arXiv subjects

M. D. Jones

Publications and source records attributed to M. D. Jones.

24 records · Page 2Linked to original sources

arXiv2005

An Empirical Tight-Binding Model for Titanium Phase Transformations

For a previously published study of the titanium hcp (alpha) to omega (omega) transformation, a tight-binding model was developed for titanium that accurately reproduces the structural energies and electron eigenvalues from all-electron density-functional calculations. We use a fitting method that matches the correctly symmetrized wavefuctions of the tight-binding model to those of the density-functional calculations at high symmetry points. The structural energies, elastic constants, phonon spectra, and point-defect energies predicted by our tight-binding model agree with density-functional calculations and experiment. In addition, a modification to the functional form is implemented to overcome the "collapse problem" of tight-binding, necessary for phase transformation studies and molecular dynamics simulations. The accuracy, transferability and efficiency of the model makes it particularly well suited to understanding structural transformations in titanium.

cond-mat.mtrl-sci↗

arXiv2003

Thermal Stabilization of the HCP Phase in Titanium

We have used a tight-binding model that is fit to first-principles electronic-structure calculations for titanium to calculate quasi-harmonic phonons and the Gibbs free energy of the hexagonal close-packed (hcp) and omega crystal structures. We show that the true zero-temperature ground-state is the omega structure, although this has never been observed experimentally at normal pressure, and that it is the entropy from the thermal population of phonon states which stabilizes the hcp structure at room temperature. We present the first completely theoretical prediction of the temperature- and pressure-dependence of the hcp-omega phase transformation and show that it is in good agreement with experiment. The quasi-harmonic approximation fails to adequately treat the bcc phase because the zero-temperature phonons of this structure are not all stable.

cond-mat.mtrl-sci↗

arXiv2003

New Pseudo-Phase Structure for $α$-Pu

In this paper we propose a new pseudo-phase crystal structure, based on an orthorhombic distortion of the diamond structure, for the ground-state $α$-phase of plutonium. Electronic-structure calculations in the generalized-gradient approximation give approximately the same total energy for the two structures. Interestingly, our new pseudo-phase structure is the same as the Pu $γ$-phase structure except with very different b/a and c/a ratios. We show how the contraction relative to the $γ$ phase, principally in the $z$ direction, leads to an $α$-like structure in the [0,1,1] plane. This is an important link between two complex structures of plutonium and opens new possibilities for exploring the very rich phase diagram of Pu through theoretical calculations.

cond-mat↗

arXiv2003

A New Mechanism for the Alpha to Omega Martensitic Transformation in Pure Titanium

We propose a new direct mechanism for the pressure driven alpha to omega martensitic transformation in pure titanium. A systematic algorithm enumerates all possible mechanisms whose energy barriers are evaluated. A new, homogeneous mechanism emerges with a barrier at least four times lower than other mechanisms. This mechanism remains favorable in a simple nucleation model.

cond-mat.mtrl-sci↗

arXiv2002

First-Principles-Based Thermodynamic Description of Solid Copper Using the Tight-Binding Approach

A tight-binding model is fit to first-principles calculations for copper that include structures distorted according to elastic constants and high-symmetry phonon modes. With the resulting model the first-principles-based phonon dispersion and the free energy are calculated in the quasi-harmonic approximation. The resulting thermal expansion, the temperature- and volume-dependence of the elastic constants, the Debye temperature, and the Gruneisen parameter are compared with available experimental data.

cond-mat.mtrl-sci↗

arXiv1999

Theoretical Atomic Volumes of fcc Thorium and Plutonium

The zero-presssure zero-temperature equilibrium volume and bulk modulus are calculated for fcc Th and Pu by two independent all-electron, full-potential, electronic-structure methods: the full-potential linear augmented-plane-wave (FLAPW) method and the linear combinations of Gaussian type orbitals-fitting function (LCGTO-FF) method. The results produced by these two distinctly different electronic-structure techniques are in excellent agreement with each other, but differ significantly from a previous calculation using the full-potential linear muffin-tin-orbital (FP-LMTO) method. The theoretically calculated equilibrium volumes are nearly 10% larger than the previous FP-LMTO calculations. While the new theoretical volume for Th is in good agreement with experiment when the generalized gradient approximation is used, the calculated equilibrium volume for fcc Pu remains substantially smaller than experiment in qualitative agreement with previous calculations.

cond-mat.mtrl-sci↗