Atomistic Simulation Study of the Order/Disorder (Monoclinic to Hexagonal) Phase Transition of Hydroxyapatite

The transformation of the low-temperature (monoclinic, P 2 1 / b ) to the high-temperature (hexagonal, P 6 3 / m ) modification of hydroxyapatite Ca 5 [(OH)(PO 4 ) 3 ] is investigated by means of molecular dynamics simulations. In the monoclinic phase the orientation of the hydroxide ions is strictly ordered. Above the critical temperature of about 200 °C orientational changes of the hydroxide ions are observed. In the course of each reorientation event a hydroxide ion passes through the surrounding calcium triangle. From an Arrhenius fit the related activation energy is calculated. In the high-temperature phase the hydroxide ions are statistically disordered. Out of the two possible concepts for the formation and structure of hexagonal hydroxyapatite, our simulations clearly identify the disordering of hydroxide ions orientation to occur in a nonconcerted manner, i.e., collective reorientation of OH - ion rows is not observed.