First-principles investigations of Ti-substituted hydroxyapatite electronic structure

The electronic structure of Ti-substituted hydroxyapatite is investigated using density functional theory within a periodic slab model. Two sorption mechanisms have been considered: i.e., Ti⁴⁺ and Ti(OH) ₂²⁺ as the likely species to exchange with Ca ²⁺. Ti⁴⁺ has a small ionic radius compared to Ca ²⁺ and can dope into both distinct sites, showing no site preference; however, when two H were removed from the OH channel to obtain charge compensation, preferential site II substitution appears, accompanied with a large O shift forming a strong Ti-O bond. The species Ti(OH)₂ ²⁺ displays a strong site preference: substitution by Ti(OH) ₂²⁺ on the hydroxyl channel (site II) is exothermic and favored strongly over the Ca column (site I). Ti(OH)₂²⁺ substitution for Ca²⁺ induces a large geometry relaxation and distortion, especially within the OH channel and Ca²⁺ column, with a considerable shift of Ti compared to the Ca sites in pure HA. These results are consistent with the experimental observation that material synthesis with high Ti doping (atomic ratio > 0.1) shows irregular particles formation with reduced crystallinity. The calculated cell shape and volume relaxations indicate that the volume and cell parameters both expand in all the substituted HA models. The site preference and volume expansion differences found are attributed to the metal ion shift caused in meeting the requirement of strong Ti-O coordination in site I and site II polyhedra.