# Sub-Band-Gap Absorption Mechanisms Involving Oxygen Vacancies in Hydroxyapatite

## TL;DR

This study uses hybrid density-functional calculations to explore how oxygen vacancies in hydroxyapatite contribute to sub-band-gap UV absorption, elucidating mechanisms behind its photocatalytic activity.

## Contribution

It provides detailed atomic-level insights into oxygen vacancy structures and their electronic transitions in hydroxyapatite, advancing understanding of its photocatalytic properties.

## Key findings

- Oxygen vacancies occur as neutral or double plus charge states.
- Vacancies are negative-U defects with metastable single plus charge state.
- Electron promotion involves a 3.6-3.9 eV transition, explaining UV absorption onset.

## Abstract

Hydroxyapatite (HAp) is a widely used biomaterial for the preparation of bone and dental implants. Despite the relevance of HAp in medicine, exciting applications involving this material as a bio-compatible photocatalyst, depend on how well we understand its fundamental properties. Experimental evidence suggests that oxygen vacancies play a critical role in the production of surface radicals upon exposure of HAp to ultra-violet (UV) light. However, very little is known about the underlying physical and chemical details. We present a hybrid density-functional study of the structural and electronic properties of oxygen vacancies in large HAp supercells within the plane-wave formalism. We find that under equilibrium conditions, vacancies occur either as a simple vacant oxygen site (in the neutral charge state), or as extended structures replacing several crystalline moieties (in the double plus charge state). Large atomic relaxations upon ionization make the oxygen vacancy a negative-$U$ defect, where the single plus charge state is metastable, being only accessible under UV excitation. From inspection of the transition levels, we find that electron promotion from the valence band top to the donor state of the vacancy, involves a zero-phonon transition of 3.6-3.9 eV. This mechanism is the most likely explanation to the 3.4-4.0 eV absorption onset for the observation of photocatalysis using HAp under persistent UV illumination.