Mechanism of Mn incorporation into hydroxyapatite: Insights from SR-XRD, Raman, XAS, and DFT calculation

Abstract Hydroxyapatite (HAp) is ubiquitous in soils, sediments, and rocks in nature and also found to be the main mineral component of biological bones and teeth. HAp is structurally robust and chemically diverse with substitution by cations and/or anions, contributing to its important geological, environmental, and technological applications. Incorporation of Manganese (Mn), one of the common impurities in HAp, generally alters the physicochemical properties of HAp. It is thus fundamentally important to understand the incorporation mechanism of Mn in the HAp structure, as well as induced structural changes. In this study, HAps with various Mn contents have been synthesized by a co-precipitation method and then characterized with various techniques. It is indicated that as high as 10 mol% Mn can be successfully incorporated into HAp, and Mn incorporation decreases particle sizes of HAps. The interplanar spacing of (002) decreases linearly with increasing Mn content. The decrease in the crystallinity with Mn incorporation has been disclosed by the linear redshift and broadening of the ν1(PO4) in Raman spectra. Fourier Transform Infrared Spectroscopy (FTIR) analysis suggests significant covalent character of Mn-OH. Total electron yield (TEY) spectra and X-ray absorption near edge structure (XANES) analysis confirm that the incorporated Mn is in the oxidation state of Mn2+. Extended X-ray absorption fine structure (EXAFS) analyses of Ca and Mn in HAp suggest that Mn prefers to occupy the Ca2 site over the Ca1 site. Furthermore, density function theory (DFT) calculations show that the Ca2 site is more energetically favored for Mn2+ incorporation.