Co-substitution behaviour of iron and silicon into apatite lattice during sintering process

Multiple ion co-substitution in hydroxyapatite (HAp) has emerged as a transformative strategy in bone tissue engineering for developing advanced bioceramics that mimic the complex composition of natural bone. Although the individual effects of silicon (Si) and iron (Fe) have been studied, their combined structural impact remains unexplored. This study systematically investigates the defect chemistry and structural evolution of Fe- and Si-co-substituted HAp (FeSiHAp) synthesized and sintered at 1200 °C. Microstructural analyses revealed that the dopants initially existed as amorphous surface species delaying crystallization. However, upon complete solid solution at 1200 °C, co-substitution exerted a combined inhibitory effect on grain growth, yielding a fine and dense microstructure. Crystallographic and quantitative FTIR deconvolution analyses elucidated distinct substitution mechanisms: Si 4+ substitution at PO 4 3− sites, inducing volume contraction and OH − vacancies. Conversely, our data strongly suggest that Fe 3+ predominantly occupies the 12 i interstitial sites, which is consistent with the observed anomalous unit cell expansion, profound dehydroxylation, and O 2− defect generation. Crucially, FTIR deconvolution provided evidence consistent with local lattice distortion (splitting of ν 3 PO 4 3− bands) and the possible formation of a Fe–O–Si vibrational network, supporting the hypothesis of a strong structural interaction between interstitial Fe 3+ and substitutional SiO 4 4− . These fundamental insights into tailoring the grain size and defect chemistry offer a theoretical foundation for the future design of advanced bioceramics.