Experimental Visualization of Chemical Bonding and Structural Disorder in Hydroxyapatite through Charge and Nuclear-Density Analysis

Abstract

Calcium hydroxyapatite (HAp, Ca10(PO4)6(OH)2) is the principal inorganic component of bone and teeth, bioactive material, and proton (H+) conductor. However, the chemical bonding and structural disorder of HAp are unclear. Here we report precise charge- and nuclear-density distributions of HAp at 298 and 673 K. The present results clearly demonstrate the covalent P–O and O–H bonds, more ionic Ca–O bonds, and the charge transfers from P to O atoms and from H to O atoms in HAp. This work shows that the hexagonal–monoclinic phase transition of HAp is accompanied by the occupational and orientational ordering of OH– ions, the tilting of the PO4 tetrahedron, and the Ca displacements. Diffusion paths of proton are visualized along the c-axis in hexagonal HAp. We anticipate that the visualization of chemical bonding and structural disorder of HAp will contribute greatly to our understanding of biominerals, reactions, biological organisms, and the diffusion process in HAp-based proton conductors.