quasiharmonic equations of state for dynamically-stabilized soft-mode materials

We introduce a method for treating soft modes within the analytical framework of the quasiharmonic equation of state. The corresponding double-well energy-displacement relation is fitted to a functional form that is harmonic in both the low- and high-energy limits. Using density-functional calculations and statistical physics, we apply the quasiharmonic methodology to solid periclase. We predict the existence of a B1--B2 phase transition at high pressures and temperatures

First-principles study of the structural energetics of PdTi and PtTi

The structural energetics of PdTi and PtTi have been studied using first-principles density-functional theory with pseudopotentials and a plane-wave basis. We predict that in both materials, the experimentally reported orthorhombic $B19$ phase will undergo a low-temperature phase transition to a monoclinic $B19'$ ground state. Within a soft-mode framework, we relate the $B19$ structure to the cubic $B2$ structure, observed at high temperature, and the $B19'$ structure to $B19$ via phonon modes strongly coupled to strain. In contrast to NiTi, the $B19$ structure is extremely close to hcp. We draw on the analogy to the bcc-hcp transition to suggest likely transition mechanisms in the present case.Comment: 8 pages 5 figure

Ab-initio density-functional lattice-dynamics studies of ice

We present the results of first-principles computational studies of the dynamical properties of hexagonal ice using both the ab-initio pseudopotential method and the full-potential augmented plane-wave method. Properties obtained using both the generalized gradient approximation (GGA) and the meta-GGA in density-functional theory are compared. The lattice-dynamical properties of the structures are obtained using a finite-difference evaluation of the dynamical matrix and force-constant matrix from atomic forces. Phonon dispersion is evaluated by the direct determination of the force-constant matrix in supercells derived from the primitive molecule unit cells with the assumption that force constants are zero beyond the second molecular nearest neighbors. The k-dependent phonon frequencies are then obtained from the force-constant matrix and dispersion relations, and the Brillouin-zone integrated density of states is evaluated. The importance of phonon dispersion in the various regions of the phonon spectra is then assessed and compared to existing neutron-scattering data. Frozen-phonon calculations are used to compare phonon frequencies evaluated in both the GGA and meta-GGA

Ab initio and finite-temperature molecular dynamics studies of lattice resistance in tantalum

This manuscript explores the apparent discrepancy between experimental data and theoretical calculations of the lattice resistance of bcc tantalum. We present the first results for the temperature dependence of the Peierls stress in this system and the first ab initio calculation of the zero-temperature Peierls stress to employ periodic boundary conditions, which are those best suited to the study of metallic systems at the electron-structure level. Our ab initio value for the Peierls stress is over five times larger than current extrapolations of experimental lattice resistance to zero-temperature. Although we do find that the common techniques for such extrapolation indeed tend to underestimate the zero-temperature limit, the amount of the underestimation which we observe is only 10-20%, leaving open the possibility that mechanisms other than the simple Peierls stress are important in controlling the process of low temperature slip.Comment: 12 pages and 9 figure

Simulating radiation damage in a bcc Fe system with embedded yttria nanoparticles

This paper was accepted for publication in the journal Journal of Nuclear Materials and the definitive published version is available at http://dx.doi.org/10.1016/j.jnucmat.2013.02.016We present a molecular dynamics study of radiation damage arising from nuclear collisions close to embedded yttria nanoparticles in a bcc Fe matrix. The model assumes a perfect body-centred cubic (bcc) iron matrix in which yttria nano-particles are embedded as a simplified model of an Oxide Dispersion Strengthened steel. It is shown how the nano-particles interact with nearby initiated collision cascades, through cascade blocking and absorbing energy. Fe defects accumulate at the interface both directly from the ballistic collisions and also by attraction of defects generated close by. The nano-particles generally remain intact during a radiation event and release absorbed energy over times longer than the ballistic phase of the collision cascade

Theoretical study of high-density phases of covalent semiconductors. I. Ab initio treatment.

We present detailed calculations using the total-energy pseudopotential method in the local-density approximation of the relative stability and pressure-induced behavior of complex tetrahedrally bonded structures formed metastably in silicon and germanium by depressurization from their metallic phases. The corresponding structures in carbon are also investigated. These calculations present the first direct atomistic relaxation of BC8 under the influence of Hellmann-Feynman forces, and the first calculations on the ST12 structure using any form of relaxation. We also present evidence to show that in both Si and Ge the BC8 and ST12 structures are covalently bonded, while the equivalent structures in carbon cannot support such covalent bonding

Tetrahedral structures and phase transitions in III-V semiconductors.

The BC8 structure (body-centered cubic with eight atoms per cell) is a known pressure-induced modification of both silicon and germanium. However, its diatomic analogue [the SC16 structure (a simple cubic lattice with a basis of 16 atoms)] has never been found in compound semiconductors. We find from total-energy pseudopotential calculations that the SC16 structure is a stable high-pressure polymorph of the III-V semiconductors GaAs, InAs, and AlSb. We report ab initio calculations of the structural, electronic, and vibrational properties of SC16-GaAs. The wurtzite structure is found to be unstable at all pressures for each compound considered. We consider possible transition routes consistent with our high-pressure x-ray diffraction results and propose that the formation of the SC16 structure in compounds is kinetically inhibited by the formation of wrong bonds at the structural transition

Ab initio calculations and interatomic potentials for iron and iron alloys: Achievements within the Perfect Project

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