Eleanor E. Jay

Prediction and characterisation of radiation damage in fluorapatite

Molecular dynamics simulations, used in conjunction with a set of classical pair potentials, have been employed to examine simulated radiation damage cascades in the fluorapatite structure. Regions of damage have subsequently been assessed for their ability to recover and the effect that damage has on the important structural units defining the crystal structure, namely phosphate tetrahedra and calcium meta-prisms. Damage was considered by identifying how the phosphorous coordination environment changed during a collision cascade. This showed that PO4 units are substantially retained, with only a very small number of under or over coordinated phosphate units being observed, even at peak radiation damage. By comparison the damaged region of the material showed a marked change in the topology of the phosphate polyhedra, which polymerised to form chains up to seven units in length. Significantly, the fluorine channels characteristic of the fluorapatite structure and defined by the structures calcium meta-prisms stayed almost entirely intact throughout. This meant that the damaged region could be characterised as amorphous phosphate chains interlaced with regular features of the original undamaged apatite structure.

Migration of fluorine in fluorapatite – a concerted mechanism

Molecular dynamics simulations, used in conjunction with a set of classical pair potentials, have been employed to investigate the transport of fluorine in fluorapatite. A new coupled interstitial migration mechanism is identified with a migration activation energy of 0.55 eV in the temperature range 1100–1500 K. A full description of the mechanism is provided, which differs markedly from previously proposed vacancy mechanisms for fluorine transport.

A concerted mechanism for Cl− migration in chlorapatite

A highly anisotropic concerted vacancy mediated mechanism is identified for Cl− transport in chlorapatite. This was revealed in molecular dynamics simulations of stoichiometric and CaCl2 deficient chlorapatite. The mechanism was established within the temperature range 1000–1400 K, with an activation energy of 2.37 ± 0.07 eV. A considerably lower activation energy is predicted in the CaCl2 deficient material, 0.54 ± 0.16 eV, due to the availability of Cl− vacancies. The transport process involves the concerted migration of two to four Cl− ions directly along the c axis halide channel and is contrasted with the F− interstitial mechanism shown previously in fluorapatite.