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][doi]
![Coupled migration sequence of two Cl− ions within a single anion channel at 1250 K over 0.64 ps viewed in (a) the [100] and (b) the [010] 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.
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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 CaCl~2~ 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.
[doi]: http://dx.doi.org/10.1039/C4TA03275F
[image]: /posts/new-paper-a-concerted-mechanism-for-cl-migration-in-chlorapatite/channels.png