Pristine and hydrated fluoroapatite (0001)

The surface structure of fluoro­apatite (0001) (FAp(0001)) under quasi-dry and humid conditions has been probed with surface X-ray diffraction (SXRD). Lateral and perpendicular atomic relaxations corresponding to the FAp(0001) termination before and after H(2)O exposure and the location of the adsorbed water molecules have been determined from experimental analysis of the crystal truncation rod (CTR) intensities. The surface under dry conditions exhibits a bulk termination with relaxations in the outermost atomic layers. The hydrated surface is formed by a disordered partially occupied H(2)O layer containing one water molecule (33% surface coverage) adsorbed at each of the three surface Ca atoms, and is coupled with one OH group randomly bonded to each of the three topmost P atoms with a 33% surface coverage.