Thermodynamics of mono and di-vacancies in barium titanate

TL;DR

This study uses density-functional theory to analyze the thermodynamic and kinetic properties of vacancies in barium titanate, revealing how vacancy types and clusters form under different conditions, impacting defect distribution and aging.

Contribution

It provides new insights into vacancy charge states, binding energies, and migration barriers in barium titanate, enhancing understanding of defect behavior in ferroelectric materials.

Findings

Vacancies occur in their nominal charge states across most of the band gap.

Di-vacancy binding energies are negative and constant in the dominant band gap range.

Oxygen vacancies migrate readily at typical growth temperatures, facilitating di-vacancy formation.

Abstract

The thermodynamic and kinetic properties of mono and di-vacancy defects in cubic (para-electric) barium titanate are studied by means of density-functional theory calculations. It is determined which vacancy types prevail for given thermodynamic boundary conditions. The calculations confirm the established picture that vacancies occur in their nominal charge states almost over the entire band gap. For the dominating range of the band gap the di-vacancy binding energies are constant and negative. The system, therefore, strives to achieve a state in which under metal-rich (oxygen-rich) conditions all metal (oxygen) vacancies are bound in di-vacancy clusters. The migration barriers are calculated for mono-vacancies in different charge states. Since oxygen vacancies are found to readily migrate at typical growth temperatures, di-vacancies can be formed at ease. The key results of the present study with respect to the thermodynamic behavior of mono and di-vacancies influence the initial defect distribution in the ferroelectric phases and therefore the conditions for aging.