Crystal growth and structure analysis of twin-free monoclinic hydroxyapatite

Single crystals of hydroxyapatite were grown by a flux method using Ca(3)(PO(4))(2) and Ca(OH)(2) under 100 MPa of Ar gas. The crystals obtained had stoichiometric composition of Ca(10)(PO(4))(6)(OH)(2) and some of them were twin-free single crystals. From X-ray diffraction analyses, the space group was confirmed to be monoclinic P2(1)/b with cell parameters a=0.9419(3) nm, b=1.8848(6) nm, c=0.6884(2) nm, and gamma=119.98(2) degrees. The detailed crystal structure was determined with a reliability factor R(w)=0.033; the O atoms of OH were located just on the 2(1) axis while the H atoms of OH occupied the positions a little deviated from the 2(1) axis. The origin of this structure was ascribed to the formation of hydrogen bonds between the H atoms of the OH ions and the specific O atoms of the PO(4) ions.