Structure and Transformation of Amorphous Calcium Phosphate ACP

It was also observed that ACP transforms not only on contact with SBF-H (Table 7.8) but also during exposure to the electron beam used for imaging in the STEM (Scanning transmission electron microscope) (Heimann and Wirth, 2006) as well as by postspraying treatment with a C02 laser beam (Ranz et al., 1998). The former happens only in those areas of the ACP containing substantial amounts of protons that appear to act as a driving force of the crystallisation 

However, despite these experimental achievements reviewed, for example by Dorozhkin (2012) and Combes and Rey (2010) stubborn mysteries still surround the structure of ACP (Mathew and Tagaki, 2001 Celotti et al., 2006). While the 

A more recent formula of ACP was provided by Heughebaert (1977) and Heughe-baert and Montel (1982) as Ca9(P()4)6 x(I lP()4)x(OI l)x. The presence of HP042-ions in the structure of ACP was confirmed by solid-state NMR investigation (Jager, Maltsev and Karrasch, 2006). 

Once HAp has been nucleated after OCP is brought in contact with either water or SBF the epitaxial mechanism discussed earlier proceeds quickly and unimpeded (Dorozhkin, 2012). However, nucleation of HAp is the rate-determining step of this reaction and delays conversion of OCP to HAp even in solutions supersaturated with respect to HAp (Ito et al., 2014). 

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