Displacement threshold and Frenkel pair formation energy in ionic systems

Displacement threshold energies (E_d) and Frenkel pair formation energies (E_Fp) are investigated in detail by molecular dynamics computer simulation for three different ionic systems with the same crystal structure, MgO, SrO and NaCl in order to see if there is a functional relationship between them. It is found that there are wide variations in the values of E_d depending on the direction in which energy is imparted to a static atom in the lattice. Large values of E_d are found along the major crystallographic directions and lower values elsewhere. Typically these thresholds are between 5 and 9 times bigger than the Frenkel pair formation energies E_Fp with no observable dependence on mass or ion charge. The differences in the interaction potentials also means that for any given direction, there is only limited correlation between values of E_d in the different systems studied and no quantifiable relationship with E_Fp.