A physical method for investigating defect chemistry in solid metal oxides

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.

Is Reduced Strontium Titanate a Semiconductor or a Metal?

“…It is predicted that in a regime with a high oxygen partial pressure, SrTiO 3 behaves as a p-type semiconductor, whereas n-type conductivity prevails under reducing conditions [12]. This behavior has been experimentally confirmed using ceramics and single crystals of SrTiO 3 in many studies over the last 30 years [9,11,[13][14][15][16].…”

“…While the temperature was being held at 1000 • C, the oxygen pressure was increased in different steps, up to a final pressure of 200 mbar (Figure 3b). Before starting each step, the resistance was monitored until near equilibrium conditions were achieved, as per the procedure previously described [11]. The bulk resistance measured in (near) equilibrium was used to calculate the macroscopic conductivity σ, which is displayed in Figure 3c as a function of the oxygen partial pressure.…”

“…The nearly current free measurement of the potential drop between the inner electrodes can be used to determine the bulk resistance, while the potential drops between inner and outer electrodes on each side (I and II) are related to the interface resistance, which is mainly determined by the resistance of the surface layer [31]. As is described in detail elsewhere [11], the sample was placed in a quartz tube furnace connected to an ultrahigh vacuum pumping system. The oxygen activity was controlled by filling pure oxygen into the vacuum chamber.…”

“…Although stoichiometric SrTiO 3 is an insulator, it can be turned into a semiconductor by means of doping with extrinsic donors or self-doping with oxygen vacancies [8]. The redox behavior of SrTiO 3 has been described in theoretical terms using point defect chemistry [9][10][11]. As the dominant type of disorder in SrTiO 3 is of the Schottky variety, the properties of the surface/surface layer play a key role in exchange processes.…”

“…Figure 3. (a) Resistance during reannealing to 1000 °C under vacuum conditions following previous oxidation; (b) step-wise exposure to oxygen at 1000 °C; (c) Brouwer diagram of the macroscopic conductivity measured at different temperatures by step-wise oxidation (the differently toned data points indicate repeated measurement runs); and (d) Brower diagram measured on Verneuil grown SrTiO3 for comparison[11].…”