340 research outputs found
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
Phase mapping of aging process in InN nanostructures: oxygen incorporation and the role of the zincblende phase
Uncapped InN nanostructures undergo a deleterious natural aging process at
ambient conditions by oxygen incorporation. The phases involved in this process
and their localization is mapped by Transmission Electron Microscopy (TEM)
related techniques. The parent wurtzite InN (InN-w) phase disappears from the
surface and gradually forms a highly textured cubic layer that completely wraps
up a InN-w nucleus which still remains from original single-crystalline quantum
dots. The good reticular relationships between the different crystals generate
low misfit strains and explain the apparent easiness for phase transformations
at room temperature and pressure conditions, but also disable the classical
methods to identify phases and grains from TEM images. The application of the
geometrical phase algorithm in order to form numerical moire mappings, and RGB
multilayered image reconstructions allows to discern among the different phases
and grains formed inside these nanostructures. Samples aged for shorter times
reveal the presence of metastable InN:O zincblende (zb) volumes, which acts as
the intermediate phase between the initial InN-w and the most stable cubic
In2O3 end phase. These cubic phases are highly twinned with a proportion of
50:50 between both orientations. We suggest that the existence of the
intermediate InN:O-zb phase should be seriously considered to understand the
reason of the widely scattered reported fundamental properties of thought to be
InN-w, as its bandgap or superconductivity.Comment: 18 pages 7 figure
Free energies of crystalline solids: a lattice-switch Monte Carlo method
We present a method for the direct evaluation of the difference between the
free energies of two crystalline structures, of different symmetry. The method
rests on a Monte Carlo procedure which allows one to sample along a path,
through atomic-displacement-space, leading from one structure to the other by
way of an intervening transformation that switches one set of lattice vectors
for another. The configurations of both structures can thus be sampled within a
single Monte Carlo process, and the difference between their free energies
evaluated directly from the ratio of the measured probabilities of each. The
method is used to determine the difference between the free energies of the fcc
and hcp crystalline phases of a system of hard spheres.Comment: 5 pages Revtex, 3 figure
Theoretical and computational study of high pressure structures in barium
Recent high pressure work has suggested that elemental barium forms a high
pressure self-hosting structure (Ba IV) involving two `types' of barium atom.
Uniquely among reported elemental structures it cannot be described by a single
crystalline lattice, instead involving two interpenetrating incommensurate
lattices. In this letter we report pseudopotential calculations demonstrating
the stability and the potentially disordered nature of the `guest' structure.
Using band structures and nearly-free electron theory we relate the appearance
of Ba IV to an instability in the close-packed structure, demonstrate that it
has a zero energy vibrational mode, and speculate about the structure's
stability in other divalent elements.Comment: 4 pages and 5 figures. To appear in PR
Dynamical properties of Au from tight-binding molecular-dynamics simulations
We studied the dynamical properties of Au using our previously developed
tight-binding method. Phonon-dispersion and density-of-states curves at T=0 K
were determined by computing the dynamical-matrix using a supercell approach.
In addition, we performed molecular-dynamics simulations at various
temperatures to obtain the temperature dependence of the lattice constant and
of the atomic mean-square-displacement, as well as the phonon density-of-states
and phonon-dispersion curves at finite temperature. We further tested the
transferability of the model to different atomic environments by simulating
liquid gold. Whenever possible we compared these results to experimental
values.Comment: 7 pages, 9 encapsulated Postscript figures, submitted to Physical
Review
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
The Effect of Lattice Vibrations on Substitutional Alloy Thermodynamics
A longstanding limitation of first-principles calculations of substitutional
alloy phase diagrams is the difficulty to account for lattice vibrations. A
survey of the theoretical and experimental literature seeking to quantify the
impact of lattice vibrations on phase stability indicates that this effect can
be substantial. Typical vibrational entropy differences between phases are of
the order of 0.1 to 0.2 k_B/atom, which is comparable to the typical values of
configurational entropy differences in binary alloys (at most 0.693 k_B/atom).
This paper describes the basic formalism underlying ab initio phase diagram
calculations, along with the generalization required to account for lattice
vibrations. We overview the various techniques allowing the theoretical
calculation and the experimental determination of phonon dispersion curves and
related thermodynamic quantities, such as vibrational entropy or free energy. A
clear picture of the origin of vibrational entropy differences between phases
in an alloy system is presented that goes beyond the traditional bond counting
and volume change arguments. Vibrational entropy change can be attributed to
the changes in chemical bond stiffness associated with the changes in bond
length that take place during a phase transformation. This so-called ``bond
stiffness vs. bond length'' interpretation both summarizes the key phenomenon
driving vibrational entropy changes and provides a practical tool to model
them.Comment: Submitted to Reviews of Modern Physics 44 pages, 6 figure
Defect and solute properties in dilute Fe-Cr-Ni austenitic alloys from first principles
We present results of an extensive set of first-principles density functional
theory calculations of point defect formation, binding and clustering energies
in austenitic Fe with dilute concentrations of Cr and Ni solutes.Comment: 24 pages, 10 figures, published in Phys. Rev.
Chronotropic incompetence and myocardial injury after non-cardiac surgery: planned secondary analysis of a prospective observational international cohort study
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