Site preference and local structural stability of Bi(iii) substitution in hydroxyapatite using first-principles simulations

Abstract

Bismuth (Bi(III)) substitution in hydroxyapatite (HAp) lattice confers unique properties such as antibacterial, catalytic, radiosensitization, and conductive properties while preserving the innate bioactivity. Understanding the local structural changes upon Bi³⁺ substitution is essential for controlling the stability and optimizing the properties of HAp. Despite numerous experimental studies, the precise substitution behaviors, such as site preference and structural stability, remain incompletely understood. In this study, the substitution behavior of Bi(III) into the HAp lattice with formula of Ca₉Bi(PO₄)₆(O)(OH) was investigated via first-principles simulation by implementing density functional theory. Energy calculations showed that Bi³⁺ preferentially occupies the Ca(2) site with an energy difference of ∼0.02 eV per atom. Local structure analysis revealed higher bond population values and an oxygen coordination shift from 7 to 6 for the Ca(2) site, attributed to the greater covalent interactions and its flexible environment accommodating the bulky Bi³⁺ ion and its stereochemically active lone pair. This work provides the first comprehensive investigation on Bi³⁺ ion substitution site preference in HAp using first-principles simulations.