Damage structure in the ionic crystal LiF irradiated with swift heavy ions

Abstract

In many insulators, swift heavy ions in the MeV to GeV energy regime create latent tracks characterized by irreversible structural and chemical changes. Based on a large data set, the present report will give a detailed description of the damage structure and defect morphology of ion tracks in lithium fluoride. The results were obtained by different complementary techniques including optical absorption spectroscopy, small-angle X-ray scattering (SAXS), chemical etching, scanning force microscopy, and surface profilometry. In a large cylindrical halo of several tens of nanometers around the ion trajectory, single defects such as F- and F2-centers are evidenced by optical absorption spectroscopy, similar to the damage known from conventional irradiations. Above a critical electronic stopping power of the ions of around 10 keV/nm, new effects occur, namely the formation of more complex defects in a very small core region with a radius of 1–2 nm. The damage in this zone is responsible for a characteristic anisotropic X-ray scattering and for chemical etching. Several observations indicate that this core consists of a quasi-cylindrical discontinuous array of complex defect aggregates (presumably small Li colloids, molecular fluorine and vacancy clusters). Profilometer measurements reveal substantial ion-induced volume expansion. This swelling can be assigned to a track radius of about 5–10 nm, an intermediate zone between the track core and the halo, and appears at a much lower threshold of around 4 keV/nm. Track data (radii and threshold) linked to the core and to swelling can be described within the frame of the thermal spike model assuming two different criteria, namely quenching of a vapor and a melt phase, respectively created along the ion path.