CO₃^2- mobility in carbonate apatite as revealed by density functional modeling

Carbonate apatite is a material of the utmost importance as it represents the inorganic fraction of biological hard tissues in bones and teeth. Here we study the static and dynamic features of CO₃^2- ion in the apatitic channel of carbonate apatite (A-type substitution), by applying both static and dynamic quantum mechanical calculations based on density functional methods with B3LYP-D* and PBE functionals. The static calculations reveal a number of almost energetically equivalent carbonate configurations in the channel, leading to cell parameters compatible with the P_3Ì... space group assigned by the experimental X-ray structure determination. Ab initio isothermal-isobaric molecular dynamics simulations provide insights on the CO₃^2- mobility, showing that at the temperature of the experimental structural determination the CO₃^2- moiety undergoes a dynamic disorder, as the carbonate group is almost free to move within the apatitic channel enhancing its exchangeability with other anions.