxygen vacancies under a wet atmosphere were probed by in situ XAS for Sc and Zr pre-edges at elevated temperatures. Here, the microscopic hydration linearly responds to the macroscopic degree of hydration. RXMC sampling further supports the presence of Sc-VO-Zr and Sc-VO-Sc environments. An initial hydration occurs in the Sc-VO-Zr environment at and above 600 °C, but the Sc-VO-Sc environment contribution is greater at higher degrees of hydration. The Zr-Vo-Zr environment is the least abundant among them for the whole temperature range examined and thus has a negligible impact.

Study of physical properties of the quaternary full-Heusler alloy: FeMnCuSi

In this work, we investigated the physical properties of the Quaternary Full-Heusler alloy FeCuMnSi, including: the structural, magnetic, electronic, optical, and thermoelectric characteristics using the density functional theory (DFT) that has been implemented in the Wien2k package. The exchange-correlation potential has been performed in combination with the GGA-PBE and the modified Becke-Johnson (mBJ) semi-local interchange potential. Among others, we find that the studied compound presents a half-metal character. Moreover, we calculated and discussed the energy loss of electrons, the coefficient of absorption, the real dielectric tensor, the imaginary dielectric tensor, the real optical conductivity, and the imaginary optical conductivity. In addition, we examined the electrical conductivity, the Seebeck coefficient, the electronic, and the lattice thermal conductivity. Through Monte Carlo simulations, we have calculated the Curie temperature. By combining ab initio calculations and Monte Carlo simulations (MCS), we illustrated the magnetic properties and the magneto-caloric effects of the Quaternary Full-Heusler alloy FeCuMnSi. Explicitly, we have determined and examined the transition temperature, the change in magnetic entropy, and the relative cooling power coefficient.

Enhancing NO Uptake in Metal-Organic Frameworks via Linker Functionalization. A Multi-Scale Theoretical Study

In the present work, ab initio calculations and Monte Carlo simulations were combined to investigate the effect of linker functionalization on nitric oxide (NO)’s storage ability of metal–organic frameworks (MOFs). The binding energy (BE) of nitric oxide with a set of forty-two strategically selected, functionalized benzenes was investigated using Density Functional Theory calculations at the RI-DSD-BLYP/def2-TZVPP level of theory. It was found that most of the functional groups (FGs) increased the interaction strength compared to benzene. Phenyl hydrogen sulfate (–OSO3H) was the most promising among the set of ligands, with an enhancement of 150%. The organic linker of IRMOF-8 was modified with the three top-performing functional groups (–OSO3H, –OPO3H2, –SO3H). Their ability for NO adsorption was investigated using Grand Canonical Monte Carlo (GCMC) simulations at an ambient temperature and a wide pressure range. The results showed great enhancement in NO uptake constituting the above-mentioned FGs, suggesting them to be promising modification candidates in a plethora of porous materials.

Multiscale simulation of surface-assisted synthesis of 7-armchair graphene nanoribbons

The on-surface synthesis method is a remarkably promising technology in producing nanometer-wide graphene nanoribbons (GNRs) whose edge structures are well preserved. Controlling the quality of GNRs in terms of length is important for various GNR-based applications. We studied the surface-assisted synthesis of 7-armchair graphene nanoribbons (7-AGNRs) using a multiscale simulation method combining ab initio molecular dynamics simulations (AIMD) and kinetic Monte Carlo (KMC) simulations. The effect of monomer coverage and substrate type on the polymerization process are investigated. The length of the synthesized polyanthracene/GNRs on both Au(111) and Ag(111) substrates is shown to be significantly influenced by the deposited monomer amount. Ag(111) is better than Au(111) in facilitating the formation of longer polyanthracene/GNRs. An empirical mathematical model for predicting the percentage of long GNRs in the growth process is developed based on reaction parameters such as monomer coverage, the bonding and diffusion activation energies of DBBA radicals on the substrate, and temperature. This work provides a robust means for predicting and controlling the synthesis of 7-AGNRs on various substrates and presents a general multiscale framework for the synthesis of various GNRs.

Electronic phase transitions in quasi-one-dimensional atomic chains: Au wires on Si(553)

The Si(553) surface, covered with half a monolayer of gold, forms a double strand of well-ordered atomic Au chains in each miniterrace. It represents one of the smallest possible realizations of quasi-one-dimensional (1D) systems. In this prototype system, by combining DC conductance and low-energy electron diffraction measurements with density functional calculations and ab initio Monte Carlo simulations, we demonstrate that nonlocal electronic correlations, together with thermal excitations, lead to peculiar phase transitions without long-range order. They are characterized by marginal (average) geometric relaxations, but with huge variations of the electronic band structure and concomitant strong temperature-dependent modifications of the density of states close to the Fermi level. Similar phenomena are expected in the large class of quasi-1D conductors and open a wide range of possibilities for their controlled manipulation. It is the increasing hybridization between spin-polarized Au and Si edge states that makes the Si dangling bond states at the step edge conducting, first as a transient between two insulating phases and finally opening a permanent new 1D conduction channel at high temperatures.

Chemical Etching of Screw Dislocated Transition Metal Dichalcogenides.

Chemical etching can create novel structures inaccessible by growth and provide complementary understanding on the growth mechanisms of complex nanostructures. Screw dislocation-driven growth influences the layer stackings of transition metal dichalcogenides (MX2) resulting in complex spiral morphologies. Herein, we experimentally and theoretically study the etching of screw dislocated WS2 and WSe2 nanostructures using H2O2 etchant. The kinetic Wulff constructions and Monte Carlo simulations establish the etching principles of single MX2 layers. Atomic force microscopy characterization reveals diverse etching morphology evolution behaviors around the dislocation cores and along the exterior edges, including triangular, hexagonal, or truncated hexagonal holes and smooth or rough edges. These behaviors are influenced by the edge orientations, layer stackings, and the strain of screw dislocations. Ab initio calculation and kinetic Monte Carlo simulations support the experimental observations and provide further mechanistic insights. This knowledge can help one to understand more complex structures created by screw dislocations through etching.

Structure and electronic properties of copper oxide clusters and the effect of reacting with water investigated using Monte Carlo simulations and DFT calculations

Copper oxide clusters are particularly effective catalysts for the formation of polychlorinated-dibenzo-p-dioxins and -dibenzofurans and other pollutants. In this study, ab initio Monte Carlo simulations and density functional calculations are performed to study the structures and electronic properties of copper oxide clusters, CuOn (n = 1–6), and their reactions with a single water molecule. The lowest-energy structures of CuOn and CuOn-water clusters for n = 1–6 are optimized using the aug-cc-pVDZ and LANL2DZ basis sets of the B3LYP functional. The lowest-energy structures of copper oxide clusters are found to be planar or nearly planar. Selected electronic properties including the binding energies, second differences of the energies, HOMO-LUMO gaps, adiabatic and vertical ionization energies, adiabatic electron affinities, reaction energies, and Bader charges are calculated and examined.

Effect of surface structural change on adsorption behavior on InAs wetting layer surface grown on GaAs(001) substrate

The effect of surface structural change on the adsorption behavior of In adatom during the growth of InAs wetting layer (WL) on GaAs(001) substrate is theoretically investigated on the basis of ab initio calculations and Monte Carlo (MC) simulations. The ab initio calculations for InAs WL surface on GaAs(001) substrate reveal that the adsorption of In adatom is promoted on the (4×7) surface, which corresponds to the boundary between the (2×4) and (4×3) surfaces. The promotion of In adsorption on the (4×7) surface is due to the relaxation of atoms located near the In adatom. Furthermore, the MC simulations using the calculated total energies of various surface structures obtained by ab initio calculations demonstrate that the number of stable adsorption sites of In adatom becomes the largest around θ=1.1 monolayer (ML). This is because the area of the (4×7) surface with many stable adsorption sites increases with InAs coverage for 0.9 ⩽θ⩽1.15 ML. The calculated results manifest that the boundary between (2×4) and (4×3) surfaces is crucial for the nucleation and growth on InAs WL surface grown on the GaAs(001) substrate.

Mechanistic insights into carbon-carbon coupling on NiAu and PdAu single-atom alloys.

Carbon-carbon coupling is an important step in many catalytic reactions, and performing sp3-sp3 carbon-carbon coupling heterogeneously is particularly challenging. It has been reported that PdAu single-atom alloy (SAA) model catalytic surfaces are able to selectively couple methyl groups, producing ethane from methyl iodide. Herein, we extend this study to NiAu SAAs and find that Ni atoms in Au are active for C-I cleavage and selective sp3-sp3 carbon-carbon coupling to produce ethane. Furthermore, we perform ab initio kinetic Monte Carlo simulations that include the effect of the iodine atom, which was previously considered a bystander species. We find that model NiAu surfaces exhibit a similar chemistry to PdAu, but the reason for the similarity is due to the role the iodine atoms play in terms of blocking the Ni atom active sites. Specifically, on NiAu SAAs, the iodine atoms outcompete the methyl groups for occupancy of the Ni sites leaving the Me groups on Au, while on PdAu SAAs, the binding strengths of methyl groups and iodine atoms at the Pd atom active site are more similar. These simulations shed light on the mechanism of this important sp3-sp3 carbon-carbon coupling chemistry on SAAs. Furthermore, we discuss the effect of the iodine atoms on the reaction energetics and make an analogy between the effect of iodine as an active site blocker on this model heterogeneous catalyst and homogeneous catalysts in which ligands must detach in order for the active site to be accessed by the reactants.

Origin of the magnetic properties of MnFe2O4 spinel ferrite: Ab initio and Monte Carlo simulation

In this work, the electronic and magnetic properties of bulk spinel ferrite MnFe2O4 were carried out using ab initio calculations and Monte Carlo simulation. The electronic properties as well as, the exchange coupling interactions and the magnetic anisotropy were calculated using Density functional theory. The obtained exchange couplings between nearest n eighbours (JAB = −21.8 K, JBB = 13.7 K and JAA = 8.6 K) and the magnetic anisotropy were used as input parameters in a classical Ising model to study the finite-temperature effects on the magnetic properties of bulk MnFe2O4 with different size. It is shown that MnFe2O4 has a magnetic transition around 559.26 K.

Room-temperature large magnetocaloric, electronic and magnetic properties in La0.75Sr0.25MnO3 manganite: Ab initio calculations and Monte Carlo simulations

By using the Ab initio and Monte Carlo calculations, we have studied the magnetism of the crystalline La0.75Sr0.25MnO3 perovskite. The ferromagnetic phase of La0.75Sr0.25MnO3 is half-metallic, which is important in the relation to the colossal magnetoresistance properties of this compound. The total magnetic moment and the exchange couplings deduced from ab initio calculations lead, by using Monte Carlo simulations, to a quantitative agreement with the experimental transition temperatures. The maximum magnetic entropy change and the specific heat are found to be 9.23 J K−1 kg −1 and 191 J mol −1 K−1, respectively for H=6 T. Our results suggest that this material a promising candidate for magnetic refrigeration application near to room temperature at moderate fields.

Ab initio Monte Carlo simulations of structure and electronic properties of copper-tin clusters

This study investigated the structural and electronic properties of (CuSn)n clusters with n = 1–6 using ab initio Monte Carlo Simulations and density functional theory calculations. Monte Carlo simulations were performed using a large number of initial structures of neutral, cationic, and anionic copper-tin clusters. Their stable structures were determined using B3LYP/def2-TZVP model chemistry and the most stable structures of neutral, cationic, and anionic copper-tin clusters were established. The values of bond angles and bond lengths, atomization energies, second differences of the energies, HOMO-LUMO gaps, Mulliken population analysis, adiabatic ionization potentials, and adiabatic electronic affinities were determined as a function of n.

Dielectric ordering of water molecules arranged in a dipolar lattice

Intermolecular hydrogen bonds impede long-range (anti-)ferroelectric order of water. We confine H2O molecules in nanosized cages formed by ions of a dielectric crystal. Arranging them in channels at a distance of ~5 Å with an interchannel separation of ~10 Å prevents the formation of hydrogen networks while electric dipole-dipole interactions remain effective. Here, we present measurements of the temperature-dependent dielectric permittivity, pyrocurrent, electric polarization and specific heat that indicate an order-disorder ferroelectric phase transition at T0 ≈ 3 K in the water dipolar lattice. Ab initio molecular dynamics and classical Monte Carlo simulations reveal that at low temperatures the water molecules form ferroelectric domains in the ab-plane that order antiferroelectrically along the channel direction. This way we achieve the long-standing goal of arranging water molecules in polar order. This is not only of high relevance in various natural systems but might open an avenue towards future applications in biocompatible nanoelectronics.