A concerted mechanism for Cl− migration in chlorapatite

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.