A physical method for investigating defect chemistry in solid metal oxides

TL;DR

This paper introduces a novel physical method for studying defect chemistry in solid metal oxides by controlling oxygen partial pressure in an ultra-high vacuum environment and measuring conductivity with high sensitivity, enabling detailed surface chemistry analysis.

Contribution

The paper presents an alternative approach using pure oxygen dosing in ultra-high vacuum to investigate defect chemistry, overcoming limitations of traditional buffer gas methods.

Findings

Method accurately reproduces redox behavior of SrTiO3

Allows direct analysis of oxygen dose effects

Minimizes electrochemical polarization effects

Abstract

The investigation of the defect chemistry of solid oxides is of central importance for the understanding of redox processes. This can be performed by measuring conductivity as a function of the oxygen partial pressure, which is conventionally established by using buffer gas mixtures or oxygen pumps based on zirconia. However, this approach has some limitations, such as difficulty regulating oxygen partial pressure in some intermediate-pressure regions or the possibility of influencing the redox process by gases that can also be incorporated into the oxide or react with the surface via heterogeneous catalysis. Herein, we present an alternative physical method in which the oxygen partial pressure is controlled by dosing pure oxygen inside an ultra-high vacuum chamber. To monitor the conductivity of the oxide under investigation, we employ a dedicated four-probe measurement system that relies on the application of a very small AC voltage, in combination with lock-in data acquisition using highly sensitive electrometers, minimizing the electrochemical polarization or electro-reduction and degradation effects. By analyzing the model material SrTiO3, we demonstrate that its characteristic redox behavior can be reproduced in good agreement with the theory when performing simultaneous electrical conductivity relaxation (ECR) and high-temperature equilibrium conductivity (HTEC) measurements. We show that the use of pure oxygen allows for a direct analysis of the characteristic oxygen dose, which opens up various perspectives for a detailed analysis of the surface chemistry of redox processes.