Energetics of Sn2+ isomorphic substitution into hydroxylapatite: first-principles predictions

Abstract

The energetics of Sn²⁺ substitution into the Ca²⁺ sublattice of hydroxylapatite (HA), Ca₁₀(PO₄)₆(OH)₂, has been investigated within the framework of density functional theory. Calculations reveal that Sn²⁺ incorporation via coupled substitutions at Ca(II) sites is energetically favourable up to a composition of Sn₆Ca₄(PO₄)₆(OH)₂, and further substitutions at Ca(I) sites proceed once full occupancy of Ca(II) sites by Sn²⁺ is achieved. Compositions of Sn_xCa_10−x(PO₄)₆(OH)₂ (x = 4–9) are predominant, with an optimal stoichiometry of Sn₈Ca₂(PO₄)₆(OH)₂, and Sn-substituted HA follows approximately Vegard's law across the entire composition range.