Apatites are functional materials with important potential applications as solid electrolytes, and also as biomaterials, ceramics for nuclear waste immobilisation, and catalysts. In apatite-type solid electrolytes, the conductivity depends on the number and the location of oxide ion interstitials, as well as energetically favourable carrier migration pathways within the structure. The location of interstitial oxygen atoms in apatite silicates has been controversial, with different pieces of work, based on powder diffraction data, reporting excess oxygen atoms at different sites. The proposed experiments will be carried out on high-quality single crystals and will provide an accurate structural characterisation of these materials ad a better understanding of the structure-property relationships.