Ion tracks and microstructures in barium titanate irradiated with swift heavy ions: A combined experimental and computational study

摘要

Tetragonally structured barium titanate (BaTiO_3) single crystals were irradiated using 635 MeV ~(238)U~+ ions to fluences of 1 × 10~7, 5 × 10~(10) and 1.4 × 10~(12) ions cm~(-2) at room temperature. Irradiated samples were characterized using ion channeling, X-ray diffraction, helium ion microscopy and transmission electron microscopy. The results show that the ion-entry spot on the surface has an amorphous core of up to ~10 nm in diameter, surrounded by a strained lattice structure. Satellite-like defects around smaller cores are also observed and are attributed to the imperfect epitaxial recrystallization of thermal-spike-induced amorphization. The critical value of the electronic stopping power for creating observable amorphous cores is determined to be ~22 keV nm~(-1). Molecular dynamics simulations show an amorphous track of ~1.2 nm in radius under thermal energy deposition at 5 keV nm~(-1); the radius increases to ~4.5 nm at 20 keV nm~(-1). A linear fit of the core diameter as a function of the square root of the energy deposition rate suggests a reduction in the diameter by an average of ~8.4 nm due to thermal recrystallization if electron-phonon coupling efficiency of 100 is assumed. The simulation also reveals details of the bonding environments and shows different densities of the amorphous zones produced at different energy deposition rates.