2,237 research outputs found
Configurational order-disorder induced metal-nonmetal transition in BC studied with first-principles superatom-special quasirandom structure method
Due to a large discrepancy between theory and experiment, the electronic
character of crystalline boron carbide BC has been a controversial
topic in the field of icosahedral boron-rich solids. We demonstrate that this
discrepancy is removed when configurational disorder is accurately considered
in the theoretical calculations. We find that while ordered ground state
BC is metallic, configurationally disordered BC,
modeled with a superatom-special quasirandom structure method, goes through a
metal to non-metal transition as the degree of disorder is increased with
increasing temperature. Specifically, one of the chain-end carbon atoms in the
CBC chains substitutes a neighboring equatorial boron atom in a B
icosahedron bonded to it, giving rise to a BC(BBC) unit. The
atomic configuration of the substitutionally disordered BC thus
tends to be dominated by a mixture between B(CBC) and
BC(BBC). Due to splitting of valence states in
BC(BBC), the electron deficiency in B(CBC) is gradually
compensated
Lattice dynamics of anharmonic solids from first principles
An accurate and easily extendable method to deal with lattice dynamics of
solids is offered. It is based on first-principles molecular dynamics
simulations and provides a consistent way to extract the best possible harmonic
- or higher order - potential energy surface at finite temperatures. It is
designed to work even for strongly anharmonic systems where the traditional
quasiharmonic approximation fails. The accuracy and convergence of the method
are controlled in a straightforward way. Excellent agreement of the calculated
phonon dispersion relations at finite temperature with experimental results for
bcc Li and bcc Zr is demonstrated
Electronic properties and magnetism of iron at the Earth's inner core conditions
We employ state-of-the-art ab initio simulations within the dynamical
mean-field theory to study three likely phases of iron (hexogonal close-packed,
hcp, face centered cubic, fcc, and body centered cubic, bcc) at the Earth's
core conditions. We demonstrate that the correction to the electronic free
energy due to correlations can be significant for the relative stability of the
phases. The strongest effect is observed in bcc Fe, which shows a non-Fermi
liquid behaviour, and where a Curie-Weiss behaviour of the uniform
susceptbility hints at a local magnetic moment still existing at 5800 K and 300
GPa. We predict that all three structures have sufficiently high magnetic
susceptibility to stabilize the geodynamo.Comment: 7 pages, 6 figures. Changes in version 2: the magnetic susceptibility
has been recalculated for a larger temperature range and with a higher
accuracy (Fig. 4), comparison with the enhanced Stoner model added, some
other minor correction to the tex
- …