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.