Ferroelectric polarization of hydroxyapatite from density functional theory

Abstract

The theoretical ferroelectric polarization of the low-temperature (monoclinic, P2₁) phase and the high-temperature (hexagonal, P6₃) phase of hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ is calculated based on the density functional theory (DFT). In the monoclinic structure, the value of ferroelectric polarization is found to be 9.87 μC cm⁻² along the [001] direction. In the hexagonal structure, the ferroelectric polarization is 7.05 μC cm⁻² along the [001] direction. The main contribution to the electric polarization comes from ordered hydroxyl OH⁻ anions for both phases, although the inorganic Ca₅(PO₄)₃ apatite framework also gives a non-negligible contribution. A detailed analysis of ferroelectric polarization and structural change of the hydroxyapatite is presented for a better understanding of this important biomaterial.