70 research outputs found
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
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
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
Structural Disorder in Doped Zirconias, Part II: Vacancy Ordering Effects and the Conductivity Maximum.
Polarizable interaction potentials, parametrized using ab initio electronic structure calculations, have been used in molecular dynamics simulations to study the conduction mechanism in doped zirconias. The influence of vacancy-vacancy and vacancy-cation interactions on the conductivity of these materials has been characterized. Although the latter can be minimized by using dopant cations with radii which match those of Zr4+ (as in the case of Sc3+), the former appears as an intrinsic characteristic of the fluorite lattice that cannot be avoided and that 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 Sc2O 3-doped zirconia confirms that this material is likely to present the highest conductivity achievable in zirconias. © 2011 American Chemical Society
Thermal conductivity of ionic systems from equilibrium molecular dynamics.
Thermal conductivities of ionic compounds (NaCl, MgO, Mg(2)SiO(4)) are calculated from equilibrium molecular dynamics simulations using the Green-Kubo method. Transferable interaction potentials including many-body polarization effects are employed. Various physical conditions (solid and liquid states, high temperatures, high pressures) relevant to the study of the heat transport in the Earth's mantle are investigated, for which experimental measures are very challenging. By introducing a frequency-dependent thermal conductivity, we show that important coupled thermoelectric effects occur in the energy conduction mechanism in the case of liquid systems
A dipole polarizable potential for reduced and doped CeO <sub>2</sub> obtained from first principles
In this paper we present the parameterization of a new interionic potential
for stoichiometric, reduced and doped CeO. We use a dipole-polarizable
potential (DIPPIM) and optimize its parameters by fitting them to a series of
DFT calculations. The resulting potential was tested by calculating a series of
fundamental properties for CeO and by comparing them to experimental
values. The agreement for all the calculated properties (thermal and chemical
expansion coefficients, lattice parameters, oxygen migration energies, local
crystalline structure and elastic constants) is within 10-15% of the
experimental one, an accuracy comparable to that of ab initio calculations.
This result suggests the use of this new potential for reliably predicting
atomic-scale properties of CeO in problems where ab initio calculations are
not feasible due to their size-limitations.Comment: 12 pages, 6 figures and 5 tables. Accepted on Journal of Physics:
Condensed Matte
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