Final Activity Report Summary - APAMGNMR (Structural study of magnesium substituted hydroxyapatites by solid state NMR : towards biological and catalytic applications) Calcium hydroxyapatite (HA), the main inorganic phase of bone tissue, is an important material for bone substitution and other non-medical applications such as catalysis. In natural HA, magnesium (Mg) is one of the main substitutions for calcium. The main objective of this project was to determine the detailed consequences of Mg incorporation on the bulk and surface structure of hydroxyapatites, in order to try to improve understand of its biological role. Solid state nuclear magnetic resonance (NMR) was the key tool utilised in this work, because this technique allowed for the local environment around an atom to be investigated in detail. 25Mg and 43Ca solid state NMR studies were performed on a set of synthetic magnesium-substituted hydroxyapatites, which were used as model samples, and proved, for the first time, that magnesium preferentially entered one of the two positions in the HA crystal structure. In addition, it was shown that proximities between calcium or magnesium cations and neighbouring hydrogen atoms could be investigated in these materials, and that it was possible to analyse their structure in detail at the molecular level. In the long term, this work would enable a more rational elaboration of HA-based functional biomaterials and catalysts, by allowing for better tailoring of their structure, in particular through Mg substitution.