Molecular dynamics simulation of low-energy recoil events in titanate pyrochlores

Abstract

Molecular dynamics simulations of low-energy displacement events in titanate pyrochlores A₂Ti₂O₇ (A = Lu, Er, Gd, Eu, Ce, La) have been carried out along three main crystallographic directions, [100], [110] and [111], to determine threshold displacement energies (E_d) for A, Ti and O, corresponding defect configurations, and defect formation dynamics. The A-site cation size was found to have an important effect on the value of E_d for each type of ion in titanate pyrochlores. E_d depended on the knock-on direction, ion type, and Wyckoff position (48f vs. 8b site for O). The influence of A-site cation size on E_d for Ti and O_48f was significantly larger than on E_d for A and O_8b in titanate pyrochlores. Besides, the value of E_d for each type of atom in titanate pyrochlores was highly anisotropic. The largest change of E_d was for Ti, while the smallest change of E_d was for O_8b. The energy required to displace the A cation was less than that to displace Ti. The lowest displacement energy was for the anion in titanate pyrochlores. The easiest displacement direction for Ti⁴⁺ and O_48f was along [100], while the easiest directions for A³⁺ and O_8b in titanate pyrochlores varied with the choice of A-site cation. Among the surviving defects, Frenkel pairs were dominant at the end of low-energy recoils. Cation displacements produced both cation and anion defects, while anion displacements produced only anion defects. Thus, cation displacement, which occurs at higher recoil energies, holds the key to amorphization. With increasing A-site cation radius in titanate pyrochlores A₂Ti₂O₇ (A = Lu, Er, Gd, Eu), the variation of the E_d of Ti along [110] was similar to the variation of the experimental critical amorphization temperature (T_c). The displacement of Ti along [110] may play an important role in the amorphization of titanate pyrochlores.