Role of water in the surface relaxation of the fluorapatite (100) surface by grazing incidence x-ray diffraction

The structure of the fluorapatite (100) surface in humid ambient N₂ with grazing incidence x-ray diffraction is investigated and compared with results on the same surface in dry ambient conditions. Measurement of specular and nonspecular crystal truncation rods provided atomic scale information about the surface structure and the adsorption sites of the water molecules. In humid environment (75% relative humidity), a laterally ordered monolayer of four water molecules per unit cell is formed at about 1.8 (1)0.3em0ex above the relaxed surface reducing the magnitude of atomic relaxations observed on surface in dry conditions.