Calcium hydroxyapatite (HAp, Ca 10 (PO 4 ) 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 chargeand nuclear-density distributions of HAp at 298 and 673 K. The present results clearly demonstrate the covalent PÀO and OÀH bonds, more ionic CaÀO bonds, and the charge transfers from P to O atoms and from H to O atoms in HAp. This work shows that the hexagonalÀmonoclinic phase transition of HAp is accompanied by the occupational and orientational ordering of OH À ions, the tilting of the PO 4 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.
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