Crystal growth and structure analysis of twin-free monoclinic hydroxyapatite

Single crystals of hydroxyapatite were grown by a flux method using Ca₃(PO₄)₂ and Ca(OH)₂ under 100 MPa of Ar gas. The crystals obtained had stoichiometric composition of Ca₁₀(PO₄)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 P2₁/b with cell parameters a=0.9419(3) nm, b=1.8848(6) nm, c=0.6884(2) nm, and =119.98(2)°. The detailed crystal structure was determined with a reliability factor R_w=0.033; the O atoms of OH were located just on the 2₁ axis while the H atoms of OH occupied the positions a little deviated from the 2₁ 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 PO₄ ions.