Abstract
Hydroxyapatite constitutes a large part of the human calcified tissues (e.g. teeth and bones). Owing to its great biological interest, much effort has been devoted for many years to the study of the structure of this crystalline compound, notably by means of medium-voltage electron microscopes designed for high-resolution imaging. Foregoing studies have already revealed that specimens of hydroxyapatite [(Ca2+)10(PO3− 4)6(OH−)2, space group P63/m] irradiated in a high-resolution electron microscope by 300 to 400 keV electrons undergo various types of damages due notably to knock-on collisions, inelastic energy loss events liable to give rise to radiolysis, and heating. In order to describe the ballistic aspect of the interaction modes, the electrostatic binding energies of the ions in a unit cell are calculated by summing the coulombic contributions of the neighbours up to convergence of the series. These binding energies, eventually increased by the covalent bond energy in the case of the breaking of a complex ion, are considered as displacement energies in the computation of the collision cross-sections by means of the McKinley-Feshbach approximation. It appears that at the beginning of the radiation hydroxyl and oxygen ions can be ejected. After creation of vacancies in the crystal unit cell, in particular by the departure of the two hydroxyl ions, calcium ions become more loosely bound and can then also be displaced, while our calculations show that phosphate ions are stable and seem not to be liable to be displaced. The difference in extraction probability between calcium and phosphate ions has strong implications for structural and compositional studies of hydroxyapatite by medium voltage electron microscopy. The irradiation-induced diffusion of ions can explain, at least partly, the formation of calcium oxide observed on the micrographs described in a previous study.