Comment on ‘Large swelling and percolation in irradiated zircon’

Abstract

A recent model for the large radiation-induced swelling exhibited by irradiated zircon(ZrSiO4) is partially based on results of molecular dynamics (MD) simulations of the partial overlapof two collision cascades that predict a densified boundary of polymerized silica and thescattering of the second cascade away from the densified boundary (Trachenko et al 2003J. Phys.: Condens. Matter 15 L1). These MD simulations are based on an atomic interactionpotential for zircon (Trachenko et al 2001 J. Phys.: Condens. Matter 13 1947), which,according to our analysis, only reproduces some of the crystallographic properties atequilibrium and does not adequately describe the atomic scattering physics for zircon, andon simulation methodologies for energetic events that are ill defined. In fact, theinteratomic potential model used by Trachenko et al yields a significantly more rigidstructure, with very high Frenkel defect formation energies and extremely low entropy andspecific heat capacity. Consequently, the reported results of the cascade simulations, whichare events far from equilibrium, may be artifacts of both the potential model andsimulation methodologies employed. Thus, the structural changes predicted by thesimulations must be viewed cautiously, as these simulation results cannot betaken as confirmation of a new scattering physics process that is the basis forthe proposed swelling model. In this comment, the deficiencies in the atomicinteraction potential and methodologies employed by these authors are criticallyreviewed, and the validity of the cascade overlap simulations and proposed physics isdiscussed.