Importance of phonon contribution to the free migration energy of the O vacancy in BaZrO3

TL;DR

This study demonstrates that phonon excitations significantly increase the free migration energy of oxygen vacancies in BaZrO3, explaining its high-temperature stability and low diffusivity relevant for fuel cell applications.

Contribution

It reveals the crucial role of phonon contributions in the migration energy of oxygen vacancies, a factor often neglected in previous models.

Findings

Phonon excitations increase the effective migration barrier at high temperatures.

Calculated diffusivity aligns with experimental data at operating temperatures.

High-temperature stability is mainly due to phonon contributions to vacancy migration energy.

Abstract

BaZrO3 exhibits excellent proton conductivity and good high-temperature stability. It is therefore a promising electrolyte material for solid oxide fuel cells. The stability of BaZrO3 at high temperatures is generally explained by the low diffusivity of the O vacancy. Present first-principle Density-Functional-Theory calculations show that the slow migration of the doubly positively charged O vacancy at high temperature cannot be solely caused by the ground-state migration energy but by the contribution of phonon excitations to the free migration energy. With increasing temperature, the effective barrier for oxygen vacancy migration increases. At about 1000K, which is the operating temperature of fuel cells, the calculated O vacancy diffusivity is more than one order of magnitude lower than that determined using solely the ground-state migration barrier. The calculated diffusivity data agree well with experimental results from literature. The present work reveals that the high-temperature stability of BaZrO3 is mainly due to the phonon contribution to the free migration energy of the O vacancy.