arXiv subjects

David G. Pettifor

Publications and source records attributed to David G. Pettifor.

5 recordsLinked to original sources

arXiv2021

Tight-binding bond parameters for dimers across the periodic table from density-functional theory

We obtain parameters for non-orthogonal and orthogonal TB models from two-atomic molecules for all combinations of elements of period 1 to 6 and group 3 to 18 of the periodic table. The TB bond parameters for 1711 homoatomic and heteroatomic dimers show clear chemical trends. In particular, using our parameters we compare to the rectangular d-band model, the reduced sp TB model as well as canonical TB models for sp- and d-valent systems which have long been used to gain qualitative insight into the interatomic bond. The transferability of our dimer-based TB bond parameters to bulk systems is discussed exemplarily for the bulk ground-state structures of Mo and Si. Our dimer-based TB bond parameters provide a well-defined and promising starting point for developing refined TB parameterizations and for making the insight of TB available for guiding materials design across the periodic table.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure based descriptor for characterizing local atomic environments

A quantitative descriptor of local atomic environments is often required for the analysis of atomistic data. Descriptors of the local atomic environment ideally provide physically and chemically intuitive insight. This requires descriptors that are low-dimensional representations of the interplay between atomic geometry and electronic bond formation. The moments of the local density of states (DOS) relate the atomic structure to the electronic structure and bond chemistry. This makes it possible to construct electronic structure based descriptors of the local atomic environment that have an immediate relation to the binding energy. We show that a low-dimensional moments-descriptor is sufficient as the lowest moments, calculated from the closest atomic neighborhood, carry the largest contributions to the local bond energy. Here, we construct moments-descriptors that project the space of local atomic environments on a 2-D map. We discuss in detail the separation of various atomic environments and their connections in the map. The distances in the map may be related to energy differences between local atomic environments as we show by analytic considerations based on analytic bond-order potentials (BOP) and by numerical assessment using TB and density-functional theory calculations. Possible applications of the proposed moments-descriptors include the classification of local atomic environments in molecular-dynamic simulations, the selection of structure sets for developing and testing interatomic potentials, as well as the construction of descriptors for machine-learning applications.

cond-mat.mtrl-sci↗

arXiv2016

Topologically close-packed phases in binary transition-metal compounds: matching high-throughput ab initio calculations to an empirical structure map

In steels and single-crystal superalloys the control of the formation of topologically close-packed (TCP) phases is critical for the performance of the material. The structural stability of TCP phases in multi-component transition-metal alloys may be rationalised in terms of the average valence-electron count $\bar{N}$ and the composition-dependent relative volume-difference $\overline{ΔV/V}$. We elucidate the interplay of these factors by comparing density-functional theory calculations to an empirical structure map based on experimental data. In particular, we calculate the heat of formation for the TCP phases A15, C14, C15, C36, $χ$, $μ$, and $σ$ for all possible binary occupations of the Wyckoff positions. We discuss the isovalent systems V/Nb-Ta to highlight the role of atomic-size difference and observe the expected stabilisation of C14/C15/C36/$μ$ by $\overline{ΔV/V}$ at $ΔN=0$ in V-Ta. In the systems V/Nb-Re, we focus on the well-known trend of A15$- σ- χ$ stability with increasing $\bar{N}$ and show that the influence of $\overline{ΔV/V}$ is too weak to stabilise C14/C15/C36/$μ$ in Nb-Re. As an example for a significant influence of both $\bar{N}$ and $\overline{ΔV/V}$, we also consider the systems Cr/Mo-Co. Here the sequence A15$- σ- χ$ is observed in both systems but in Mo-Co the large size-mismatch stabilises C14/C15/C36/$μ$. We also include V/Nb-Co that cover the entire valence range of TCP stability and also show the stabilisation of C14/C15/C36/$μ$. Moreover, the combination of a large volume difference with a large mismatch in valence-electron count reduces the stability of the A15/$σ$/$χ$ phases in Nb-Co as compared to V-Co. By comparison to non-magnetic calculations we also find that magnetism is of minor importance for the structural stability of TCP phases in Cr/Mo-Co and in V/Nb-Co.

cond-mat.mtrl-sci↗

arXiv2008

Deriving molecular bonding from macromolecular self-assembly

Macromolecules can form regular structures on inert surfaces. We have developed a combined empirical and modeling approach to derive the bonding. From experimental scanning tunneling microscopy (STM) images of structures formed on Au(111) by melamine, by PTCDA, and by a 2:3 mixture of the two, we determine the molecular bonding morphologies. Within these bonding morphologies and recognizing the distinction between cohesive and adhesive molecular interactions we simultaneously simulated different molecular structures using a lattice Monte Carlo method. Within these bonding morphologies there is a distinction between cohesive and adhesive molecular interactions. We have simulated different molecular structures using a lattice Monte Carlo method.

cond-mat.mtrl-sci↗

arXiv2007

Modelling spin qubits in carbon peapods

We have calculated electron spin interactions in chains of Sc@C82 endohedral fullerenes in isolation and inserted into a semiconducting or metallic single-walled carbon nanotube to form a peapod. Using hybrid density functional theory (DFT), we find that the spin resides mainly on the fullerene cage, whether or not the fullerenes are in a nanotube. The spin interactions decay exponentially with fullerene separation, and the system can be described by a simple antiferromagnetic Heisenberg spin chain. A generalised Hubbard-Anderson model gives an exchange parameter J and a Coulomb parameter U in good agreement with the DFT values. Within the accuracy of the calculations, neither semiconducting nor metallic nanotubes affect the interactions between the fullerene electron spins.

cond-mat.mtrl-sci↗