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 <i>c</i>-axis in the hexagonal channel and (2) two-dimensional proton migration pathway network on the <i>ab</i>-planes at <i>z</i> = 0 and 1/2. The proton diffusion and reorientation of hydroxide ions (OH^–) are a complex sinusoidal process in the hexagonal channel along the <i>c</i>-axis, which is consistent with the anisotropic nuclear-density distribution of proton obtained by the neutron diffraction and maximum-entropy method