988 research outputs found
Ab initio studies of structural instabilities in magnesium silicate perovskite
Density-functional simulations are used to calculate structural properties
and high-symmetry phonons of the hypothetical cubic phase, the stable
orthorhombic phase and an intermediate tetragonal phase of magnesium silicate
perovskite. We show that the structure of the stable phase is well described by
freezing in a small number of unstable phonons into the cubic phase. We use the
frequencies of these unstable modes to estimate transition temperatures for
cubic--tetragonal and tetragonal--orthorhombic phase transitions. These are
investigated further to find that the coupling with the strain suggests that
phonons give a better representation than rigid unit modes. The phonons of an
intermediate tetragonal phase were found to be stable except for two rotational
modes. The eigenvectors of the most unstable mode of each of the cubic and
tetragonal phases account for all the positional parameters of the orthorhombic
phase. The phase boundary for the orthorhombic--tetragonal transition
intersects possible mantle geotherms, suggesting that the tetragonal phase may
be present in the lower mantle.Comment: 16 pages, REVTEX, 7 postscript figures (Fig 1 very large, contact
Authors if required); submitted to Physics and Chemistry of Mineral
The MOLDY short-range molecular dynamics package
We describe a parallelised version of the MOLDY molecular dynamics program.
This Fortran code is aimed at systems which may be described by short-range
potentials and specifically those which may be addressed with the embedded atom
method. This includes a wide range of transition metals and alloys. MOLDY
provides a range of options in terms of the molecular dynamics ensemble used
and the boundary conditions which may be applied. A number of standard
potentials are provided, and the modular structure of the code allows new
potentials to be added easily. The code is parallelised using OpenMP and can
therefore be run on shared memory systems, including modern multicore
processors. Particular attention is paid to the updates required in the main
force loop, where synchronisation is often required in OpenMP implementations
of molecular dynamics. We examine the performance of the parallel code in
detail and give some examples of applications to realistic problems, including
the dynamic compression of copper and carbon migration in an iron-carbon alloy
Local size segregation in polydisperse hard sphere fluids
The structure of polydisperse hard sphere fluids, in the presence of a wall,
is studied by the Rosenfeld density functional theory. Within this approach,
the local excess free energy depends on only four combinations of the full set
of density fields. The case of continuous polydispersity thereby becomes
tractable. We predict, generically, an oscillatory size segregation close to
the wall, and connect this, by a perturbation theory for narrow distributions,
with the reversible work for changing the size of one particle in a
monodisperse reference fluid.Comment: RevTeX, 4 pages, 3 figures, submitted to Phys. Rev. Let
Temperature dependence in interatomic potentials and an improved potential for Ti
The process of deriving an interatomic potentials represents an attempt to
integrate out the electronic degrees of freedom from the full quantum
description of a condensed matter system. In practice it is the derivatives of
the interatomic potentials which are used in molecular dynamics, as a model for
the forces on a system. These forces should be the derivative of the free
energy of the electronic system, which includes contributions from the entropy
of the electronic states. This free energy is weakly temperature dependent, and
although this can be safely neglected in many cases there are some systems
where the electronic entropy plays a significant role. Here a method is
proposed to incorporate electronic entropy in the Sommerfeld approximation into
empirical potentials. The method is applied as a correction to an existing
potential for titanium. Thermal properties of the new model are calculated, and
a simple method for fixing the melting point and solid-solid phase transition
temperature for existing models fitted to zero temperature data is presented.Comment: CCP 201
Total energy calculation of high pressure selenium: The origin of incommensurate modulations in Se-IV and the instability of proposed Se-II
We present calculation of the high pressure crystal structures in selenium,
including rational approximants to the recently reported incommensurate phases.
We show how the incommensurate phases can be intuitively explained in terms of
imaginary phonon frequencies arising from Kohn anomalies in the putative
undistorted phase. We also find inconsistencies between the calculated and
experimental Se-II phase - the calculations show it to be a metastable metal
while the experiment finds a stable semiconductor. We propose that the
experimentally reported structure is probably in error.Comment: 4 pages 4 figure
Origin of complex crystal structures of elements at pressure
We present a unifying theory for the observed complex structures of the
sp-bonded elements under pressure based on nearly free electron picture (NFE).
In the intermediate pressure regime the dominant contribution to crystal
structure arises from Fermi-surface Brillouin zone (FSBZ) interactions -
structures which allow this are favoured. This simple theory explains the
observed crystal structures, transport properties, the evolution of internal
and unit cell parameters with pressure. We illustrate it with experimental data
for these elements and ab initio calculation for Li.Comment: 4 pages 5 figure
Microscopic model of diffusion limited aggregation and electrodeposition in the presence of levelling molecules
A microscopic model of the effect of unbinding in diffusion limited
aggregation based on a cellular automata approach is presented. The geometry
resembles electrochemical deposition - ``ions'' diffuse at random from the top
of a container until encountering a cluster in contact with the bottom, to
which they stick. The model exhibits dendritic (fractal) growth in the
diffusion limited case. The addition of a field eliminates the fractal nature
but the density remains low. The addition of molecules which unbind atoms from
the aggregate transforms the deposit to a 100% dense one (in 3D). The molecules
are remarkably adept at avoiding being trapped. This mimics the effect of
so-called ``leveller'' molecules which are used in electrochemical deposition
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