Comment on 'large swelling and percolation in irradiated zircon'

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 overlap of two collision cascades that predict a densified boundary of polymerized silica and the scattering of the second cascade away from the densified boundary (Trachenko et al 2003 J. Phys.: Condens. Matter 15 L1). These MD simulations are based on an atomic interaction potential for zircon (Trachenko et al 2001 J. Phys.: Condens. Matter 13 1947), which, according to our analysis, only reproduces some of the crystallographic properties at equilibrium and does not adequately describe the atomic scattering physics for zircon, and on simulation methodologies for energetic events that are ill defined. In fact, the interatomic potential model used by Trachenko et al yields a significantly more rigid structure, with very high Frenkel defect formation energies and extremely low entropy and specific heat capacity. Consequently, the reported results of the cascade simulations, which are events far from equilibrium, may be artifacts of both the potential model and simulation methodologies employed. Thus, the structural changes predicted by the simulations must be viewed cautiously, as these simulation results cannot be taken as confirmation of a new scattering physics process that is the basis for the proposed swelling model. In this comment, the deficiencies in the atomic interaction potential and methodologies employed by these authors are critically reviewed, and the validity of the cascade overlap simulations and proposed physics is discussed.