14 research outputs found
Thermal effects on lattice strain in hcp Fe under pressure
We compute the c/a lattice strain versus temperature for nonmagnetic hcp iron
at high pressures using both first-principles linear response quasiharmonic
calculations based on the full potential linear-muffin-tin-orbital (LMTO)
method and the particle-in-cell (PIC) model for the vibrational partition
function using a tight-binding total-energy method. The tight-binding model
shows excellent agreement with the all-electron LMTO method. When hcp structure
is stable, the calculated geometric mean frequency and Helmholtz free energy of
hcp Fe from PIC and linear response lattice dynamics agree very well, as does
the axial ratio as a function of temperature and pressure. On-site
anharmonicity proves to be small up to the melting temperature, and PIC gives a
good estimate of its sign and magnitude. At low pressures, hcp Fe becomes
dynamically unstable at large c/a ratios, and the PIC model might fail where
the structure approaches lattice instability. The PIC approximation describes
well the vibrational behavior away from the instability, and thus is a
reasonable approach to compute high temperature properties of materials. Our
results show significant differences from earlier PIC studies, which gave much
larger axial ratio increases with increasing temperature, or reported large
differences between PIC and lattice dynamics results.Comment: 9 figure
First-principles thermoelasticity of bcc iron under pressure
We investigate the elastic and isotropic aggregate properties of
ferromagnetic bcc iron as a function of temperature and pressure by computing
the Helmholtz free energies for the volume-conserving strained structures using
the first-principles linear response linear-muffin-tin-orbital method and the
generalized-gradient approximation. We include the electronic excitation
contributions to the free energy from the band structures, and phonon
contributions from quasi-harmonic lattice dynamics. We make detailed
comparisons between our calculated elastic moduli and their temperature and
pressure dependences with available experimental and theoretical data.Comment: 5 figures, 2 table
Elastic isotropy of hcp-Fe under Earth core conditions
Our first-principles calculations show that both the compressional and shear
waves of hcp-Fe become elastically isotropic under the high temperatures of
Earth inner core conditions, with the variation in sound velocities along
different angles from the c axis within 1%. We computed the thermoelasticity at
high pressures and temperatures from quasiharmonic linear response
linear-muffin-tin-orbital calculations in the generalized-gradient
approximation. The calculated anisotropic shape and magnitude in hcp-Fe at
ambient temperature agree well with previous first-principles predictions, and
the anisotropic effects show strong temperature dependences. This implies that
other mechanisms, rather than the preferential alignment of the hcp-Fe crystal
along the Earth rotation axis, account for the seismic P-wave travel time
anomalies. Either the inner core is not hcp iron, and/or the seismologically
observed anisotropy is caused by inhomogeneity, i.e. multiple phases.Comment: 16 pages, 3 figure
First-principles thermal equation of state and thermoelasticity of hcp Fe at high pressures
We investigate the equation of state and elastic properties of hcp iron at
high pressures and high temperatures using first principles linear response
linear-muffin-tin-orbital method in the generalized-gradient approximation. We
calculate the Helmholtz free energy as a function of volume, temperature, and
volume-conserving strains, including the electronic excitation contributions
from band structures and lattice vibrational contributions from quasi-harmonic
lattice dynamics. We perform detailed investigations on the behavior of elastic
moduli and equation of state properties as functions of temperature and
pressure, including the pressure-volume equation of state, bulk modulus, the
thermal expansion coefficient, the Gruneisen ratio, and the shock Hugoniot.
Detailed comparison has been made with available experimental measurements and
theoretical predictions.Comment: 33 pages, 12 figure
Atomic structural and electronic bandstructure calculations for borophene
Density of states (DOS) and electronic bandstructure diagrams with ε ( k ) versus k are found for particular allotropes of borophene with much improved accuracy by ab initio quantum calculations using hybrid functionals of several types. The particular types of hybrid functionals are delineated in detail. Varying levels of k-point discretization are utilized to evaluate accuracy. Structural relaxation has been carefully applied prior to electronic bandstructure simulations. Results indicate whether or not one has regions in k-space which display Dirac type non-gapped behavior or parabolic gapped behavior. This work is required in order to determine what types of electronic uses 2D single atomic layer borophene is appropriate for in modern nanoscopic devices