Swift heavy ion tracks in alkali tantalate crystals: a combined experimental and computational study

The formation of latent tracks with different damage morphologies in alkali tantalate crystals (KTaO₃ and LiTaO₃) under the action of the extreme electronic energy loss induced by 358 MeV ⁵⁸Ni¹⁹⁺ irradiation was studied by experimental characterizations of the lattice damage and numerical calculations using the inelastic thermal spike model. Prism coupling measurements were used to analyze of the refractive index profiles of irradiated regions. This approach is effective and very accurate for determination of the in-depth damage profile and its correlation with the energy loss curves. The calculated spatio-temporal evolution of the energy deposition densities and lattice temperatures theoretically demonstrate the experimentally observed latent tracks in Ni¹⁹⁺-irradiated crystals. Based on the observed damage morphologies of individual and overlapped spherical defects, and discontinuous and continuous tracks, the corresponding threshold values of the electronic energy loss for track damage in alkali tantalate crystals were assessed. For irradiating ions with an energy of 6.17 MeV amu^–1, a threshold of ~12.0 keV nm⁻¹ for the production of spherical defects in KTaO₃ crystals is indicated, and the threshold for LiTaO₃ crystals is less than 12.0 keV nm⁻¹. For irradiating ions with an energy of 2.15 MeV amu^–1, owing to the ion-velocity dependence effect, an electronic energy loss of ~13.8 keV nm⁻¹ leads to overlapped spherical defects and discontinuous tracks in KTaO₃ and continuous tracks in LiTaO₃. Compared with LiTaO₃, a relatively higher damage tolerance and critical threshold for track formation in KTaO₃ crystals are proven. The determined lattice temperature threshold for continuous track production is 3410 K for KTaO₃ and slightly less than 3250 K for LiTaO₃, demonstrating that, compared with the melting point, a much higher lattice temperature in the region surrounding the ion path needs to be achieved to produce stable track damage due to the non-negligible effect of melting damage caused by annealing during the cooling process. (paper)