Defect evolution and electrical response in HAp-TiO2-derived calcium phosphate-titanate composites investigated by positron annihilation lifetime spectroscopy.

The structural evolution of hydroxyapatite (HAp)-TiO2 composites was investigated as a function of TiO2 content (0-5 wt%) and sintering temperature (1000-1300 °C). X-ray diffraction revealed that TiO2 no longer exists as a separate phase above ∼1100 °C owing to solid-state reactions with HAp, resulting in the formation of β-tricalcium phosphate (β-TCP) and calcium titanate phases (CaTiO3/Ca4Ti3O10). Consequently, the resulting materials consist of multiphase calcium phosphate-titanate composites rather than TiO2-doped HAp. SEM observations showed significant temperature-dependent microstructural evolution, including densification, grain growth, and the formation of interfacial regions between the constituent phases. Positron annihilation lifetime spectroscopy (PALS) revealed systematic variations in the defect-related lifetime (τ 2 = 0.44-0.62 ns) and intensity, suggesting an evolution from pore-dominated defects to increasingly localized interfacial trapping sites during phase transformation. Electrical measurements showed low bulk conductivity, with the non-monotonic DC conductivity behavior reflecting the combined influence of defect evolution, interfacial heterogeneity, and microstructural changes, while the AC conductivity response suggested localized hopping and polarization processes. The combined use of XRD, SEM, PALS, and electrical measurements provides new insight into the interplay among phase evolution, defect structure, microstructure, and electrical behavior in HAp-TiO2-derived calcium phosphate-titanate composites.