Solid‐State NMR Study of Mn ²⁺ for Ca ²⁺ Substitution in Thermally Processed Hydroxyapatites

Abstract

In the present study calcium hydroxyapatites enriched at 0.08 wt% in Mn 2+ ions (Mn– HA ) and their unsubstituted forms ( HA ) were synthesized using the same standard wet chemical route. Mn‐ HA and HA were both calcined at 800°C to give Mn‐ HA c and HA c, respectively or sintered at 1250°C, to give Mn‐ HA s and HA s, respectively. The influence of the heat treatment on physicochemical properties of Mn‐ HA was investigated using powder X‐ray diffraction ( PXRD ), scanning, and transmission electron microscopy ( SEM and TEM ), and solid‐state nuclear magnetic resonance (ss NMR ). Mn‐ HA c and Mn‐ HA s were compared to each other and to HA c and HA s, respectively. Assignment of the proton ss NMR peaks from high‐temperature‐treated apatites has been revised. It was found that Mn– HA c and HA c were nanocrystalline, while Mn‐ HA s and HA s comprised micrometer sized, partially fused particles ( SEM and TEM ). PXRD and ss NMR demonstrated that the incorporation of Mn 2+ into the crystal lattice of hydroxyapatite significantly facilitates its dehydroxylation and decomposition to oxyhydroxyapatite during calcination at 800°C, and induces its transformation to tetracalcium phosphate ( TTCP ) and alpha‐tricalcium phosphate (α‐ TCP ) at 1250°C. Contamination by CaO has also been detected. The 1 H→ 31 P NMR cross‐polarization experiments have indicated that the Mn 2+ ions preferentially occupied the Ca(I) position in the crystallographic unit cell of Mn‐ HA c. In Mn‐ HA s, the Mn 2+ ions were evenly distributed between the Ca(I) and Ca( II ) positions.