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Register a new userCharge density and redox potential of LiNiO2 using ab initio diffusion quantum Monte Carlo
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Electronic structure of layered LiNiO2 has been controversial despite numerous theoretical and experimental reports regarding its nature. We investigate the charge densities, lithium intercalation potentials and Li diffusion barrier energies of LixNiO2 (0.0 < x < 1.0) system using a truly ab-initio method, diffusion quantum Monte Carlo (DMC). We compare the charge densities from DMC and density functional theory (DFT) and show that local and semi-local DFT functionals yield spin polarization densities with incorrect sign on the oxygen atoms. SCAN functional and Hubbard-U correction improves the polarization density around Ni and O atoms, resulting in smaller deviations from the DMC densities. DMC accurately captures the p-d hybridization between the Ni-O atoms, yielding accurate lithium intercalation voltages, polarization densities and reaction barriers.
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A roadblock in utilizing InGaAs for scaled-down electronic devices is its anomalous dopant diffusion behavior; specifically, existing models are not able to explain available experimental data on beryllium diffusion consistently. In this paper, we propose a comprehensive model, taking self-interstitial migration and Be interaction with Ga and In into account. Density functional theory (DFT) calculations are first used to calculate the energy parameters and charge states of possible diffusion mechanisms. Based on the DFT results, continuum modeling and kinetic Monte Carlo simulations are then performed. The model is able to reproduce experimental Be concentration profiles. Our results suggest that the Frank-Turnbull mechanism is not likely, instead, kick-out reactions are the dominant mechanism. Due to a large reaction energy difference, the Ga interstitial and the In interstitial play different roles in the kick-out reactions, contrary to what is usually assumed. The DFT calculations also suggest that the influence of As on Be diffusion may not be negligible.
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Henry H. Wu
, Dallas R. Trinkle (Department of Materials Science andn Engineering
, University of Illinois
2009
A binary embedded-atom method (EAM) potential is optimized for Cu on Ag(111) by fitting to ab initio data. The fitting database consists of DFT calculations of Cu monomers and dimers on Ag(111), specifically their relative energies, adatom heights, and dimer separations. We start from the Mishin Cu-Ag EAM potential and first modify the Cu-Ag pair potential to match the FCC/HCP site energy difference then include Cu-Cu pair potential optimization for the entire database. The optimized EAM potential reproduce DFT monomer and dimer relative energies and geometries correctly. In trimer calculations, the potential produces the DFT relative energy between FCC and HCP trimers, though a different ground state is predicted. We use the optimized potential to calculate diffusion barriers for Cu monomers, dimers, and trimers. The predicted monomer barrier is the same as DFT, while experimental barriers for monomers and dimers are both lower than predicted here. We attribute the difference with experiment to the overestimation of surface adsorption energies by DFT and a simple correction is presented. Our results show that the optimized Cu-Ag EAM can be applied in the study of larger Cu islands on Ag(111).
In this work, we adopt first-principle calculations based on density functional theory and Kinetic Monte Carlo simulations to investigate the adsorption and diffusion of lithium in bilayer graphene (BLG) as anodes in lithium-ion batteries. Based on energy barriers directly obtained from first-principle calculations for single-Li and two-Li intercalated BLG, a new equation was deduced for predicting energy barriers considering Lis interactions for multi-Li intercalated BLG. Our calculated results indicate that Li energetically prefers to intercalate within rather than adsorb outside the bilayer graphene. Additionally, lithium exists in cationic state in the bilayer graphene. More excitingly, ultrafast Li diffusion coefficient, within AB-stacked BLG near room temperature was obtained, which reproduces the ultrafast Li diffusion coefficient measured in recent experiment. However, ultrafast Li diffusion was not found within AA-stacked BLG near room temperature. The analyses of potential distribution indicate that the stacking structure of BLG greatly affects its height of potential well within BLG, which directly leads to the large difference in Li diffusion. Furthermore, it is found that both the interaction among Li ions and the stacking, structure cause Li diffusion within AB-stacked BLG to exhibit directional preference. Finally, the temperature dependence of Li diffusion is described by the Arrhenius law. These findings suggest that the stacking structure of BLG has an important influence on Li diffusion within BLG, and changing the stacking structure of BLG is one possible way to greatly improve Li diffusion rate within BLG. At last, it is suggested that AB-stacked BLG can be an excellent candidate for anode material in Lithium-ion batteries.
We use the diffusion quantum Monte Carlo to revisit the enthalpy-pressure phase diagram of the various products from the different proposed decompositions of H$_2$S at pressures above 150~GPa. Our results entails a revision of the ground-state enthalpy-pressure phase diagram. Specifically, we find that the C2/c HS$_2$ structure is persistent up to 440~GPa before undergoing a phase transition into the C2/m phase. Contrary to density functional theory, our calculations suggest that the C2/m phase of HS is more stable than the I4$_1$/amd HS structure over the whole pressure range from 150 to 400 GPa. Moreover, we predict that the Im-3m phase is the most likely candidate for H$_3$S, which is consistent with recent experimental x-ray diffraction measurements.
Motivated by the discovery of multiferroicity in the geometrically frustrated triangular antiferromagnet CuCrO$_2$ below its Neel temperature $T_N$, we investigate its magnetic and ferroelectric properties using ab initio calculations and Monte Carlo simulations. Exchange interactions up to the third nearest neighbors in the $ab$ plane, inter-layer interaction and single ion anisotropy constants in CuCrO$_2$ are estimated by series of density functional theory calculations. In particular, our results evidence a hard axis along the [110] direction due to the lattice distortion that takes place along this direction below $T_N$. Our Monte Carlo simulations indicate that the system possesses a Neel temperature $T_Napprox27$ K very close to the ones reported experimentally ($T_N = 24-26$ K). Also we show that the ground state is a proper-screw magnetic configuration with an incommensurate propagation vector pointing along the [110] direction. Moreover, our work reports the emergence of spin helicity below $T_N$ which leads to ferroelectricity in the extended inverse Dzyaloshinskii-Moriya model. We confirm the electric control of spin helicity by simulating $P$-$E$ hysteresis loops at various temperatures.
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