Diffusion path and conduction mechanism of protons in hydroxyapatite

Calcium hydroxyapatite (HAp, Ca₁₀(PO₄) ₆(OH)₂) is the principal inorganic component of bone and teeth, one of the most important bioceramics, and a proton (H⁺) conductor with potential for energy conversion devices. The proton diffusion pathway in the HAp lattice is a key to understand the proton conduction mechanism and chemical reaction. Previous neutron-diffraction studies of HAp visualized the short-range proton diffusion pathway. In this work, we report the successful visualization of the long-range proton diffusion pathway in stoichiometric HAp at 923, 673, and 298 K through a combined technique of high-temperature neutron diffraction and bond valence method. We have visualized (1) one-dimensional proton diffusional pathways along the c-axis in the hexagonal channel and (2) two-dimensional proton migration pathway network on the ab-planes at z = 0 and 1/2. The proton diffusion and reorientation of hydroxide ions (OH^-) are a complex sinusoidal process in the hexagonal channel along the c-axis, which is consistent with the anisotropic nuclear-density distribution of proton obtained by the neutron diffraction and maximum-entropy method.