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

TL;DR

This study uses first-principles simulations to elucidate the polarization and ion conduction mechanisms in hydroxyapatite, revealing how OH$^-$ ions facilitate ion transport and polarization at elevated temperatures.

Contribution

It provides a detailed atomic-level understanding of ion conduction and polarization mechanisms in hydroxyapatite through first-principles calculations, aligning with experimental data.

Findings

Ion conduction occurs mainly along the c-axis via OH$^-$ ions.

Activation energies match experimental conductivity measurements.

Ion flipping, proton vacancy exchange, and vacancy hopping are key mechanisms.

Abstract

We report first-principles simulation of polarization mechanisms in hydroxyapatite to explain the underlying mechanism behind the reported ion conductivities and polarization under electrical poling at elevated temperatures. It is found that ion conduction occurs mainly in the column of OH$^-$ ions along the $c$-axis through a combination of the flipping of OH$^-$ ions, exchange of proton vacancies between OH$^-$ ions, and the hopping of the OH$^-$ vacancy. The calculated activation energies are consistent with those found in conductivity measurements and thermally stimulated depolarization current measurements.