Molecular Dynamics Study of a Nuclear Waste Glass Matrix With Plutonium
- PDF / 465,179 Bytes
- 7 Pages / 393.66 x 624.6 pts Page_size
- 113 Downloads / 185 Views
C o---o
+[ _
_2
where r is the distance in A between the two oxygen ions, and Co-o = 27.8 eV.A 6. A very high Co-0 value is often necessary to model certain oxides; fourth-order Buckingham potentials are used in such cases with polynomial expressions between the Bom-Mayer and r6 terms. The very high ionic polarizability of ionic solids is taken into account by the Dick-Overhauser shell model f6] in which each ion comprises a core (corresponding to the nucleus masked by the inner electrons) and a shell with a
345 Mat. Res. Soc. Symp. Proc. Vol. 556
©®1999Materials
Research Society
charge corresponding approximately to the ion valence. The interaction energy between the core and shell of the same ion, separated by a distance R, is expressed by a harmonic oscillator Ecoresheu - ½KR2 , where K is a constant. Short-range interactions between adjacent shells are expressed by Buckingham potentials, while the Ewald method is used for the purely Coulombian long-range interactions. By maintaining the oxide symmetry group and by considering the oxygen interaction potential in Eqn (1), a satisfactory model for various oxides can be obtained using short-range oxygen-cation pair interactions, completed when necessary by three-body interactions [7,8]. Cation-oxygen potentials of the Buckingham type are generally fitted in a pure oxide phase where all the local atomic environments are identical and charge transfers between cations and anions are very close from one site to another. Simulating complex glasses using the shell model is a more complicated situation, however, as the charge transfers, depending on the nature and the position of the neighbouring ions, can vary to a considerable extent, depending on the degree of complexity of the simulated composition. A highly inhomogeneous polarisability could result following the various local environments. Currently, there is no algorithm capable of taking into account such electronic charge
and polarizability variations.
Furthermore, our objective is to simulate displacement cascades in glass models, hence, a shell model is no longer satisfactory, not only for its inability to charge adjustment but also for not preserving the coreshell rigidity at high nuclei recoil momenta. Complex oxide glasses are therefore usually modeled by a set of simple Boom-Mayer-Huggins potentials [9,101 developed upon formal ion charges and completed, if necessary, by three-body terms to more accurately represent the covalent nature of certain bonds [11,121. Local atom configurations are determined mainly by first-neighbor cation-oxygen interactions, with the other potentials used for adjustment purposes. The 0-0 potential does not have the same central role as for fitting potentials in pure oxides, hence the difficulty in transferring potentials from a pure oxide to a vitreous oxide mixture. The short-range interaction energy between two oxygen ions with formal charges, generally used for the glasses, is expressed as follows[9,101:
Eg (2).(V) r) 0- = 352.6Sexp((-0.350)
(r in
A)
(2)
At range
Data Loading...
