191 research outputs found
Effect of dispersion interactions on the properties of LiF in condensed phases
Classical molecular dynamics simulations are performed on LiF in the
framework of the polarizable ion model. The overlap-repulsion and polarization
terms of the interaction potential are derived on a purely non empirical,
first-principles basis. For the dispersion, three cases are considered: a first
one in which the dispersion parameters are set to zero and two others in which
they are included, with different parameterizations. Various thermodynamic,
structural and dynamic properties are calculated for the solid and liquid
phases. The melting temperature is also obtained by direct coexistence
simulations of the liquid and solid phases. Dispersion interactions appear to
have an important effect on the density of both phases and on the melting
point, although the liquid properties are not affected when simulations are
performed in the NVT ensemble at the experimental density.Comment: 8 pages, 5 figure
Vacancy ordering effects on the conductivity of yttria- and scandia-doped zirconia
Polarizable interaction potentials, parametrized using ab initio electronic
structure calculations, have been used in molecular dynamics simulations to
study the conduction mechanism in Y2 O3 - and Sc2 O3 -doped zirconias. The
influence of vacancy-vacancy and vacancy-cation interactions on the
conductivity of these materials has been characterised. While the latter can be
avoided by using dopant cations with radii which match those of Zr4+ (as is the
case of Sc3+), the former is an intrinsic characteristic of the fluorite
lattice which cannot be avoided and which is shown to be responsible for the
occurrence of a maximum in the conductivity at dopant concentrations between 8
and 13 %. The weakness of the Sc-vacancy interactions in Sc2 O3 -doped zirconia
suggests that this material is likely to present the highest conductivity
achievable in zirconias.Comment: 17 pages, 6 figur
The construction of a reliable potential for GeO2 from first-principles
The construction of a reliable potential for GeO2, from first-principles, is
described. The obtained potential, which includes dipole polarization effects,
is able to reproduce all the studied properties (structural, dynamical and
vibrational) to a high degree of precision with a single set of parameters. In
particular, the infrared spectrum was obtained with the expression proposed for
the dielectric function of polarizable ionic solutions by Weis et al. [J.M.
Caillol, D. Levesque and J.J. Weis, J. Chem. Phys. 91, 5544 (1989)]. The
agreement with the experimental spectrum is very good, with three main bands
that are associated to tetrahedral modes of the GeO2 network. Finally, we give
a comparison with a simpler pair-additive potential.Comment: 9 pages, 8 figure
Edge dislocation slows down oxide ion diffusion in doped CeO2 by segregation of charged defects
Strained oxide thin films are of interest for accelerating oxide ion conduction in electrochemical devices. Although the effect of elastic strain has been uncovered theoretically, the effect of dislocations on the diffusion kinetics in such strained oxides is yet unclear. Here we investigate a 1/2{100} edge dislocation by performing atomistic simulations in 4–12% doped CeO₂ as a model fast ion conductor. At equilibrium, depending on the size of the dopant, trivalent cations and oxygen vacancies are found to simultaneously enrich or deplete either in the compressive or in the tensile strain fields around the dislocation. The associative interactions among the point defects in the enrichment zone and the lack of oxygen vacancies in the depletion zone slow down oxide ion transport. This finding is contrary to the fast diffusion of atoms along the dislocations in metals and should be considered when assessing the effects of strain on oxide ion conductivity.United States. Department of Energy (DE-SC0002633)National Science Foundation (U.S.) (TG-DMR110004)National Science Foundation (U.S.) (TG-DMR120025
Cation composition effects on oxide conductivity in the Zr_2Y_2O_7-Y_3NbO_7 system
Realistic, first-principles-based interatomic potentials have been used in
molecular dynamics simulations to study the effect of cation composition on the
ionic conductivity in the Zr2Y2O7-Y3NbO7 system and to link the dynamical
properties to the degree of lattice disorder. Across the composition range,
this system retains a disordered fluorite crystal structure and the vacancy
concentration is constant. The observed trends of decreasing conductivity and
increasing disorder with increasing Nb5+ content were reproduced in simulations
with the cations randomly assigned to positions on the cation sublattice. The
trends were traced to the influences of the cation charges and relative sizes
and their effect on vacancy ordering by carrying out additional calculations in
which, for example, the charges of the cations were equalised. The simulations
did not, however, reproduce all the observed properties, particularly for
Y3NbO7. Its conductivity was significantly overestimated and prominent diffuse
scattering features observed in small area electron diffraction studies were
not always reproduced. Consideration of these deficiencies led to a preliminary
attempt to characterise the consequence of partially ordering the cations on
their lattice, which significantly affects the propensity for vacancy ordering.
The extent and consequences of cation ordering seem to be much less pronounced
on the Zr2Y2O7 side of the composition range.Comment: 22 pages, 8 figures, submitted to Journal of Physics: Condensed
Matte
Studying the conduction mechanism of stabilised zirconias by means of molecular dynamics simulations
Stabilised zirconias have a remarkable variety of technological and commercial applications,
e.g., thermal barrier coatings, gas sensors, solid oxide fuel cells, ceramic knives and even fashion
jewelry. This amazing versatility seems to originate from the creation of atomic defects
(oxide ion vacancies) in the zirconia crystal. Indeed, these vacancies, and their interactions
with other vacancies or cations, dramatically affect the structural, thermal, mechanical and
electrical properties of zirconia. This thesis is concerned with the study of the role of the vacancy
interactions on the conducting properties of these materials. This study was performed
by using realistic, first-principles based molecular dynamics simulations.
The first system studied in this thesis is Zr0:50:5xY0:5+0:25xNb0:25xO7. This has a fixed
number of vacancies across the series but its conductivity changes by almost two orders of magnitude
as a function of x. For this reason, Zr0:50:5xY0:5+0:25xNb0:25xO7 represents an ideal
test-bed for the role of the cation species on the defect interactions and therefore on the ionic
conductivity of these materials. Realistic inter-atomic potentials for Zr0:50:5xY0:5+0:25xNb0:25xO7
were developed on a purely first-principles basis. The observed trends of decreasing conductivity
and increasing disorder with increasing Nb5+ content were successfully reproduced. These
trends were traced to the influences of the cation charges and relative sizes and their effect on
vacancy ordering by carrying out additional calculations in which, for instance, the charges of
the cations were equalised. The effects of cation ordering were considered as well and their
influence on the conductivity understood.
The second part of this thesis deals with Sc2O3–doped (ScSZ) and Y2O3–doped (YSZ)
zirconias. These systems are of great academic and technological interest as they find use in
solid oxide fuel cells. Inter-atomic potentials were parametrised and used to predict the structural
and conducting properties of these materials, which were found to agree very well with
the experimental evidence. The simulations were then used to study the role of the vacancy interactions
on the conducting properties of these materials. Two factors were found to influence
the ionic conductivity in these materials: cation-vacancy and vacancy-vacancy interactions.
The former is responsible for the difference in conductivity observed between YSZ and ScSZ.
Vacancies, in fact, prefer to bind to the smaller Zr4+ ions in YSZ whereas there is not a strong
preference in the case of ScSZ, since the cations have similar sizes in this case. This effect is
observed at temperatures as high as T = 1500 K. Finally, it was found that vacancies tend to
order so that they can minimise their mutual interaction and that this ordering tendency is what
ultimately is responsible for the observed anomalous decrease of the ionic conductivity with
increasing dopant concentration. The consequences of such a behaviour are discussed
High-pressure behaviour of GeO2: a simulation study
In this work we study the high pressure behaviour of liquid and glassy GeO2
by means of molecular dynamics simulations. The interaction potential, which
includes dipole polarization effects, was parameterized from first-principles
calculations. Our simulations reproduce the most recent experimental data to a
high degree of precision. The proportion of the various GeOn polyhedra is
determined as a function of the pressure: a smooth transition from tetrahedral
to octahedral network is observed. Finally, the study of high-pressure, liquid
germania confirms that this material presents an anomalous behaviour of the
diffusivity as observed in analog systems such as silica and water. The
importance of penta-coordinated germanium ions for such behaviour is stressed.Comment: 16 pages, 4 figures, accepted as a Fast Track Communication on
Journal of Physics: Condensed Matte
Impact of uniaxial strain and doping on oxygen diffusion in CeO2
Doped ceria is an important electrolyte for solid oxide fuel cell applications. Molecular dynamics simulations have been used to investigate the impact of uniaxial strain along the directions and rare-earth doping (Yb, Er, Ho, Dy, Gd, Sm, Nd, and La) on oxygen diffusion. We introduce a new potential model that is able to describe the thermal expansion and elastic properties of ceria to give excellent agreement with experimental data. We calculate the activation energy of oxygen migration in the temperature range 900-1900K for both unstrained and rare-earth doped ceria systems under tensile strain. Uniaxial strain has a considerable effect in lowering the activation energies of oxygen migration. A more pronounced increase in oxygen diffusivities is predicted at the lower end of the temperature range for all the dopants considered
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