Diffusion Path and Conduction Mechanism of Protons in Hydroxyapatite

Abstract

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.

Calcium orthophosphates (CaPO 4 ): Occurrence and properties

“…A shell model was developed to study the lattice dynamics of HA (Calderin et al 2005), while a cluster growth model was created to illustrate its growth (Onuma and Ito 1998). Polarization characteristics (Tanaka et al 2010a, b), pyroelectric (Tofail et al 2009(Tofail et al , 2015 and piezoelectric (Tofail et al 2015;Bystrov 2015) properties of HA as well as diffusion of protons inside the HA crystal lattice (Yashima et al 2014) were investigated. The thermodynamic properties of HA and other types of orthophosphate-based apatites were summarized (Drouet 2015).…”

First-principles investigation of polarization and ion conduction mechanisms in hydroxyapatite

“…Understanding of the microscopic mechanisms is highly desirable for optimizing this material for electret applications such as micro-energy harvesting and biological tissue engineering. Figure 1 shows the hexagonal P 6 3 /m structure of HAp, which is the most frequently encountered structure, especially at elevated temperatures 5,6 . The crystal structure consists of columns of hydroxyl (OH − ) ions along the c-axis, PO 3− 4 tetrahedra, and Ca 2+ ions.…”

Dissolving Hydroxyolite: A DNA Molecule into Its Hydroxyapatite Mold

“…2 PO H at the HOli surface as well as to mimic the proton transfer process experimentally observed in HAp, [17] every five time steps, 5t= 50 ns, the conversion of an internal HAp with different acids. [10,18] On the other hand, results reflect a polynuclear detachment of Ca 2+ (Figure 3).…”