Oxygen vacancies in BaTiO₃ have a surprising impact on dielectric and ferroelectric properties

Barium titanate (BaTiO₃) is an interesting material for applications such as ceramic capacitors and positive temperature coefficient thermistors because of its dielectric and ferroelectric properties. Different synthesis paths can lead to stoichiometric or oxygen-deficient materials, which behave quite differently, suggesting an important role of oxygen vacancies.

Now, a research group is intentionally creating oxygen vacancies in BaTiO₃ to explore how it changes the dielectric and ferroelectric properties of a system. In the Journal of Applied Physics, they report the surprising discovery that the large change in dielectric constant (the “dielectric anomaly”) and the ferroelectricity, which are associated with the structural transition, become masked by the presence of large leakage current of the samples.

The authors compared ferroelectric measurements of BaTiO₃ samples in which they systematically created increasing numbers of oxygen vacancies and observed an increase of leakage in samples with higher oxygen deficiency. They attributed the increased leakage to the presence of defect states, which they found via diffuse reflectance spectroscopy measurements. “These states form extra energy states within the band gap, which provides a path for conducting electrons — and increased leakage within the sample,” said co-author Archna Sagdeo. “This modifies the dielectric and ferroelectric properties of the sample.”

But the group found no significant structural changes even for oxygen-deficient samples. “All samples had nearly the same band gap, but the oxygen-deficient sample shows no dielectric anomaly or ferroelectric loop,” she said. “When ferroelectricity isn’t visible within the sample and the dielectric anomaly has disappeared, the structural phase transition should also vanish.” But surprisingly, they found that this structural transition persists, calling for further investigation.

Source: “Disappearance of dielectric anomaly in spite of presence of structural phase transition in reduced BaTiO₃: Effect of defect states within the bandgap,” by Archna Sagdeo, Anjali Nagwanshi, Preeti Pokhriyal, A. K. Sinha, Parasmani Rajput, Vikash Mishra, and P. R. Sagdeo, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5010870 .