New Paper - "A Concerted Mechanism for Cl Migration in Chlorapatite"

We are pleased to announce the publication of our new paper on Cl- diffusion in chlorapatite, a material that is being considered as a host for halide bearing nuclear waste:

  • M.L. Jackson, E.E. Jay, M.J.D. Rushton and R.W. Grimes, “A concerted mechanism for Cl- in chlorapatite”, Journal of Materials Chemistry A 2 (2014) 16157. doi:10.1039/C4TA03275F
Coupled migration sequence of two Cl− ions within a single anion channel at 1250 K over 0.64 ps viewed in (a) the  and (b) the  direction. The colour of the migrating ion denotes time such that ions of the same colour represent different ions at the same time. Cl− lattice sites are denoted by black dotted circles.

In this work the potential set previously developed for fluorapatite has been extended to include chlorine and thus chlorapatite. Previously, fluorine was observed to migrate in a quasi–1D concerted mechanism along channels in the fluorapatite [001] direction, with migration mediated by interstitial sites. Chlorine appears to migrate by a similar but distinct mechanism in chlorapatite, whereby instead of being mediated by interstitial sites migration proceeds directly along the [001] direction via a vacancy mediated mechanism, with several ions moving in concert. Vacancies are created through the formation of Frenkel pairs, with the interstitial position roughly equivalent to that of fluorine in fluorapatite.

Abstract

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.