Abstract
The response of titanate pyrochlores (A2Ti2O7, A = Y, Gd and Sm) to electronic excitation is investigated utilizing an ab initio molecular dynamics method. All the titanate pyrochlores are found to undergo a crystalline-to-amorphous structural transition under a low concentration of electronic excitations. The transition temperature at which structural amorphization starts to occur depends on the concentration of electronic excitations. During the structural transition, O2-like molecules are formed, and this anion disorder further drives cation disorder that leads to an amorphous state. This study provides new insights into the mechanisms of amorphization in titanate pyrochlores under laser, electron and ion irradiations.
Highlights
The response of titanate pyrochlores (A2Ti2O7, A 5 Y, Gd and Sm) to electronic excitation is investigated utilizing an ab initio molecular dynamics method
Moll et al compared the structural transitions in Gd2Ti2O7 single crystals irradiated with high- and low-energy heavy ions (870-MeV Xe and 4-MeV Au), and suggested that at high energy the structural amorphization is dominated by high electronic excitation, while at low energy the amorphization is driven by ballistic nuclear energy deposition from the ions[35]
It is shown that structural amorphization starts at t 5 0.3 ps, and the structure is completely amorphized at t 5 3 ps
Summary
Correspondence and requests for materials should be addressed to H.Y.X. Lian et al have investigated the irradiation response of A2Ti2O7 (A 5 Sm to Lu, and Y) pyrochlores by 1-MeV Kr ions at temperatures from 293 to 1073 K, and found that most of the titanate pyrochlores are readily amorphized[23] Several possible mechanisms, such as direct-impact (in-cascade) amorphization within an individual collision cascade, the local accumulation of high defect concentrations due to the overlap of collision cascades, or a combination of these processes have been proposed to explain the origin of irradiation-induced amorphization[24,25]. This technique has been employed to study the electronic effects on preamorphized SrTiO3 and Sr2Nd8(SiO4)6O2, and electron-beam-induced recrystallization was observed[39,40,41], demonstrating that electronic excitation or ionization from electron irradiation can have substantial effects on the structure of materials This raises the question of the role of electronic excitation at lower ion energies on the irradiation-induced crystalline-to-amorphous transition in pyrochlores, i.e., whether electronic excitation contributes to the amorphization or not. The simulation times are 12 ps and 3 ps for electronic excitation and electron-hole recombination, respectively
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