We compare computational methods for determining the force between carbon atoms as a function of bond length, in order to establish which ones are capable of accurately simulating carbon-carbon bonds ...breaking due to applied mechanical strain in nanomaterials. Results from Tight-binding, density-functional theory and molecular mechanics potentials are compared to Møller-Plesset perturbation theory and complete-active-space self-consistent-field method through application to bond breaking in small molecules. Of the two molecular mechanics and three tight-binding parameter sets chosen only DFTB3 gives results which are broadly similar to those from the first-principles methods; the others fail to give physically meaningful variation of the forces with internuclear separation. This method and the molecular mechanics potentials are then applied to a periodic carbon nanoribbon under tensile strain. The molecular mechanics methods fail even qualitatively to reproduce the single catastrophic failure shortly after the peak stress indicated by DFTB3. This shows the importance of the electronic behaviour for the carbon-carbon interatomic forces relevant to the determination of the mechanical strength of materials at atomic-length scales.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Abstract Ab initio calculations in forsterite (Mg $$_2$$ 2 SiO $$_4$$ 4 ) are used to gain insight into the formation of point defects and incorporation of noble gases. We calculate the enthalpies of ...incorporation both at pre-existing vacancies in symmetrically non-equivalent sites, and at interstitial positions. At high pressure, most structural changes affect the MgO $$_{6}$$ 6 units and the enthalpies of point defects increase, with those involving Mg and Si vacancies increasing more than those involving O sites. At 15 GPa Si vacancies and Mg interstitials have become the predominant intrinsic defects. We use these calculated enthalpies to estimate the total uptake of noble gases into the bulk crystal as a function of temperature and pressure both in the presence and absence of other heterovalent trace elements. For He and Ne our calculated solubilities point to atoms occupying mainly interstitial sites in agreement with previous experimental work. In contrast, Ar most likely substitutes for Mg due to its larger size and the deformation it causes within the crystal. Incorporation energies, as well as atomic distances suggest that the incorporation mainly depend on the size mismatch between host and guest atoms. Polarization effects arising from the polarizability of the noble gas atom or the presence of charged defects are minimal and do not contribute significantly to the uptake. Finally, the discrepancies between our results and recent experiments suggest that there are other incorporation mechanisms such as adsorption at internal and external interfaces, voids and grain boundaries which must play a major role in noble gas storage and solubility.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
3.
Fast-ion conduction and local environments in BIMEVOX Stroud, Harry J.; Mohn, Chris E.; Hernandez, Jean-Alexis ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
11/2021, Volume:
379, Issue:
2211
Journal Article
Peer reviewed
Open access
The energy landscape of the fast-ion conductor Bi
4
V
2
O
11
is studied using density functional theory. There are a large number of energy minima, dominated by low-lying thermally accessible ...configurations in which there are equal numbers of oxygen vacancies in each vanadium–oxygen layer, a range of vanadium coordinations and a large variation in Bi–O and V–O distances. By dividing local minima in the energy landscape into sets of configurations, we then examine diffusion in each different layer using
ab initio
molecular dynamics. These simulations show that the diffusion mechanism mainly takes place in the 〈110〉 directions in the vanadium layers, involving the cooperative motion of the oxide ions between the O(2) and O(3) sites in these layers, but not O(1) in the Bi–O layers, in agreement with experiment. O(1) vacancies in the Bi–O layers are readily filled by the migration of oxygens from the V–O layers. The calculated ionic conductivity is in reasonable agreement with the experiment. We compare ion conduction in δ-Bi
4
V
2
O
11
with that in δ-Bi
2
O
3
.
This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.
Density functional theory calculations were performed to model the adsorption of up to 1 monolayer (ML) of aluminium on the bare and O-terminated (1 0 0) diamond surface. Large adsorption energies of ...up to −6.36 eV per atom are observed for the Al-adsorbed O-terminated diamond surface. Most adsorption sites give a negative electron affinity (NEA), with the largest NEAs −1.47 eV on the bare surface (1 ML coverage) and −1.36 eV on the O-terminated surface (0.25 ML coverage). The associated adsorption energies per Al atom for these sites are −4.11 eV and −5.24 eV, respectively. Thus, with suitably controlled coverage, Al on diamond shows promise as a thermally-stable surface for electron emission applications.
Molecular dynamics simulations were performed to study the behavior of nanoscale water droplets at solid surfaces. Simulations of droplets on heterogeneous patterned surfaces show that the relative ...sizes of the domains and the droplets play an important role as do the interactions between the solid and the liquid, particularly when the domain width is comparable to the droplet radius. For pillar surfaces, a transition is observed between the Wenzel and the Cassie and Baxter regimes with increasing pillar height. The effects of pillar width and the gap between the pillars were also examined. The simulations show clearly the importance of the detailed topography and composition of the solid surface.
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DFT calculations of the Li substitutional defect in diamond based on the B3LYP functional and a 64-atom supercell indicate that (i) the quartet (
S
z
= 3/2) state is lower in energy than the doublet ...(
S
z
= 1/2) state by 0.07 eV (810 K) for fully relaxed static structures and by 0.09 eV (1045 K) with the inclusion of zero-point vibrations, (ii) the effective charges at the Li and four neighbouring C sites are similar in the two spin states, but there are substantial differences in the corresponding spin distributions, and (iii) there are unprecedented differences in the Raman spectra of the two spin states, in terms of both frequency distributions and intensities, that can most reasonably be attributed to strong spin-phonon coupling, in view of the very similar charge distributions in the two states. These differences are an order of magnitude greater than those reported previously for any bulk transition metal or rare-earth compound. The basis sets and functional used in these calculations predict many of the relevant constants (
a
0
,
c
11
,
c
44
) of diamond mostly to within 1% of the experimental values, most notably the TO(X) Raman frequency and the phonon density of states. Comparisons with the calculated Raman spectra of the quintet (
S
z
= 2) and singlet (
S
z
= 0) spin states of the neutral vacancy defect, which have similar spin distributions at the four neighbouring C atoms (C
n
) to the vacancy site as those at the corresponding C
n
sites in the quartet and singlet states of the Li defect, show that the differences in the two Raman spectra of the latter defect are closely related to those in the former.
DFT calculations of the Li substitutional defect in diamond indicate that the quartet spin state is lower in energy than the doublet, with unprecedented differences in the frequency and intensity in the Raman spectra of the two states.
Using density functional theory electronic structure calculations, the equation of state, thermodynamic and elastic properties, and sound wave velocities of Fe3S at pressures up to 250 GPa have been ...determined. Fe3S is found to be ferromagnetic at ambient conditions but becomes non-magnetic at pressures above 50 GPa. This magnetic transition changes the c/a ratio leading to more isotropic compressibility, and discontinuities in elastic constants and isotropic sound velocities. Thermal expansion, heat capacity, and Grüneisen parameters are calculated at high pressures and elevated temperatures using the quasiharmonic approximation. We estimate Fe-Fe and Fe-S force constants, which vary with Fe environment, as well as the 56Fe/54Fe equilibrium reduced partition function in Fe3S and compare these results with recently reported experimental values. Finally, our calculations under the conditions of the Earth's inner core allow us to estimate a S content of 2.7 wt% S, assuming the only components of the inner core are Fe and Fe3S, a linear variation of elastic properties between end-members Fe and Fe3S, and that Fe3S is kinetically stable. Possible consequences for the core-mantle boundary of Mars are also discussed.
Recent computational work has shown that light metals adsorbed onto the oxygenated diamond (1 0 0) surface have the potential to give diamond a temperature-stable negative electron affinity (NEA). ...Here, we use density functional theory to study three of these metals, lithium, magnesium and aluminium, on the (1 1 1) surface. We show that all three of these metals adsorbed onto the ketone O-terminated diamond surface can possess a large NEA and adsorption energies above that of H-termination at monolayer (ML) or sub-ML coverages. Adsorption onto the ether O-terminated surface gives similarly large NEAs but lower adsorption energies. These results are promising for the development of novel NEA surfaces such as those required for thermionic devices.
Cooperative excitations in superionic PbF2 Mohn, Chris E; Krynski, Marcin; Kob, Walter ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
11/2021, Volume:
379, Issue:
2211
Journal Article
Methyl carboxylate esters promote the formation of dimethyl ether (DME) from the dehydration of methanol in H-ZSM-5 zeolite. We employ a multilevel quantum method to explore the possible associative ...and dissociative mechanisms in the presence, and absence, of six methyl ester promoters. This hybrid method combines density functional theory, with dispersion corrections (DFT-D3), for the full periodic system, with second-order Møller-Plesset perturbation theory (MP2) for small clusters representing the reaction site, and coupled cluster with single, double, and perturbative triple substitution (CCSD(T)) for the reacting molecules. The calculated adsorption enthalpy of methanol, and reaction enthalpies of the dehydration of methanol to DME within H-ZSM-5, agree with experiment to within chemical accuracy (∼4 kJ mol
−1
). For the promoters, a reaction pathway
via
an associative mechanism gives lower overall reaction enthalpies and barriers compared to the reaction with methanol only. Each stage of this mechanism is explored and related to experimental data. We provide evidence that suggests the promoter's adsorption to the Brønsted acid site is the most important factor dictating its efficiency.
Multilevel
ab initio
calculations reveal the role of promoters in the dehydration of methanol in H-ZSM-5.
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